For all these reasons, so long as storminess remained great, the Pleistocene snowfields, according to the solar hypothesis, must have deepened and expanded. In duetime some of the snow was converted into glacial ice. When that occurred, the growth of the snowfield as well as of the ice cap must have been accelerated by glacial movement. Under such circumstances, as the ice crowded southward toward the source of the moisture by which it grew, the area of high pressure produced by its low temperature would expand. This would force the storm track southward in spite of the contrary tendency due to the sun. When the ice sheet had become very extensive, the track would be crowded relatively near to the northern margin of the trade-wind belt. Indeed, the Pleistocene ice sheets, at the time of their maximum extension, reached almost as far south as the latitude now marking the northern limit of the trade-wind belt in summer. As the storm track with its frequent low pressure and the subtropical belt with its high pressure were forced nearer and nearer together, the barometric gradient between the two presumably became greater, winds became stronger, and the storms more intense.This zonal crowding would be of special importance in summer, at which time it would also be most pronounced. In the first place, the storms would be crowded far upon the ice cap which would then be protected from the sun by a cover of fog and cloud more fully than at any other season. Furthermore, the close approach of the trade-wind belt to the storm belt would result in a great increase in the amount of moisture drawn from the belt of evaporation which the trade winds dominate. In the trade-wind belt, clear skies and high temperature make evaporation especially rapid. Indeed, in spite of the vast deserts it is probable that more than three-fourths of the total evaporation now taking place on the earth occurs in the belt of trades, an area which includes about one-half of the earth's surface.The agency which could produce this increased drawing northward of moisture from the trade-wind belt would be the winds blowing into the lows. According to the cyclonic hypothesis, many of these lows would be so strong that they would temporarily break down the subtropical belt of high pressure which now usually prevails between the trades and the zone of westerly winds. This belt is even now often broken by tropical cyclones. If the storms of more northerly regions temporarily destroyed the subtropical high-pressure belt, even though they still remained on its northern side, they would divert part of the trade winds. Hence the air which now is carried obliquely equatorward by those winds would be carried spirally northward into the cyclonic lows. Precipitation in the storm track on the margin of the relatively cold ice sheet would thus be much increased, for most winds from low latitudes carry abundant moisture. Such a diversion of moisture from low latitudes probably explains the deficiency of precipitation along the heat equator at times of solar activity, as shown in Fig. 3. Taken as a whole, the summer conditions, according to the cyclonic hypothesis, would be such that increased evaporation in low latitudes would coöperate with increased storminess, cloudiness, and fog in higher latitudes to preserve and increase the accumulation of ice upon the borders of the ice sheet. The greater the storminess, the more this would be true and the more the ice sheet would be able to hold its own against melting in summer. Such a combination of precipitation and of protection from the sun is especially important if an ice sheet is to grow.The meteorologist needs no geologic evidence that the storm track was shoved equatorward by the growth of the ice sheet, for he observes a similar shifting whenever a winter's snow cap occupies part of the normal stormtract. The geologist, however, may welcome geologic evidence that such an extreme shift of the storm track actually occurred during the Pleistocene. Harmer, in 1901, first pointed out the evidence which was repeated with approval by Wright of the Ireland Geological Survey in 1914.[44]According to these authorities, numerous boulders of a distinctive chalk were deposited by Pleistocene icebergs along the coast of Ireland. Their distribution shows that at the time of maximum glaciation the strong winds along the south coast of Ireland were from the northeast while today they are from the southwest. Such a reversal could apparently be produced only by a southward shift of the center of the main storm track from its present position in northern Ireland, Scotland, and Norway to a position across northern France, central Germany, and middle Russia. This would mean that while now the centers of the lows commonly move northeastward a short distance north of southern Ireland, they formerly moved eastward a short distance south of Ireland. It will be recalled that in the northern hemisphere the winds spiral into a low counter-clockwise and that they are strongest near the center. When the centers pass not far north of a given point, the strong winds therefore blow from the west or southwest, while when the centers pass just south of that point, the strong winds come from the east or northeast.In addition to the consequences of the crowding of the storm track toward the trade-wind belt, several other conditions presumably operated to favor the growth of the ice sheet. For example, the lowering of the sea level by the removal of water to form the snowfields and glaciers interfered with warm currents. It also increased the rate of erosion, for it was equivalent to an uplift ofall the land. One consequence of erosion and weathering was presumably a diminution of the carbon dioxide in the atmosphere, for although the ice covered perhaps a tenth of the lands and interfered with carbonation to that extent, the removal of large quantities of soil by accelerated erosion on the other nine-tenths perhaps more than counterbalanced the protective effect of the ice. At the same time, the general lowering of the temperature of the ocean as well as the lands increased the ocean's capacity for carbon dioxide and thus facilitated absorption. At a temperature of 50°F. water absorbs 32 per cent more carbon dioxide than at 68°. The high waves produced by the severe storms must have had a similar effect on a small scale. Thus the percentage of carbon dioxide in the atmosphere was presumably diminished. Of less significance than these changes in the lands and the air, but perhaps not negligible, was the increased salinity of the ocean which accompanied the removal of water to form snow, and the increase of the dissolved mineral load of the rejuvenated streams. Increased salinity slows up the deep-sea circulation, as we shall see in a later chapter. This increases the contrasts from zone to zone.At times of great solar activity the agencies mentioned above would apparently coöperate to cause an advance of ice sheets into lower latitudes. The degree of solar activity would have much to do with the final extent of the ice sheets. Nevertheless, certain terrestrial conditions would tend to set limits beyond which the ice would not greatly advance unless the storminess were extraordinarily severe. The most obvious of these conditions is the location of oceans and of deserts or semi-arid regions. The southwestward advance of the European ice sheet and the southeastward advance of the Labradorean sheet in America were stopped by the Atlantic. The semi-aridityof the Great Plains, produced by their position in the lee of the Rocky Mountains, stopped the advance of the Keewatin ice sheet toward the southwest. The advance of the European ice sheet southeast seems to have been stopped for similar reasons. The cessation of the advance would be brought about in such an area not alone by the light precipitation and abundant sunshine, but by the dryness of the air, and also by the power of dust to absorb the sun's heat. Much dust would presumably be drawn in from the dry regions by passing cyclonic storms and would be scattered over the ice.The advance of the ice is also slowed up by a rugged topography, as among the Appalachians in northern Pennsylvania. Such a topography besides opposing a physical obstruction to the movement of the ice provides bare south-facing slopes which the sun warms effectively. Such warm slopes are unfavorable to glacial advance. The rugged topography was perhaps quite as effective as the altitude of the Appalachians in causing the conspicuous northward dent in the glacial margin in Pennsylvania. Where glaciers lie in mountain valleys the advance beyond a certain point is often interfered with by the deployment of the ice at the mouths of gorges. Evaporation and melting are more rapid where a glacier is broad and thin than where it is narrow and thick, as in a gorge. Again, where the topography or the location of oceans or dry areas causes the glacial lobes to be long and narrow, the elongation of the lobe is apparently checked in several ways. Toward the end of the lobe, melting and evaporation increase rapidly because the planetary westerly winds are more likely to overcome the glacial winds and sweep across a long, narrow lobe than across a broad one. As they cross the lobe, they accelerate evaporation, and probably lessen cloudiness, with a consequentaugmentation of melting. Moreover, although lows rarely cross a broad ice sheet, they do cross a narrow lobe. For example, Nansen records that strong lows occasionally cross the narrow southern part of the Greenland ice sheet. The longer the lobe, the more likely it is that lows will cross it, instead of following its margin. Lows which cross a lobe do not yield so much snow to the tip as do those which follow the margin. Hence elongation is retarded and finally stopped even without a change in the earth's general climate.Because of these various reasons the advances of the ice during the several epochs of a glacial period might be approximately equal, even if the durations of the periods of storminess and low temperature were different. Indeed, they might be sub-equal, even if the periods differed in intensity as well as length. Differences in the periods would apparently be manifested less in the extent of the ice than in the depth of glacial erosion and in the thickness of the terminal moraines, outwash plains, and other glacial or glacio-fluvial formations.Having completed the consideration of the conditions leading to the advance of the ice, let us now consider the condition of North America at the time of maximum glaciation.[45]Over an area of nearly four million square miles, occupying practically all the northern half of the continent and part of the southern half, as appears in Fig. 6, the surface was a monotonous and almost level plain of ice covered with snow. When viewed from a high altitude, all parts except the margins must have presented a uniformly white and sparkling appearance. Along the margins, however, except to the north, thewhiteness was irregular, for the view must have included not only fresh snow, but moving clouds and dirty snow or ice. Along the borders where melting was in progress there was presumably more or less spottedness due to morainal material or glacial débris brought to the surface by ice shearage and wastage. Along the dry southwestern border it is also possible that there were numerous dark spots due to dust blown onto the ice by the wind.Fig. 6Fig. 6. Distribution of Pleistocene ice sheets.(After Schuchert.)The great white sheet with its ragged border was roughly circular in form, with its center in central Canada. Yet there were many departures from a perfectly circular form. Some were due to the oceans, for, except in northern Alaska, the ice extended into the ocean all the way from New Jersey around by the north to Washington. On the south, topographic conditions made the margin depart from a simple arc. From New Jersey to Ohio it swung northward. In the Mississippi Valley it reached far south; indeed most of the broad wedge between the Ohio and the Missouri rivers was occupied by ice. From latitude 37° near the junction of the Missouri and the Mississippi, however, the ice margin extended almost due north along the Missouri to central North Dakota. It then stretched westward to the Rockies. Farther west lowland glaciation was abundant as far south as western Washington. In the Rockies, the Cascades, and the Sierra Nevadas glaciation was common as far south as Colorado and southern California, respectively, and snowfields were doubtless extensive enough to make these ranges ribbons of white. Between these lofty ranges lay a great unglaciated region, but even in the Great Basin itself, in spite of its present aridity, certain ranges carried glaciers, while great lakes expanded widely.In this vast field of snow the glacial ice slowly crept outward, possibly at an average speed of half a foot a day, but varying from almost nothing in winter at the north, to several feet a day in summer at the south.[46]The force which caused the movement was the presence of the ice piled up not far from the margins. Almost certainly, however, there was no great dome from the center in Canada outward, as some early writers assumed. Such a dome would require that the ice be many thousands of feet thick near its center. This is impossible because of the fact that ice is more voluminous than water (about 9 per cent near the freezing point). Hence when subjected to sufficient pressure it changes to the liquid form. As friction and internal heat tend to keep the bottom of a glacier warm, even in cold regions, the probabilities are that only under very special conditions was a continental ice sheet much thicker than about 2500 feet. In Antarctica, where the temperature is much lower than was probably attained in the United States, the ice sheet is nearly level, several expeditions having traveled hundreds of miles with practically no change in altitude. In Shackleton's trip almost to the South Pole, he encountered a general rise of 3000 feet in 1200 miles. Mountains, however, projected through the ice even near the pole and the geologists conclude that the ice is not very thick even at the world's coldest point, the South Pole.Along the margin of the ice there were two sorts of movement, much more rapid than the slow creep of the ice. One was produced by the outward drift of snow carried by the outblowing dry winds and the other and more important was due to the passage of cyclonic storms. Along the border of the ice sheet, except at thenorth, storm presumably closely followed storm. Their movement, we judge, was relatively slow until near the southern end of the Mississippi lobe, but when this point was passed they moved much more rapidly, for then they could go toward instead of away from the far northern path which the sun prescribes when solar activity is great. The storms brought much snow to the icefield, perhaps sometimes in favored places as much as the hundred feet a year which is recorded for some winters in the Sierras at present. Even the unglaciated intermontane Great Basin presumably received considerable precipitation, perhaps twice as much as its present scanty supply. The rainfall was enough to support many lakes, one of which was ten times as large as Great Salt Lake; and grass was doubtless abundant upon many slopes which are now dry and barren. The relatively heavy precipitation in the Great Basin was probably due primarily to the increased number of storms, but may also have been much influenced by their slow eastward movement. The lows presumably moved slowly in that general region not only because they were retarded and turned from their normal path by the cold ice to the east, but because during the summer the area between the Sierra snowfields on the west and the Rocky Mountain and Mississippi Valley snowfields on the east was relatively warm. Hence it was normally a place of low pressure and therefore of inblowing winds. Slow-moving lows are much more effective than fast-moving ones in drawing moisture northwestward from the Gulf of Mexico, for they give the moisture more time to move spirally first northeast, under the influence of the normal southwesterly winds, then northwest and finally southwest as it approaches the storm center. In the case of the present lows, before much moisture-laden air can describe sucha circuit, first eastward and then westward, the storm center has nearly always moved eastward across the Rockies and even across the Great Plains. A result of this is the regular decrease in precipitation northward, northwestward, and westward from the Gulf of Mexico.Along the part of the glacial margins where for more than 3000 miles the North American ice entered the Atlantic and the Pacific oceans, myriads of great blocks broke off and floated away as stately icebergs, to scatter boulders far over the ocean floor and to melt in warmer climes. Where the margin lay upon the lands numerous streams issued from beneath the ice, milk-white with rock flour, and built up great outwash plains and valley trains of gravel and sand. Here and there, just beyond the ice, marginal lakes of strange shapes occupied valleys which had been dammed by the advancing ice. In many of them the water level rose until it reached some low point in the divide and then overflowed, forming rapids and waterfalls. Indeed, many of the waterfalls of the eastern United States and Canada were formed in just this way and not a few streams now occupy courses through ridges instead of parallel to them, as in pre-glacial times.In the zone to the south of the continental ice sheet, the plant and animal life of boreal, cool temperate, and warm temperate regions commingled curiously. Heather and Arctic willow crowded out elm and oak; musk ox, hairy mammoth, and marmot contested with deer, chipmunk, and skunk for a chance to live. Near the ice on slopes exposed to the cold glacial gales, the immigrant boreal species were dominant, but not far away in more protected areas the species that had formerly lived there held their own. In Europe during the last two advances of the great ice sheet the caveman also struggled withfierce animals and a fiercer climate to maintain life in an area whose habitability had long been decreasing.The next step in our history of glaciation is to outline the disappearance of the ice sheets. When a decrease in solar activity produced a corresponding decrease in storminess, several influences presumably combined to cause the disappearance of the ice. Most of their results are the reverse of those which brought on glaciation. A few special aspects, however, some of which have been discussed inEarth and Sun, ought to be brought to mind. A diminution in storminess lessens upward convection, wind velocity, and evaporation, and these changes, if they occurred, must have united to raise the temperature of the lower air by reducing the escape of heat. Again a decrease in the number and intensity of tropical cyclones presumably lessened the amount of moisture carried into mid-latitudes, and thus diminished the precipitation. The diminution of snowfall on the ice sheets when storminess diminished was probably highly important. The amount of precipitation on the sheets was presumably lessened still further by changes in the storminess of middle latitudes. When storminess diminishes, the lows follow a less definite path, as Kullmer's maps show, and on the average a more southerly path. Thus, instead of all the lows contributing snow to the ice sheet, a large fraction of the relatively few remaining lows would bring rain to areas south of the ice sheet. As storminess decreased, the trades and westerlies probably became steadier, and thus carried to high latitudes more warm water than when often interrupted by storms. Steadier southwesterly winds must have produced a greater movement of atmospheric as well as oceanic heat to high latitudes. The warming due to these two causes was probably the chief reason for the disappearance of the European ice sheetand of those on the Pacific coast of North America. The two greater American ice sheets, however, and the glaciers elsewhere in the lee of high mountain ranges, probably disappeared chiefly because of lessened precipitation. If there were no cyclonic storms to draw moisture northward from the Gulf of Mexico, most of North America east of the Rocky Mountain barrier would be arid. Therefore a diminution of storminess would be particularly effective in causing the disappearance of ice sheets in these regions.That evaporation was an especially important factor in causing the ice from the Keewatin center to disappear, is suggested by the relatively small amount of water-sorted material in its drift. In South Dakota, for example, less than 10 per cent of the drift is stratified.[47]On the other hand, Salisbury estimates that perhaps a third of the Labradorean drift in eastern Wisconsin is crudely stratified, about half of that in New Jersey, and more than half of the drift in western Europe.When the sun's activity began to diminish, all these conditions, as well as several others, would coöperate to cause the ice sheets to disappear. Step by step with their disappearance, the amelioration of the climate would progress so long as the period of solar inactivity continued and storms were rare. If the inactivity continued long enough, it would result in a fairly mild climate in high latitudes, though so long as the continents were emergent this mildness would not be of the extreme type. The inauguration of another cycle of increased disturbance of the sun, with a marked increase in storminess, would inaugurate another glacial epoch. Thus a succession of glacial and inter-glacial epochs might continue so long as the sun was repeatedly disturbed.CHAPTER VIIISOME PROBLEMS OF GLACIAL PERIODSHaving outlined in general terms the coming of the ice sheets and their disappearance, we are now ready to discuss certain problems of compelling climatic interest. The discussion will be grouped under five heads: (I) the localization of glaciation; (II) the sudden coming of glaciation; (III) peculiar variations in the height of the snow line and of glaciation; (IV) lakes and other evidences of humidity in unglaciated regions during the glacial epochs; (V) glaciation at sea level and in low latitudes in the Permian and Proterozoic eras. The discussion of perhaps the most difficult of all climatic problems of glaciation, that of the succession of cold glacial and mild inter-glacial epochs, has been postponed to the next to the final chapter of this book. It cannot be properly considered until we take up the history of solar disturbances.I. The first problem, the localization of the ice sheets, arises from the fact that in both the Pleistocene and the Permian periods glaciation was remarkably limited. In neither period were all parts of high latitudes glaciated; yet in both cases glaciation occurred in large regions in lower latitudes. Many explanations of this localization have been offered, but most are entirely inadequate. Even hypotheses with something of proven worth, such as those of variations in volcanic dust and in atmosphericcarbon dioxide, fail to account for localization. The cyclonic form of the solar hypothesis, however, seems to afford a satisfactory explanation.The distribution of the ice in the last glacial period is well known, and is shown in Fig. 6. Four-fifths of the ice-covered area, which was eight million square miles, more or less, was near the borders of the North Atlantic in eastern North America and northwestern Europe. The ice spread out from two great centers in North America, the Labradorean east of Hudson Bay, and the Keewatin west of the bay. There were also many glaciers in the western mountains, especially in Canada, while subordinate centers occurred in Newfoundland, the Adirondacks, and the White Mountains. The main ice sheet at its maximum extension reached as far south as latitude 39° in Kansas and Kentucky, and 37° in Illinois. Huge boulders were transferred more than one thousand miles from their source in Canada. The northward extension was somewhat less. Indeed, the northern margin of the continent was apparently relatively little glaciated and much of Alaska unglaciated. Why should northern Kentucky be glaciated when northern Alaska was not?In Europe the chief center from which the continental glacier moved was the Scandinavian highlands. It pushed across the depression now occupied by the Baltic to southern Russia and across the North Sea depression to England and Belgium. The Alps formed a center of considerable importance, and there were minor centers in Scotland, Ireland, the Pyrenees, Apennines, Caucasus, and Urals. In Asia numerous ranges also contained large glaciers, but practically all the glaciation was of the alpine type and very little of the vast northern lowland was covered with ice.In the southern hemisphere glaciation at low latitudeswas less striking than in the northern hemisphere. Most of the increase in the areas of ice was confined to mountains which today receive heavy precipitation and still contain small glaciers. Indeed, except for relatively slight glaciation in the Australian Alps and in Tasmania, most of the Pleistocene glaciation in the southern hemisphere was merely an extension of existing glaciers, such as those of south Chile, New Zealand, and the Andes. Nevertheless, fairly extensive glaciation existed much nearer the equator than is now the case.In considering the localization of Pleistocene glaciation, three main factors must be taken into account, namely, temperature, topography, and precipitation. The absence of glaciation in large parts of the Arctic regions of North America and of Asia makes it certain that low temperature was not the controlling factor. Aside from Antarctica, the coldest place in the world is northeastern Siberia. There for seven months the average temperature is below 0°C., while the mean for the whole year is below -10°C. If the temperature during a glacial period averaged 6°C. lower than now, as is commonly supposed, this part of Siberia would have had a temperature below freezing for at least nine months out of the twelve even if there were no snowfield to keep the summers cold. Yet even under such conditions no glaciation occurred, although in other places, such as parts of Canada and northwestern Europe, intense glaciation occurred where the mean temperature is much higher.The topography of the lands apparently had much more influence upon the localization of glaciation than did temperature. Its effect, however, was always to cause glaciation exactly where it would be expected and not in unexpected places as actually occurred. For example, in North America the western side of the Canadian Rockiessuffered intense glaciation, for there precipitation was heavy because the westerly winds from the Pacific are forced to give up their moisture as they rise. In the same way the western side of the Sierra Nevadas was much more heavily glaciated than the eastern side. In similar fashion the windward slopes of the Alps, the Caucasus, the Himalayas, and many other mountain ranges suffered extensive glaciation. Low temperature does not seem to have been the cause of this glaciation, for in that case it is hard to see why both sides of the various ranges did not show an equal percentage of increase in the size of their icefields.From what has been said as to temperature and topography, it is evident that variations in precipitation have had much more to do with glaciation than have variations in temperature. In the Arctic lowlands and on the leeward side of mountains, the slight development of glaciation appears to have been due to scarcity of precipitation. On the windward side of mountains, on the other hand, a notable increase in precipitation seems to have led to abundant glaciation. Such an increase in precipitation must be dependent on increased evaporation and this could arise either from relatively high temperature or strong winds. Since the temperature in the glacial period was lower than now, we seem forced to attribute the increased precipitation to a strengthening of the winds. If the westerly winds from the Pacific should increase in strength and waft more moisture to the western side of the Canadian Rockies, or if similar winds increased the snowfall on the upper slopes of the Alps or the Tian-Shan Mountains, the glaciers would extend lower than now without any change in temperature.Although the incompetence of low temperature to cause glaciation, and the relative unimportance of the mountainsin northeastern Canada and northwestern Europe throw most glacial hypotheses out of court, they are in harmony with the cyclonic hypothesis. The answer of that hypothesis to the problem of the localization of ice sheets seems to be found in certain maps of storminess and rainfall in relation to solar activity. In Fig. 2 a marked belt of increased storminess at times of many sunspots is seen in southern Canada. A comparison of this with a series of maps given inEarth and Sunshows that the stormy belt tends to migrate northward in harmony with an increase in the activity of the sun's atmosphere. If the sun were sufficiently active the belt of maximum storminess would apparently pass through the Keewatin and Labradorean centers of glaciation instead of well to the south of them, as at present. It would presumably cross another center in Greenland, and then would traverse the fourth of the great centers of Pleistocene glaciation in Scandinavia. It would not succeed in traversing northern Asia, however, any more than it does now, because of the great high-pressure area which develops there in winter. When the ice sheets expanded from the main centers of glaciation, the belt of storms would be pushed southward and outward. Thus it might give rise to minor centers of glaciers such as the Patrician between Hudson Bay and Lake Superior, or the centers in Ireland, Cornwall, Wales, and the northern Ural Mountains. As the main ice sheets advanced, however, the minor centers would be overridden and the entire mass of ice would be merged into one vast expanse in the Atlantic portion of each of the two continents.In this connection it may be well to consider briefly the most recent hypothesis as to the growth and hence the localization of glaciation. In 1911 and more fully in 1915,Hobbs,[48]advanced the anti-cyclonic hypothesis of the origin of ice sheets. This hypothesis has the great merit of focusing attention upon the fact that ice sheets are pronounced anti-cyclonic regions of high pressure. This is proved by the strong outblowing winds which prevail along their margins. Such winds must, of course, be balanced by inward-moving winds at high levels. Abundant observations prove that such is the case. For example, balloons sent up by Barkow near the margin of the Antarctic ice sheet reveal the occurrence of inblowing winds, although they rarely occur below a height of 9000 meters. The abundant data gathered by Guervain on the coast of Greenland indicate that outblowing winds prevail up to a height of about 4000 meters. At that height inblowing winds commence and increase in frequency until at an altitude of over 5000 meters they become more common than outblowing winds. It should be noted, however, that in both Antarctica and Greenland, although the winds at an elevation of less than a thousand meters generally blow outward, there are frequent and decided departures from this rule, so that "variable winds" are quite commonly mentioned in the reports of expeditions and balloon soundings.The undoubted anti-cyclonic conditions which Hobbs thus calls to the attention of scientists seem to him to necessitate a peculiar mechanism in order to produce the snow which feeds the glaciers. He assumes that the winds which blow toward the centers of the ice sheets at high levels carry the necessary moisture by which the glaciers grow. When the air descends in the centers of the highs, it is supposed to be chilled on reaching the surfaceof the ice, and hence to give up its moisture in the form of minute crystals. This conclusion is doubtful for several reasons. In the first place, Hobbs does not seem to appreciate the importance of the variable winds which he quotes Arctic and Antarctic explorers as describing quite frequently on the edges of the ice sheets. They are one of many signs that cyclonic storms are fairly frequent on the borders of the ice though not in its interior. Thus there is a distinct and sufficient form of precipitation actually at work near the margin of the ice, or exactly where the thickness of the ice sheet would lead us to expect.Another consideration which throws grave doubt on the anti-cyclonic hypothesis of ice sheets is the small amount of moisture possible in the highs because of their low temperature. Suppose, for the sake of argument, that the temperature in the middle of an ice sheet averages 20°F. This is probably much higher than the actual fact and therefore unduly favorable to the anti-cyclonic hypothesis. Suppose also that the decrease in temperature from the earth's surface upward proceeds at the rate of 1°F. for each 300 feet, which is 50 per cent less than the actual rate for air with only a slight amount of moisture, such as is found in cold regions. Then at a height of 10,000 feet, where the inblowing winds begin to be felt, the temperature would be -20°F. At that temperature the air is able to hold approximately 0.166 grain of moisture per cubic foot when fully saturated. This is an exceedingly small amount of moisture and even if it were all precipitated could scarcely build a glacier. However, it apparently would not be precipitated because when such air descends in the center of the anti-cyclone it is warmed adiabatically, that is, by compression. On reaching the surface it would have a temperature of 20°and would be able to hold 0.898 grain of water vapor per cubic foot; in other words, it would have a relative humidity of about 18 per cent. Under no reasonable assumption does the upper air at the center of an ice sheet appear to reach the surface with a relative humidity of more than 20 or 25 per cent. Such air cannot give up moisture. On the contrary, it absorbs it and tends to diminish rather than increase the thickness of the sheet of ice and snow. But after the surplus heat gained by descent has been lost by radiation, conduction, and evaporation, the air may become super-saturated with the moisture picked up while warm. Hobbs reports that explorers in Antarctica and Greenland have frequently observed condensation on their clothing. If such moisture is not derived directly from the men's own bodies, it is apparently picked up from the ice sheet by the descending air, and not added to the ice sheet by air from aloft.The relation of all this to the localization of ice sheets is this. If Hobbs' anti-cyclonic hypothesis of glacial growth is correct, it would appear that ice sheets should grow up where the temperature is lowest and the high-pressure areas most persistent; for instance, in northern Siberia. It would also appear that so far as the topography permitted, the ice sheets ought to move out uniformly in all directions; hence the ice sheet ought to be as prominent to the north of the Keewatin and Labradorean centers as to the south, which is by no means the case. Again, in mountainous regions, such as the glacial areas of Alaska and Chile, the glaciation ought not to be confined to the windward slope of the mountains so closely as is actually the fact. In each of these cases the glaciated region was large enough so that there was probably a true anti-cyclonic area comparable with that now prevailing over southern Greenland. In both placesthe correlation between glaciation and mountain ranges seems much too close to support the anti-cyclonic hypothesis, for the inblowing winds which on that hypothesis bring the moisture are shown by observation to occur at heights far greater than that of all but the loftiest ranges.II. The sudden coming of glaciation is another problem which has been a stumbling-block in the way of every glacial hypothesis. In hisClimates of Geologic Times, Schuchert states that the fossils give almost no warning of an approaching catastrophe. If glaciation were solely due to uplift, or other terrestrial changes aside from vulcanism, Schuchert holds that it would have come slowly and the stages preceding glaciation would have affected life sufficiently to be recorded in the rocks. He considers that the suddenness of the coming of glaciation is one of the strongest arguments against the carbon dioxide hypothesis of glaciation.According to the cyclonic hypothesis, however, the suddenness of the oncoming of glaciation is merely what would be expected on the basis of what happens today. Changes in the sun occur suddenly. The sunspot cycle is only eleven or twelve years long, and even this short period of activity is inaugurated more suddenly than it declines. Again the climatic record derived from the growth of trees, as given in Figs. 4 and 5, also shows that marked changes in climate are initiated more rapidly than they disappear. In this connection, however, it must be remembered that solar activity may arise in various ways, as will appear more fully later. Under certain conditions storminess may increase and decrease slowly.III. The height of the snow line and of glaciation furnishes another means of testing glacial hypotheses. It is well established that in times of glaciation the snow linewas depressed everywhere, but least near the equator. For example, according to Penck, permanent snow extended 4000 feet lower than now in the Alps, whereas it stood only 1500 feet below the present level near the equator in Venezuela. This unequal depression is not readily accounted for by any hypothesis depending solely upon the lowering of temperature. By the carbon dioxide and the volcanic dust hypotheses, the temperature presumably was lowered almost equally in all latitudes, but a little more at the equator than elsewhere. If glaciation were due to a temporary lessening of the radiation received from the sun, such as is demanded by the thermal solar hypothesis, and by the longer periods of Croll's hypothesis, the lowering would be distinctly greatest at the equator. Thus, according to all these hypotheses, the snow line should have been depressed most at the equator, instead of least.The cyclonic hypothesis explains the lesser depression of the snow line at the equator as due to a diminution of precipitation. The effectiveness of precipitation in this respect is illustrated by the present great difference in the height of the snow line on the humid and dry sides of mountains. On the wet eastern side of the Andes near the equator, the snow line lies at 16,000 feet; on the dry western side, at 18,500 feet. Again, although the humid side of the Himalayas lies toward the south, the snow line has a level of 15,000 feet, while farther north, on the dry side, it is 16,700 feet.[49]The fact that the snow line is lower near the margin of the Alps than toward the center points in the same direction. The bearing of all this on the glacial period may be judged by looking again at Fig. 3 in Chapter V. This shows that at times of sunspot activity and hence of augmented storminess, the precipitationdiminishes near the heat equator, that is, where the average temperature for the whole year is highest. At present the great size of the northern continents and their consequent high temperature in summer, cause the heat equator to lie north of the "real" equator, except where Australia draws it to the southward.[50]When large parts of the northern continents were covered with ice, however, the heat equator and the true equator were probably much closer than now, for the continents could not become so hot. If so, the diminution in equatorial precipitation, which accompanies increased storminess throughout the world as a whole, would take place more nearly along the true equator than appears in Fig. 3. Hence so far as precipitation alone is concerned, we should actually expect that the snow line near the equator would rise a little during glacial periods. Another factor, however, must be considered. Köppen's data, it will be remembered, show that at times of solar activity the earth's temperature falls more at the equator than in higher latitudes. If this effect were magnified it would lower the snow line. The actual position of the snow line at the equator during glacial periods thus appears to be the combined effect of diminished precipitation, which would raise the line, and of lower temperature, which would bring it down.Before leaving this subject it may be well to recall that the relative lessening of precipitation in equatorial latitudes during the glacial epochs was probably caused by the diversion of moisture from the trade-wind belt. This diversion was presumably due to the great number of tropical cyclones and to the fact that the cyclonic storms of middle latitudes also drew much moisture from the trade-wind belt in summer when the northern position ofthe sun drew that belt near the storm track which was forced to remain south of the ice sheet. Such diversion of moisture out of the trade-wind belt must diminish the amount of water vapor that is carried by the trades to equatorial regions; hence it would lessen precipitation in the belt of so-called equatorial calms, which lies along the heat equator rather than along the geographical equator.Another phase of the vertical distribution of glaciation has been the subject of considerable discussion. In the Alps and in many other mountains the glaciation of the Pleistocene period appears to have had its upper limit no higher than today. This has been variously interpreted. It seems, however, to be adequately explained as due to decreased precipitation at high altitudes during the cold periods. This is in spite of the fact that precipitation in general increased with increased storminess. The low temperature of glacial times presumably induced condensation at lower altitudes than now, and most of the precipitation occurred upon the lower slopes of the mountains, contributing to the lower glaciers, while little of it fell upon the highest glaciers. Above a moderate altitude in all lofty mountains the decrease in the amount of precipitation is rapid. In most cases the decrease begins at a height of less than 3000 feet above the base of the main slope, provided the slope is steep. The colder the air, the lower the altitude at which this occurs. For example, it is much lower in winter than in summer. Indeed, the higher altitudes in the Alps are sunny in winter even where there are abundant clouds lower down.IV. The presence of extensive lakes and other evidences of a pluvial climate during glacial periods in non-glaciated regions which are normally dry is another of the facts which most glacial hypotheses fail to explain satisfactorily.Beyond the ice sheets many regions appear to have enjoyed an unusually heavy precipitation during the glacial epochs. The evidence of this is abundant, including numerous abandoned strand lines of salt lakes and an abundance of coarse material in deltas and flood plains.J. D. Whitney,[51]in an interesting but neglected volume, was one of the first to marshal the evidence of this sort. More recently Free[52]has amplified this. According to him in the Great Basin region of the United States sixty-two basins either contain unmistakable evidence of lakes, or belong to one of the three great lake groups named below. Two of these, the Lake Lahontan and the Lake Bonneville groups, comprise twenty-nine present basins, while the third, the Owens-Searles chain, contained at least five large lakes, the lowest being in Death Valley. In western and central Asia a far greater series of salt lakes is found and most of these are surrounded by strands at high levels. Many of these are described inExplorations in Turkestan,The Pulse of Asia, andPalestine and Its Transformation. There has been a good deal of debate as to whether these lakes actually date from the glacial period, as is claimed by C. E. P. Brooks, for example, or from some other period. The evidence, however, seems to be convincing that the lakes expanded when the ice also expanded.According to the older glacial hypotheses the lower temperature which is postulated as the cause of glaciation would almost certainly mean less evaporation over the oceans and hence less precipitation during glacial periods. To counteract this the only way in which thelevel of the lakes could be raised would be because the lower temperature would cause less evaporation from their surfaces. It seems quite impossible, however, that the lowering of temperature, which is commonly taken to have been not more than 10°C., could counteract the lessened precipitation and also cause an enormous expansion of most of the lakes. For example, ancient Lake Bonneville was more than ten times as large as its modern remnant, Great Salt Lake, and its average depth more than forty times as great.[53]Many small lakes in the Old World expanded still more.[54]For example, in eastern Persia many basins which now contain no lake whatever are floored with vast deposits of lacustrine salt and are surrounded by old lake bluffs and beaches. In northern Africa similar conditions prevail.[55]Other, but less obvious, evidence of more abundant rainfall in regions that are now dry is found in thick strata of gravel, sand, and fine silt in the alluvial deposits of flood plains and deltas.[56]The cyclonic hypothesis supposes that increased storminess accounts for pluvial climates in regions that are now dry just as it accounts for glaciation in the regions of the ice sheets. Figs. 2 and 3, it will be remembered, illustrate what happens when the sun is active. Solar activity is accompanied by an increase in storminess in the southwestern United States in exactly the region where elevated strands of diminished salt lakes are most numerous. In Fig. 3, the same condition is seen inthe region of salt lakes in the Old World. Judging by these maps, which illustrate what has happened since careful meteorological records were kept, an increase in solar activity is accompanied by increased rainfall in large parts of what are now semi-arid and desert regions. Such precipitation would at once cause the level of the lakes to rise. Later, when ice sheets had developed in Europe and America, the high-pressure areas thus caused might force the main storm belt so far south that it would lie over these same arid regions. The increase in tropical hurricanes at times of abundant sunspots may also have a bearing on the climate of regions that are now arid. During the glacial period some of the hurricanes probably swept far over the lands. The numerous tropical cyclones of Australia, for example, are the chief source of precipitation for that continent.[57]Some of the stronger cyclones locally yield more rain in a day or two than other sources yield in a year.V. The occurrence of widespread glaciation near the tropics during the Permian, as shown in Fig. 7, has given rise to much discussion. The recent discovery of glaciation in latitudes as low as 30° in the Proterozoic is correspondingly significant. In all cases the occurrence of glaciation in low and middle latitudes is probably due to the same general causes. Doubtless the position and altitude of the mountains had something to do with the matter. Yet taken by itself this seems insufficient. Today the loftiest range in the world, the Himalayas, is almost unglaciated, although its southern slope may seem at first thought to be almost ideally located in this respect. Some parts rise over 20,000 feet and certain lower slopes receive 400 inches of rain per year. The small size of the Himalayan glaciers in spite of these favorable conditionsis apparently due largely to the seasonal character of the monsoon winds. The strong outblowing monsoons of winter cause about half the year to be very dry with clear skies and dry winds from the interior of Asia. In all low latitudes the sun rides high in the heavens at midday, even in winter, and thus melts snow fairly effectively in clear weather. This is highly unfavorable to glaciation. The inblowing southern monsoons bring all their moisture in midsummer at just the time when it is least effective in producing snow. Conditions similar to those now prevailing in the Himalayas must accompany any great uplift of the lands which produces high mountains and large continents in subtropical and middle latitudes. Hence, uplift alone cannot account for extensive glaciation in subtropical latitudes during the Permian and Proterozoic.Fig. 7Fig. 7. Permian geography and glaciation.(After Schuchert.)The assumption of a great general lowering of temperature is also not adequate to explain glaciation in subtropical latitudes. In the first place this would require a lowering of many degrees,—far more than in the Pleistocene glacial period. The marine fossils of the Permian, however, do not indicate any such condition. In the second place, if the lands were widespread as they appear to have been in the Permian, a general lowering of temperature would diminish rather than increase the present slight efficiency of the monsoons in producing glaciation. Monsoons depend upon the difference between the temperatures of land and water. If the general temperature were lowered, the reduction would be much less pronounced on the oceans than on the lands, for water tends to preserve a uniform temperature, not only because of its mobility, but because of the large amount of heat given out when freezing takes place, or consumed in evaporation. Hence the general lowering of temperaturewould make the contrast between continents and oceans less than at present in summer, for the land temperature would be brought toward that of the ocean. This would diminish the strength of the inblowing summer monsoons and thus cut off part of the supply of moisture. Evidence that this actually happened in the cold fourteenth century has already been given in Chapter VI. On the other hand, in winter the lands would be much colder than now and the oceans only a little colder, so that the dry outblowing monsoons of the cold season would increase in strength and would also last longer than at present. In addition to all this, the mere fact of low temperature would mean a general reduction in the amount of water vapor in the air. Thus, from almost every point of view a mere lowering of temperature seems to be ruled out as a cause of Permian glaciation. Moreover, if the Permian or Proterozoic glacial periods were so cold that the lands above latitude 30° were snow-covered most of the time, the normal surface winds in subtropical latitudes would be largely equatorward, just as the winter monsoons now are. Hence little or no moisture would be available to feed the snowfields which give rise to the glaciers.It has been assumed by Marsden Manson and others that increased general cloudiness would account for the subtropical glaciation of the Permian and Proterozoic. Granting for the moment that there could be universal persistent cloudiness, this would not prevent or counteract the outblowing anti-cyclonic winds so characteristic of great snowfields. Therefore, under the hypothesis of general cloudiness there would be no supply of moisture to cause glaciation in low latitudes. Indeed, persistent cloudiness in all higher latitudes would apparently deprive the Himalayas of most of their present moisture,for the interior of Asia would not become hot in summer and no inblowing monsoons would develop. In fact, winds of all kinds would seemingly be scarce, for they arise almost wholly from contrasts of temperature and hence of atmospheric pressure. The only way to get winds and hence precipitation would be to invoke some other agency, such as cyclonic storms, but that would be a departure from the supposition that glaciation arose from cloudiness.Let us now inquire how the cyclonic hypothesis accounts for glaciation in low latitudes. We will first consider the terrestrial conditions in the early Permian, the last period of glaciation in such latitudes. Geologists are almost universally agreed that the lands were exceptionally extensive and also high, especially in low latitudes. One evidence of this is the presence of abundant conglomerates composed of great boulders. It is also probable that the carbon dioxide in the air during the early Permian had been reduced to a minimum by the extraordinary amount of coal formed during the preceding period. This would tend to produce low temperature and thus make the conditions favorable for glaciation as soon as an accentuation of solar activity caused unusual storminess. If the storminess became extreme when terrestrial conditions were thus universally favorable to glaciation, it would presumably produce glaciation in low latitudes. Numerous and intense tropical cyclones would carry a vast amount of moisture out of the tropics, just as now happens when the sun is active, but on a far larger scale. The moisture would be precipitated on the equatorward slopes of the subtropical mountain ranges. At high elevations this precipitation would be in the form of snow even in summer. Tropical cyclones, however, as is shown inEarth and Sun, occur in the autumn andwinter as well as in summer. For example, in the Bay of Bengal the number recorded in October is fifty, the largest for any month; while in November it is thirty-four, and December fourteen as compared with an average of forty-two for the months of July to September. From January to March, when sunspot numbers averaged more than forty, the number of tropical hurricanes was 143 per cent greater than when the sunspot numbers averaged below forty. During the months from April to June, which also would be times of considerable snowy precipitation, tropical hurricanes averaged 58 per cent more numerous with sunspot numbers above forty than with numbers below forty, while from July to September the difference amounted to 23 per cent. Even at this season some snow falls on the higher slopes, while the increased cloudiness due to numerous storms also tends to preserve the snow. Thus a great increase in the frequency of sunspots is accompanied by increased intensity of tropical hurricanes, especially in the cooler autumn and spring months, and results not only in a greater accumulation of snow but in a decrease in the melting of the snow because of more abundant clouds. At such times as the Permian, the general low temperature due to rapid convection and to the scarcity of carbon dioxide presumably joined with the extension of the lands in producing great high-pressure areas over the lands in middle latitudes during the winters, and thus caused the more northern, or mid-latitude type of cyclonic storms to be shifted to the equatorward side of the continents at that season. This would cause an increase of precipitation in winter as well as during the months when tropical hurricanes abound. Many other circumstances would coöperate to produce a similar result. For example, the general low temperature would cause the sea to be covered withice in lower latitudes than now, and would help to create high-pressure areas in middle latitudes, thus driving the storms far south. If the sea water were fresher than now, as it probably was to a notable extent in the Proterozoic and perhaps to some slight extent in the Permian, the higher freezing point would also further the extension of the ice and help to keep the storms away from high latitudes. If to this there is added a distribution of land and sea such that the volume of the warm ocean currents flowing from low to high latitudes was diminished, as appears to have been the case, there seems to be no difficulty in explaining the subtropical location of the main glaciation in both the Permian and the Proterozoic. An increase of storminess seems to be the key to the whole situation.One other possibility may be mentioned, although little stress should be laid on it. InEarth and Sunit has been shown that the main storm track in both the northern and southern hemispheres is not concentric with the geographical poles. Both tracks are roughly concentric with the corresponding magnetic poles, a fact which may be important in connection with the hypothesis of an electrical effect of the sun upon terrestrial storminess. The magnetic poles are known to wander considerably. Such wandering gives rise to variations in the direction of the magnetic needle from year to year. In 1815 the compass in England pointed 24-½° W. of N. and in 1906 17° 45' W. Such a variation seems to mean a change of many miles in the location of the north magnetic pole. Certain changes in the daily march of electromagnetic phenomena over the oceans have led Bauer and his associates to suggest that the magnetic poles may even be subject to a slight daily movement in response to the changes in the relative positions of the earth and sun.Thus there seems to be a possibility that a pronounced change in the location of the magnetic pole in Permian times, for example, may have had some connection with a shifting in the location of the belt of storms. It must be clearly understood that there is as yet no evidence of any such change, and the matter is introduced merely to call attention to a possible line of investigation.Any hypothesis of Permian and Proterozoic glaciation must explain not only the glaciation of low latitudes but the lack of glaciation and the accumulation of red desert beds in high latitudes. The facts already presented seem to explain this. Glaciation could not occur extensively in high latitudes partly because during most of the year the air was too cold to hold much moisture, but still more because the winds for the most part must have blown outward from the cold northern areas and the cyclonic storm belt was pushed out of high latitudes. Because of these conditions precipitation was apparently limited to a relatively small number of storms during the summer. Hence great desert areas must have prevailed at high latitudes. Great aridity now prevails north of the Himalayas and related ranges, and red beds are accumulating in the centers of the great deserts, such as those of the Tarim Basin and the Transcaspian. The redness is not due to the original character of the rock, but to intense oxidation, as appears from the fact that along the edges of the desert and wherever occasional floods carry sediment far out into the midst of the sand, the material has the ordinary brownish shades. As soon as one goes out into the places where the sand has been exposed to the air for a long time, however, it becomes pink, and then red. Such conditions may have given rise to the high degree of oxidation in the famous Permian red beds. If the air of the early Permian contained an unusual percentage ofoxygen because of the release of that gas by the great plant beds which formed coal in the preceding era, as Chamberlin has thought probable, the tendency to produce red beds would be still further increased.It must not be supposed, however, that these conditions would absolutely limit glaciation to subtropical latitudes. The presence of early Permian glaciation in North America at Boston and in Alaska and in the Falkland Islands of the South Atlantic Ocean proves that at least locally there was sufficient moisture to form glaciers near the coast in relatively high latitudes. The possibility of this would depend entirely upon the form of the lands and the consequent course of ocean currents. Even in those high latitudes cyclonic storms would occur unless they were kept out by conditions of pressure such as have been described above.The marine faunas of Permian age in high latitudes have been interpreted as indicating mild oceanic temperatures. This is a point which requires further investigation. Warm oceans during times of slight solar activity are a necessary consequence of the cyclonic hypothesis, as will appear later. The present cold oceans seem to be the expectable result of the Pleistocene glaciation and of the present relatively disturbed condition of the sun. If a sudden disturbance threw the solar atmosphere into violent commotion within a few thousand years during Permian times, glaciation might occur as described above, while the oceans were still warm. In fact their warmth would increase evaporation while the violent cyclonic storms and high winds would cause heavy rain and keep the air cool by constantly raising it to high levels where it would rapidly radiate its heat into space.Nevertheless it is not yet possible to determine how warm the oceans were at the actual time of the Permianglaciation. Some faunas formerly reported as Permian are now known to be considerably older. Moreover, others of undoubted Permian age are probably not strictly contemporaneous with the glaciation. So far back in the geological record it is very doubtful whether we can date fossils within the limits of say 100,000 years. Yet a difference of 100,000 years would be more than enough to allow the fossils to have lived either before or after the glaciation, or in an inter-glacial epoch. One such epoch is known to have occurred and nine others are suggested by the inter-stratification of glacial till and marine sediments in eastern Australia. The warm currents which would flow poleward in inter-glacial epochs must have favored a prompt reintroduction of marine faunas driven out during times of glaciation. Taken all and all, the Permian glaciation seems to be accounted for by the cyclonic hypothesis quite as well as does the Pleistocene. In both these cases, as well as in the various pulsations of historic times, it seems to be necessary merely to magnify what is happening today in order to reproduce the conditions which prevailed in the past. If the conditions which now prevail at times of sunspot minima were magnified, they would give the mild conditions of inter-glacial epochs and similar periods. If the conditions which now prevail at times of sunspot maxima are magnified a little they seem to produce periods of climatic stress such as those of the fourteenth century. If they are magnified still more the result is apparently glacial epochs like those of the Pleistocene, and if they are magnified to a still greater extent, the result is Permian or Proterozoic glaciation. Other factors must indeed be favorable, for climatic changes are highly complex and are unquestionably due to a combination of circumstances. The point which is chiefly emphasized in this book is that amongthose several circumstances, changes in cyclonic storms due apparently to activity of the sun's atmosphere must always be reckoned.CHAPTER IXTHE ORIGIN OF LOESSOne of the most remarkable formations associated with glacial deposits consists of vast sheets of the fine-grained, yellowish, wind-blown material called loess. Somewhat peculiar climatic conditions evidently prevailed when it was formed. At present similar deposits are being laid down only near the leeward margin of great deserts. The famous loess deposits of China in the lee of the Desert of Gobi are examples. During the Pleistocene period, however, loess accumulated in a broad zone along the margin of the ice sheet at its maximum extent. In the Old World it extended from France across Germany and through the Black Earth region of Russia into Siberia. In the New World a still larger area is loess-covered. In the Mississippi Valley, tens of thousands of square miles are mantled by a layer exceeding twenty feet in thickness and in many places approaching a hundred feet. Neither the North American nor the European deposits are associated with a desert. Indeed, loess is lacking in the western and drier parts of the great plains and is best developed in the well-watered states of Iowa, Illinois, and Missouri. Part of the loess overlies the non-glacial materials of the great central plain, but the northern portions overlie the drift deposits of the first three glaciations. A few traces of loess are associated with the Kansan and Illinoian, the second and third glaciations, but most of the Americanloess appears to have been formed at approximately the time of the Iowan or fourth glaciation, while only a little overlies the drift sheets of the Wisconsin age. The loess is thickest near the margin of the Iowan till sheet and thins progressively both north and south. The thinning southward is abrupt along the stream divides, but very gradual along the larger valleys. Indeed, loess is abundant along the bluffs of the Mississippi, especially the east bluff, almost to the Gulf of Mexico.[58]It is now generally agreed that all typical loess is wind blown. There is still much question, however, as to its time of origin, and thus indirectly as to its climatic implications. Several American and European students have thought that the loess dates from inter-glacial times. On the other hand, Penck has concluded that the loess was formed shortly before the commencement of the glacial epochs; while many American geologists hold that the loess accumulated while the ice sheets were at approximately their maximum size. W. J. McGee, Chamberlin and Salisbury, Keyes, and others lean toward this view. In this chapter the hypothesis is advanced that it was formed at the one other possible time, namely, immediately following the retreat of the ice.These four hypotheses as to the time of origin of loess imply the following differences in its climatic relations. If loess was formed during typical inter-glacial epochs, or toward the close of such epochs, profound general aridity must seemingly have prevailed in order to kill off the vegetation and thus enable the wind to pick up sufficient dust. If the loess was formed during times of extreme glaciation when the glaciers were supplying large quantities of fine material to outflowing streams, less aridity would be required, but there must have beensharp contrasts between wet seasons in summer when the snow was melting and dry seasons in winter when the storms were forced far south by the glacial high pressure. Alternate floods and droughts would thus affect broad areas along the streams. Hence arises the hypothesis that the wind obtained the loess from the flood plains of streams at times of maximum glaciation. If the loess was formed during the rapid retreat of the ice, alternate summer floods and winter droughts would still prevail, but much material could also be obtained by the winds not only from flood plains, but also from the deposits exposed by the melting of the ice and not yet covered by vegetation.The evidence for and against the several hypotheses may be stated briefly. In support of the hypothesis of the inter-glacial origin of loess, Shimek and others state that the glacial drift which lies beneath the loess commonly gives evidence that some time elapsed between the disappearance of the ice and the deposition of the loess. For example, abundant shells of land snails in the loess are not of the sort now found in colder regions, but resemble those found in the drier regions. It is probable that if they represented a glacial epoch they would be depauperated by the cold as are the snails of far northern regions. The gravel pavement discussed below seems to be strong evidence of erosion between the retreat of the ice and the deposition of the loess.Turning to the second hypothesis, namely, that the loess accumulated near the close of the inter-glacial epoch rather than in the midst of it, we may follow Penck. The mammalian fossils seem to him to prove that the loess was formed while boreal animals occupied the region, for they include remains of the hairy mammoth, woolly rhinoceros, and reindeer. On the other hand, the typicalinter-glacial beds not far away yield remains of species characteristic of milder climates, such as the elephant, the smaller rhinoceros, and the deer. In connection with these facts it should be noted that occasional remains of tundra vegetation and of trees are found beneath the loess, while in the loess itself certain steppe animals, such as the common gopher or spermaphyl, are found. Penck interprets this as indicating a progressive desiccation culminating just before the oncoming of the next ice sheet.The evidence advanced in favor of the hypothesis that the loess was formed when glaciation was near its maximum includes the fact that if the loess does not represent the outwash from the Iowan ice, there is little else that does, and presumably there must have been outwash. Also the distribution of loess along the margins of streams suggests that much of the material came from the flood plains of overloaded streams flowing from the melting ice.Although there are some points in favor of the hypothesis that the loess originated (1) in strictly inter-glacial times, (2) at the end of inter-glacial epochs, and (3) at times of full glaciation, each hypothesis is much weakened by evidence that supports the others. The evidence of boreal animals seems to disprove the hypothesis that the loess was formed in the middle of a mild inter-glacial epoch. On the other hand, Penck's hypothesis as to loess at the end of inter-glacial times fails to account for certain characteristics of the lowest part of the loess deposits and of the underlying topography. Instead of normal valleys and consequent prompt drainage such as ought to have developed before the end of a long inter-glacial epoch, the surface on which the loess lies shows many undrained depressions. Some of these can be seenin exposed banks, while many more are inferred from the presence of shells of pond snails here and there in the overlying loess. The pond snails presumably lived in shallow pools occupying depressions in the uneven surface left by the ice. Another reason for questioning whether the loess was formed at the end of an inter-glacial epoch is that this hypothesis does not provide a reasonable origin for the material which composes the loess. Near the Alps where the loess deposits are small and where glaciers probably persisted in the inter-glacial epochs and thus supplied flood plain material in large quantities, this does not appear important. In the broad upper Mississippi Basin, however, and also in the Black Earth region of Russia there seems to be no way to get the large body of material composing the loess except by assuming the existence of great deserts to windward. But there seems to be little or no evidence of such deserts where they could be effective. The mineralogical character of the loess of Iowan age proves that the material came from granitic rocks, such as formed a large part of the drift. The nearest extensive outcrops of granite are in the southwestern part of the United States, nearly a thousand miles from Iowa and Illinois. But the loess is thickest near the ice margin and thins toward the southwest and in other directions, whereas if its source were the southwestern desert, its maximum thickness would probably be near the margin of the desert.The evidence cited above seems inconsistent not only with the hypothesis that the loess was formed at the end of an inter-glacial epoch, but also with the idea that it originated at times of maximum glaciation either from river-borne sediments or from any other source. A further and more convincing reason for this last conclusion is the probability and almost the certainty thatwhen the ice advanced, its front lay close to areas where the vegetation was not much thinner than that which today prevails under similar climatic conditions. If the average temperature of glacial maxima was only 6°C. lower than that of today, the conditions just beyond the ice front when it was in the loess region from southern Illinois to Minnesota would have been like those now prevailing in Canada from New Brunswick to Winnipeg. The vegetation there is quite different from the grassy, semi-arid vegetation of which evidence is found in the loess. The roots and stalks of such grassy vegetation are generally agreed to have helped produce the columnar structure which enables the loess to stand with almost vertical surfaces.
For all these reasons, so long as storminess remained great, the Pleistocene snowfields, according to the solar hypothesis, must have deepened and expanded. In duetime some of the snow was converted into glacial ice. When that occurred, the growth of the snowfield as well as of the ice cap must have been accelerated by glacial movement. Under such circumstances, as the ice crowded southward toward the source of the moisture by which it grew, the area of high pressure produced by its low temperature would expand. This would force the storm track southward in spite of the contrary tendency due to the sun. When the ice sheet had become very extensive, the track would be crowded relatively near to the northern margin of the trade-wind belt. Indeed, the Pleistocene ice sheets, at the time of their maximum extension, reached almost as far south as the latitude now marking the northern limit of the trade-wind belt in summer. As the storm track with its frequent low pressure and the subtropical belt with its high pressure were forced nearer and nearer together, the barometric gradient between the two presumably became greater, winds became stronger, and the storms more intense.
This zonal crowding would be of special importance in summer, at which time it would also be most pronounced. In the first place, the storms would be crowded far upon the ice cap which would then be protected from the sun by a cover of fog and cloud more fully than at any other season. Furthermore, the close approach of the trade-wind belt to the storm belt would result in a great increase in the amount of moisture drawn from the belt of evaporation which the trade winds dominate. In the trade-wind belt, clear skies and high temperature make evaporation especially rapid. Indeed, in spite of the vast deserts it is probable that more than three-fourths of the total evaporation now taking place on the earth occurs in the belt of trades, an area which includes about one-half of the earth's surface.
The agency which could produce this increased drawing northward of moisture from the trade-wind belt would be the winds blowing into the lows. According to the cyclonic hypothesis, many of these lows would be so strong that they would temporarily break down the subtropical belt of high pressure which now usually prevails between the trades and the zone of westerly winds. This belt is even now often broken by tropical cyclones. If the storms of more northerly regions temporarily destroyed the subtropical high-pressure belt, even though they still remained on its northern side, they would divert part of the trade winds. Hence the air which now is carried obliquely equatorward by those winds would be carried spirally northward into the cyclonic lows. Precipitation in the storm track on the margin of the relatively cold ice sheet would thus be much increased, for most winds from low latitudes carry abundant moisture. Such a diversion of moisture from low latitudes probably explains the deficiency of precipitation along the heat equator at times of solar activity, as shown in Fig. 3. Taken as a whole, the summer conditions, according to the cyclonic hypothesis, would be such that increased evaporation in low latitudes would coöperate with increased storminess, cloudiness, and fog in higher latitudes to preserve and increase the accumulation of ice upon the borders of the ice sheet. The greater the storminess, the more this would be true and the more the ice sheet would be able to hold its own against melting in summer. Such a combination of precipitation and of protection from the sun is especially important if an ice sheet is to grow.
The meteorologist needs no geologic evidence that the storm track was shoved equatorward by the growth of the ice sheet, for he observes a similar shifting whenever a winter's snow cap occupies part of the normal stormtract. The geologist, however, may welcome geologic evidence that such an extreme shift of the storm track actually occurred during the Pleistocene. Harmer, in 1901, first pointed out the evidence which was repeated with approval by Wright of the Ireland Geological Survey in 1914.[44]According to these authorities, numerous boulders of a distinctive chalk were deposited by Pleistocene icebergs along the coast of Ireland. Their distribution shows that at the time of maximum glaciation the strong winds along the south coast of Ireland were from the northeast while today they are from the southwest. Such a reversal could apparently be produced only by a southward shift of the center of the main storm track from its present position in northern Ireland, Scotland, and Norway to a position across northern France, central Germany, and middle Russia. This would mean that while now the centers of the lows commonly move northeastward a short distance north of southern Ireland, they formerly moved eastward a short distance south of Ireland. It will be recalled that in the northern hemisphere the winds spiral into a low counter-clockwise and that they are strongest near the center. When the centers pass not far north of a given point, the strong winds therefore blow from the west or southwest, while when the centers pass just south of that point, the strong winds come from the east or northeast.
In addition to the consequences of the crowding of the storm track toward the trade-wind belt, several other conditions presumably operated to favor the growth of the ice sheet. For example, the lowering of the sea level by the removal of water to form the snowfields and glaciers interfered with warm currents. It also increased the rate of erosion, for it was equivalent to an uplift ofall the land. One consequence of erosion and weathering was presumably a diminution of the carbon dioxide in the atmosphere, for although the ice covered perhaps a tenth of the lands and interfered with carbonation to that extent, the removal of large quantities of soil by accelerated erosion on the other nine-tenths perhaps more than counterbalanced the protective effect of the ice. At the same time, the general lowering of the temperature of the ocean as well as the lands increased the ocean's capacity for carbon dioxide and thus facilitated absorption. At a temperature of 50°F. water absorbs 32 per cent more carbon dioxide than at 68°. The high waves produced by the severe storms must have had a similar effect on a small scale. Thus the percentage of carbon dioxide in the atmosphere was presumably diminished. Of less significance than these changes in the lands and the air, but perhaps not negligible, was the increased salinity of the ocean which accompanied the removal of water to form snow, and the increase of the dissolved mineral load of the rejuvenated streams. Increased salinity slows up the deep-sea circulation, as we shall see in a later chapter. This increases the contrasts from zone to zone.
At times of great solar activity the agencies mentioned above would apparently coöperate to cause an advance of ice sheets into lower latitudes. The degree of solar activity would have much to do with the final extent of the ice sheets. Nevertheless, certain terrestrial conditions would tend to set limits beyond which the ice would not greatly advance unless the storminess were extraordinarily severe. The most obvious of these conditions is the location of oceans and of deserts or semi-arid regions. The southwestward advance of the European ice sheet and the southeastward advance of the Labradorean sheet in America were stopped by the Atlantic. The semi-aridityof the Great Plains, produced by their position in the lee of the Rocky Mountains, stopped the advance of the Keewatin ice sheet toward the southwest. The advance of the European ice sheet southeast seems to have been stopped for similar reasons. The cessation of the advance would be brought about in such an area not alone by the light precipitation and abundant sunshine, but by the dryness of the air, and also by the power of dust to absorb the sun's heat. Much dust would presumably be drawn in from the dry regions by passing cyclonic storms and would be scattered over the ice.
The advance of the ice is also slowed up by a rugged topography, as among the Appalachians in northern Pennsylvania. Such a topography besides opposing a physical obstruction to the movement of the ice provides bare south-facing slopes which the sun warms effectively. Such warm slopes are unfavorable to glacial advance. The rugged topography was perhaps quite as effective as the altitude of the Appalachians in causing the conspicuous northward dent in the glacial margin in Pennsylvania. Where glaciers lie in mountain valleys the advance beyond a certain point is often interfered with by the deployment of the ice at the mouths of gorges. Evaporation and melting are more rapid where a glacier is broad and thin than where it is narrow and thick, as in a gorge. Again, where the topography or the location of oceans or dry areas causes the glacial lobes to be long and narrow, the elongation of the lobe is apparently checked in several ways. Toward the end of the lobe, melting and evaporation increase rapidly because the planetary westerly winds are more likely to overcome the glacial winds and sweep across a long, narrow lobe than across a broad one. As they cross the lobe, they accelerate evaporation, and probably lessen cloudiness, with a consequentaugmentation of melting. Moreover, although lows rarely cross a broad ice sheet, they do cross a narrow lobe. For example, Nansen records that strong lows occasionally cross the narrow southern part of the Greenland ice sheet. The longer the lobe, the more likely it is that lows will cross it, instead of following its margin. Lows which cross a lobe do not yield so much snow to the tip as do those which follow the margin. Hence elongation is retarded and finally stopped even without a change in the earth's general climate.
Because of these various reasons the advances of the ice during the several epochs of a glacial period might be approximately equal, even if the durations of the periods of storminess and low temperature were different. Indeed, they might be sub-equal, even if the periods differed in intensity as well as length. Differences in the periods would apparently be manifested less in the extent of the ice than in the depth of glacial erosion and in the thickness of the terminal moraines, outwash plains, and other glacial or glacio-fluvial formations.
Having completed the consideration of the conditions leading to the advance of the ice, let us now consider the condition of North America at the time of maximum glaciation.[45]Over an area of nearly four million square miles, occupying practically all the northern half of the continent and part of the southern half, as appears in Fig. 6, the surface was a monotonous and almost level plain of ice covered with snow. When viewed from a high altitude, all parts except the margins must have presented a uniformly white and sparkling appearance. Along the margins, however, except to the north, thewhiteness was irregular, for the view must have included not only fresh snow, but moving clouds and dirty snow or ice. Along the borders where melting was in progress there was presumably more or less spottedness due to morainal material or glacial débris brought to the surface by ice shearage and wastage. Along the dry southwestern border it is also possible that there were numerous dark spots due to dust blown onto the ice by the wind.
Fig. 6. Distribution of Pleistocene ice sheets.(After Schuchert.)
The great white sheet with its ragged border was roughly circular in form, with its center in central Canada. Yet there were many departures from a perfectly circular form. Some were due to the oceans, for, except in northern Alaska, the ice extended into the ocean all the way from New Jersey around by the north to Washington. On the south, topographic conditions made the margin depart from a simple arc. From New Jersey to Ohio it swung northward. In the Mississippi Valley it reached far south; indeed most of the broad wedge between the Ohio and the Missouri rivers was occupied by ice. From latitude 37° near the junction of the Missouri and the Mississippi, however, the ice margin extended almost due north along the Missouri to central North Dakota. It then stretched westward to the Rockies. Farther west lowland glaciation was abundant as far south as western Washington. In the Rockies, the Cascades, and the Sierra Nevadas glaciation was common as far south as Colorado and southern California, respectively, and snowfields were doubtless extensive enough to make these ranges ribbons of white. Between these lofty ranges lay a great unglaciated region, but even in the Great Basin itself, in spite of its present aridity, certain ranges carried glaciers, while great lakes expanded widely.
In this vast field of snow the glacial ice slowly crept outward, possibly at an average speed of half a foot a day, but varying from almost nothing in winter at the north, to several feet a day in summer at the south.[46]The force which caused the movement was the presence of the ice piled up not far from the margins. Almost certainly, however, there was no great dome from the center in Canada outward, as some early writers assumed. Such a dome would require that the ice be many thousands of feet thick near its center. This is impossible because of the fact that ice is more voluminous than water (about 9 per cent near the freezing point). Hence when subjected to sufficient pressure it changes to the liquid form. As friction and internal heat tend to keep the bottom of a glacier warm, even in cold regions, the probabilities are that only under very special conditions was a continental ice sheet much thicker than about 2500 feet. In Antarctica, where the temperature is much lower than was probably attained in the United States, the ice sheet is nearly level, several expeditions having traveled hundreds of miles with practically no change in altitude. In Shackleton's trip almost to the South Pole, he encountered a general rise of 3000 feet in 1200 miles. Mountains, however, projected through the ice even near the pole and the geologists conclude that the ice is not very thick even at the world's coldest point, the South Pole.
Along the margin of the ice there were two sorts of movement, much more rapid than the slow creep of the ice. One was produced by the outward drift of snow carried by the outblowing dry winds and the other and more important was due to the passage of cyclonic storms. Along the border of the ice sheet, except at thenorth, storm presumably closely followed storm. Their movement, we judge, was relatively slow until near the southern end of the Mississippi lobe, but when this point was passed they moved much more rapidly, for then they could go toward instead of away from the far northern path which the sun prescribes when solar activity is great. The storms brought much snow to the icefield, perhaps sometimes in favored places as much as the hundred feet a year which is recorded for some winters in the Sierras at present. Even the unglaciated intermontane Great Basin presumably received considerable precipitation, perhaps twice as much as its present scanty supply. The rainfall was enough to support many lakes, one of which was ten times as large as Great Salt Lake; and grass was doubtless abundant upon many slopes which are now dry and barren. The relatively heavy precipitation in the Great Basin was probably due primarily to the increased number of storms, but may also have been much influenced by their slow eastward movement. The lows presumably moved slowly in that general region not only because they were retarded and turned from their normal path by the cold ice to the east, but because during the summer the area between the Sierra snowfields on the west and the Rocky Mountain and Mississippi Valley snowfields on the east was relatively warm. Hence it was normally a place of low pressure and therefore of inblowing winds. Slow-moving lows are much more effective than fast-moving ones in drawing moisture northwestward from the Gulf of Mexico, for they give the moisture more time to move spirally first northeast, under the influence of the normal southwesterly winds, then northwest and finally southwest as it approaches the storm center. In the case of the present lows, before much moisture-laden air can describe sucha circuit, first eastward and then westward, the storm center has nearly always moved eastward across the Rockies and even across the Great Plains. A result of this is the regular decrease in precipitation northward, northwestward, and westward from the Gulf of Mexico.
Along the part of the glacial margins where for more than 3000 miles the North American ice entered the Atlantic and the Pacific oceans, myriads of great blocks broke off and floated away as stately icebergs, to scatter boulders far over the ocean floor and to melt in warmer climes. Where the margin lay upon the lands numerous streams issued from beneath the ice, milk-white with rock flour, and built up great outwash plains and valley trains of gravel and sand. Here and there, just beyond the ice, marginal lakes of strange shapes occupied valleys which had been dammed by the advancing ice. In many of them the water level rose until it reached some low point in the divide and then overflowed, forming rapids and waterfalls. Indeed, many of the waterfalls of the eastern United States and Canada were formed in just this way and not a few streams now occupy courses through ridges instead of parallel to them, as in pre-glacial times.
In the zone to the south of the continental ice sheet, the plant and animal life of boreal, cool temperate, and warm temperate regions commingled curiously. Heather and Arctic willow crowded out elm and oak; musk ox, hairy mammoth, and marmot contested with deer, chipmunk, and skunk for a chance to live. Near the ice on slopes exposed to the cold glacial gales, the immigrant boreal species were dominant, but not far away in more protected areas the species that had formerly lived there held their own. In Europe during the last two advances of the great ice sheet the caveman also struggled withfierce animals and a fiercer climate to maintain life in an area whose habitability had long been decreasing.
The next step in our history of glaciation is to outline the disappearance of the ice sheets. When a decrease in solar activity produced a corresponding decrease in storminess, several influences presumably combined to cause the disappearance of the ice. Most of their results are the reverse of those which brought on glaciation. A few special aspects, however, some of which have been discussed inEarth and Sun, ought to be brought to mind. A diminution in storminess lessens upward convection, wind velocity, and evaporation, and these changes, if they occurred, must have united to raise the temperature of the lower air by reducing the escape of heat. Again a decrease in the number and intensity of tropical cyclones presumably lessened the amount of moisture carried into mid-latitudes, and thus diminished the precipitation. The diminution of snowfall on the ice sheets when storminess diminished was probably highly important. The amount of precipitation on the sheets was presumably lessened still further by changes in the storminess of middle latitudes. When storminess diminishes, the lows follow a less definite path, as Kullmer's maps show, and on the average a more southerly path. Thus, instead of all the lows contributing snow to the ice sheet, a large fraction of the relatively few remaining lows would bring rain to areas south of the ice sheet. As storminess decreased, the trades and westerlies probably became steadier, and thus carried to high latitudes more warm water than when often interrupted by storms. Steadier southwesterly winds must have produced a greater movement of atmospheric as well as oceanic heat to high latitudes. The warming due to these two causes was probably the chief reason for the disappearance of the European ice sheetand of those on the Pacific coast of North America. The two greater American ice sheets, however, and the glaciers elsewhere in the lee of high mountain ranges, probably disappeared chiefly because of lessened precipitation. If there were no cyclonic storms to draw moisture northward from the Gulf of Mexico, most of North America east of the Rocky Mountain barrier would be arid. Therefore a diminution of storminess would be particularly effective in causing the disappearance of ice sheets in these regions.
That evaporation was an especially important factor in causing the ice from the Keewatin center to disappear, is suggested by the relatively small amount of water-sorted material in its drift. In South Dakota, for example, less than 10 per cent of the drift is stratified.[47]On the other hand, Salisbury estimates that perhaps a third of the Labradorean drift in eastern Wisconsin is crudely stratified, about half of that in New Jersey, and more than half of the drift in western Europe.
When the sun's activity began to diminish, all these conditions, as well as several others, would coöperate to cause the ice sheets to disappear. Step by step with their disappearance, the amelioration of the climate would progress so long as the period of solar inactivity continued and storms were rare. If the inactivity continued long enough, it would result in a fairly mild climate in high latitudes, though so long as the continents were emergent this mildness would not be of the extreme type. The inauguration of another cycle of increased disturbance of the sun, with a marked increase in storminess, would inaugurate another glacial epoch. Thus a succession of glacial and inter-glacial epochs might continue so long as the sun was repeatedly disturbed.
Having outlined in general terms the coming of the ice sheets and their disappearance, we are now ready to discuss certain problems of compelling climatic interest. The discussion will be grouped under five heads: (I) the localization of glaciation; (II) the sudden coming of glaciation; (III) peculiar variations in the height of the snow line and of glaciation; (IV) lakes and other evidences of humidity in unglaciated regions during the glacial epochs; (V) glaciation at sea level and in low latitudes in the Permian and Proterozoic eras. The discussion of perhaps the most difficult of all climatic problems of glaciation, that of the succession of cold glacial and mild inter-glacial epochs, has been postponed to the next to the final chapter of this book. It cannot be properly considered until we take up the history of solar disturbances.
I. The first problem, the localization of the ice sheets, arises from the fact that in both the Pleistocene and the Permian periods glaciation was remarkably limited. In neither period were all parts of high latitudes glaciated; yet in both cases glaciation occurred in large regions in lower latitudes. Many explanations of this localization have been offered, but most are entirely inadequate. Even hypotheses with something of proven worth, such as those of variations in volcanic dust and in atmosphericcarbon dioxide, fail to account for localization. The cyclonic form of the solar hypothesis, however, seems to afford a satisfactory explanation.
The distribution of the ice in the last glacial period is well known, and is shown in Fig. 6. Four-fifths of the ice-covered area, which was eight million square miles, more or less, was near the borders of the North Atlantic in eastern North America and northwestern Europe. The ice spread out from two great centers in North America, the Labradorean east of Hudson Bay, and the Keewatin west of the bay. There were also many glaciers in the western mountains, especially in Canada, while subordinate centers occurred in Newfoundland, the Adirondacks, and the White Mountains. The main ice sheet at its maximum extension reached as far south as latitude 39° in Kansas and Kentucky, and 37° in Illinois. Huge boulders were transferred more than one thousand miles from their source in Canada. The northward extension was somewhat less. Indeed, the northern margin of the continent was apparently relatively little glaciated and much of Alaska unglaciated. Why should northern Kentucky be glaciated when northern Alaska was not?
In Europe the chief center from which the continental glacier moved was the Scandinavian highlands. It pushed across the depression now occupied by the Baltic to southern Russia and across the North Sea depression to England and Belgium. The Alps formed a center of considerable importance, and there were minor centers in Scotland, Ireland, the Pyrenees, Apennines, Caucasus, and Urals. In Asia numerous ranges also contained large glaciers, but practically all the glaciation was of the alpine type and very little of the vast northern lowland was covered with ice.
In the southern hemisphere glaciation at low latitudeswas less striking than in the northern hemisphere. Most of the increase in the areas of ice was confined to mountains which today receive heavy precipitation and still contain small glaciers. Indeed, except for relatively slight glaciation in the Australian Alps and in Tasmania, most of the Pleistocene glaciation in the southern hemisphere was merely an extension of existing glaciers, such as those of south Chile, New Zealand, and the Andes. Nevertheless, fairly extensive glaciation existed much nearer the equator than is now the case.
In considering the localization of Pleistocene glaciation, three main factors must be taken into account, namely, temperature, topography, and precipitation. The absence of glaciation in large parts of the Arctic regions of North America and of Asia makes it certain that low temperature was not the controlling factor. Aside from Antarctica, the coldest place in the world is northeastern Siberia. There for seven months the average temperature is below 0°C., while the mean for the whole year is below -10°C. If the temperature during a glacial period averaged 6°C. lower than now, as is commonly supposed, this part of Siberia would have had a temperature below freezing for at least nine months out of the twelve even if there were no snowfield to keep the summers cold. Yet even under such conditions no glaciation occurred, although in other places, such as parts of Canada and northwestern Europe, intense glaciation occurred where the mean temperature is much higher.
The topography of the lands apparently had much more influence upon the localization of glaciation than did temperature. Its effect, however, was always to cause glaciation exactly where it would be expected and not in unexpected places as actually occurred. For example, in North America the western side of the Canadian Rockiessuffered intense glaciation, for there precipitation was heavy because the westerly winds from the Pacific are forced to give up their moisture as they rise. In the same way the western side of the Sierra Nevadas was much more heavily glaciated than the eastern side. In similar fashion the windward slopes of the Alps, the Caucasus, the Himalayas, and many other mountain ranges suffered extensive glaciation. Low temperature does not seem to have been the cause of this glaciation, for in that case it is hard to see why both sides of the various ranges did not show an equal percentage of increase in the size of their icefields.
From what has been said as to temperature and topography, it is evident that variations in precipitation have had much more to do with glaciation than have variations in temperature. In the Arctic lowlands and on the leeward side of mountains, the slight development of glaciation appears to have been due to scarcity of precipitation. On the windward side of mountains, on the other hand, a notable increase in precipitation seems to have led to abundant glaciation. Such an increase in precipitation must be dependent on increased evaporation and this could arise either from relatively high temperature or strong winds. Since the temperature in the glacial period was lower than now, we seem forced to attribute the increased precipitation to a strengthening of the winds. If the westerly winds from the Pacific should increase in strength and waft more moisture to the western side of the Canadian Rockies, or if similar winds increased the snowfall on the upper slopes of the Alps or the Tian-Shan Mountains, the glaciers would extend lower than now without any change in temperature.
Although the incompetence of low temperature to cause glaciation, and the relative unimportance of the mountainsin northeastern Canada and northwestern Europe throw most glacial hypotheses out of court, they are in harmony with the cyclonic hypothesis. The answer of that hypothesis to the problem of the localization of ice sheets seems to be found in certain maps of storminess and rainfall in relation to solar activity. In Fig. 2 a marked belt of increased storminess at times of many sunspots is seen in southern Canada. A comparison of this with a series of maps given inEarth and Sunshows that the stormy belt tends to migrate northward in harmony with an increase in the activity of the sun's atmosphere. If the sun were sufficiently active the belt of maximum storminess would apparently pass through the Keewatin and Labradorean centers of glaciation instead of well to the south of them, as at present. It would presumably cross another center in Greenland, and then would traverse the fourth of the great centers of Pleistocene glaciation in Scandinavia. It would not succeed in traversing northern Asia, however, any more than it does now, because of the great high-pressure area which develops there in winter. When the ice sheets expanded from the main centers of glaciation, the belt of storms would be pushed southward and outward. Thus it might give rise to minor centers of glaciers such as the Patrician between Hudson Bay and Lake Superior, or the centers in Ireland, Cornwall, Wales, and the northern Ural Mountains. As the main ice sheets advanced, however, the minor centers would be overridden and the entire mass of ice would be merged into one vast expanse in the Atlantic portion of each of the two continents.
In this connection it may be well to consider briefly the most recent hypothesis as to the growth and hence the localization of glaciation. In 1911 and more fully in 1915,Hobbs,[48]advanced the anti-cyclonic hypothesis of the origin of ice sheets. This hypothesis has the great merit of focusing attention upon the fact that ice sheets are pronounced anti-cyclonic regions of high pressure. This is proved by the strong outblowing winds which prevail along their margins. Such winds must, of course, be balanced by inward-moving winds at high levels. Abundant observations prove that such is the case. For example, balloons sent up by Barkow near the margin of the Antarctic ice sheet reveal the occurrence of inblowing winds, although they rarely occur below a height of 9000 meters. The abundant data gathered by Guervain on the coast of Greenland indicate that outblowing winds prevail up to a height of about 4000 meters. At that height inblowing winds commence and increase in frequency until at an altitude of over 5000 meters they become more common than outblowing winds. It should be noted, however, that in both Antarctica and Greenland, although the winds at an elevation of less than a thousand meters generally blow outward, there are frequent and decided departures from this rule, so that "variable winds" are quite commonly mentioned in the reports of expeditions and balloon soundings.
The undoubted anti-cyclonic conditions which Hobbs thus calls to the attention of scientists seem to him to necessitate a peculiar mechanism in order to produce the snow which feeds the glaciers. He assumes that the winds which blow toward the centers of the ice sheets at high levels carry the necessary moisture by which the glaciers grow. When the air descends in the centers of the highs, it is supposed to be chilled on reaching the surfaceof the ice, and hence to give up its moisture in the form of minute crystals. This conclusion is doubtful for several reasons. In the first place, Hobbs does not seem to appreciate the importance of the variable winds which he quotes Arctic and Antarctic explorers as describing quite frequently on the edges of the ice sheets. They are one of many signs that cyclonic storms are fairly frequent on the borders of the ice though not in its interior. Thus there is a distinct and sufficient form of precipitation actually at work near the margin of the ice, or exactly where the thickness of the ice sheet would lead us to expect.
Another consideration which throws grave doubt on the anti-cyclonic hypothesis of ice sheets is the small amount of moisture possible in the highs because of their low temperature. Suppose, for the sake of argument, that the temperature in the middle of an ice sheet averages 20°F. This is probably much higher than the actual fact and therefore unduly favorable to the anti-cyclonic hypothesis. Suppose also that the decrease in temperature from the earth's surface upward proceeds at the rate of 1°F. for each 300 feet, which is 50 per cent less than the actual rate for air with only a slight amount of moisture, such as is found in cold regions. Then at a height of 10,000 feet, where the inblowing winds begin to be felt, the temperature would be -20°F. At that temperature the air is able to hold approximately 0.166 grain of moisture per cubic foot when fully saturated. This is an exceedingly small amount of moisture and even if it were all precipitated could scarcely build a glacier. However, it apparently would not be precipitated because when such air descends in the center of the anti-cyclone it is warmed adiabatically, that is, by compression. On reaching the surface it would have a temperature of 20°and would be able to hold 0.898 grain of water vapor per cubic foot; in other words, it would have a relative humidity of about 18 per cent. Under no reasonable assumption does the upper air at the center of an ice sheet appear to reach the surface with a relative humidity of more than 20 or 25 per cent. Such air cannot give up moisture. On the contrary, it absorbs it and tends to diminish rather than increase the thickness of the sheet of ice and snow. But after the surplus heat gained by descent has been lost by radiation, conduction, and evaporation, the air may become super-saturated with the moisture picked up while warm. Hobbs reports that explorers in Antarctica and Greenland have frequently observed condensation on their clothing. If such moisture is not derived directly from the men's own bodies, it is apparently picked up from the ice sheet by the descending air, and not added to the ice sheet by air from aloft.
The relation of all this to the localization of ice sheets is this. If Hobbs' anti-cyclonic hypothesis of glacial growth is correct, it would appear that ice sheets should grow up where the temperature is lowest and the high-pressure areas most persistent; for instance, in northern Siberia. It would also appear that so far as the topography permitted, the ice sheets ought to move out uniformly in all directions; hence the ice sheet ought to be as prominent to the north of the Keewatin and Labradorean centers as to the south, which is by no means the case. Again, in mountainous regions, such as the glacial areas of Alaska and Chile, the glaciation ought not to be confined to the windward slope of the mountains so closely as is actually the fact. In each of these cases the glaciated region was large enough so that there was probably a true anti-cyclonic area comparable with that now prevailing over southern Greenland. In both placesthe correlation between glaciation and mountain ranges seems much too close to support the anti-cyclonic hypothesis, for the inblowing winds which on that hypothesis bring the moisture are shown by observation to occur at heights far greater than that of all but the loftiest ranges.
II. The sudden coming of glaciation is another problem which has been a stumbling-block in the way of every glacial hypothesis. In hisClimates of Geologic Times, Schuchert states that the fossils give almost no warning of an approaching catastrophe. If glaciation were solely due to uplift, or other terrestrial changes aside from vulcanism, Schuchert holds that it would have come slowly and the stages preceding glaciation would have affected life sufficiently to be recorded in the rocks. He considers that the suddenness of the coming of glaciation is one of the strongest arguments against the carbon dioxide hypothesis of glaciation.
According to the cyclonic hypothesis, however, the suddenness of the oncoming of glaciation is merely what would be expected on the basis of what happens today. Changes in the sun occur suddenly. The sunspot cycle is only eleven or twelve years long, and even this short period of activity is inaugurated more suddenly than it declines. Again the climatic record derived from the growth of trees, as given in Figs. 4 and 5, also shows that marked changes in climate are initiated more rapidly than they disappear. In this connection, however, it must be remembered that solar activity may arise in various ways, as will appear more fully later. Under certain conditions storminess may increase and decrease slowly.
III. The height of the snow line and of glaciation furnishes another means of testing glacial hypotheses. It is well established that in times of glaciation the snow linewas depressed everywhere, but least near the equator. For example, according to Penck, permanent snow extended 4000 feet lower than now in the Alps, whereas it stood only 1500 feet below the present level near the equator in Venezuela. This unequal depression is not readily accounted for by any hypothesis depending solely upon the lowering of temperature. By the carbon dioxide and the volcanic dust hypotheses, the temperature presumably was lowered almost equally in all latitudes, but a little more at the equator than elsewhere. If glaciation were due to a temporary lessening of the radiation received from the sun, such as is demanded by the thermal solar hypothesis, and by the longer periods of Croll's hypothesis, the lowering would be distinctly greatest at the equator. Thus, according to all these hypotheses, the snow line should have been depressed most at the equator, instead of least.
The cyclonic hypothesis explains the lesser depression of the snow line at the equator as due to a diminution of precipitation. The effectiveness of precipitation in this respect is illustrated by the present great difference in the height of the snow line on the humid and dry sides of mountains. On the wet eastern side of the Andes near the equator, the snow line lies at 16,000 feet; on the dry western side, at 18,500 feet. Again, although the humid side of the Himalayas lies toward the south, the snow line has a level of 15,000 feet, while farther north, on the dry side, it is 16,700 feet.[49]The fact that the snow line is lower near the margin of the Alps than toward the center points in the same direction. The bearing of all this on the glacial period may be judged by looking again at Fig. 3 in Chapter V. This shows that at times of sunspot activity and hence of augmented storminess, the precipitationdiminishes near the heat equator, that is, where the average temperature for the whole year is highest. At present the great size of the northern continents and their consequent high temperature in summer, cause the heat equator to lie north of the "real" equator, except where Australia draws it to the southward.[50]When large parts of the northern continents were covered with ice, however, the heat equator and the true equator were probably much closer than now, for the continents could not become so hot. If so, the diminution in equatorial precipitation, which accompanies increased storminess throughout the world as a whole, would take place more nearly along the true equator than appears in Fig. 3. Hence so far as precipitation alone is concerned, we should actually expect that the snow line near the equator would rise a little during glacial periods. Another factor, however, must be considered. Köppen's data, it will be remembered, show that at times of solar activity the earth's temperature falls more at the equator than in higher latitudes. If this effect were magnified it would lower the snow line. The actual position of the snow line at the equator during glacial periods thus appears to be the combined effect of diminished precipitation, which would raise the line, and of lower temperature, which would bring it down.
Before leaving this subject it may be well to recall that the relative lessening of precipitation in equatorial latitudes during the glacial epochs was probably caused by the diversion of moisture from the trade-wind belt. This diversion was presumably due to the great number of tropical cyclones and to the fact that the cyclonic storms of middle latitudes also drew much moisture from the trade-wind belt in summer when the northern position ofthe sun drew that belt near the storm track which was forced to remain south of the ice sheet. Such diversion of moisture out of the trade-wind belt must diminish the amount of water vapor that is carried by the trades to equatorial regions; hence it would lessen precipitation in the belt of so-called equatorial calms, which lies along the heat equator rather than along the geographical equator.
Another phase of the vertical distribution of glaciation has been the subject of considerable discussion. In the Alps and in many other mountains the glaciation of the Pleistocene period appears to have had its upper limit no higher than today. This has been variously interpreted. It seems, however, to be adequately explained as due to decreased precipitation at high altitudes during the cold periods. This is in spite of the fact that precipitation in general increased with increased storminess. The low temperature of glacial times presumably induced condensation at lower altitudes than now, and most of the precipitation occurred upon the lower slopes of the mountains, contributing to the lower glaciers, while little of it fell upon the highest glaciers. Above a moderate altitude in all lofty mountains the decrease in the amount of precipitation is rapid. In most cases the decrease begins at a height of less than 3000 feet above the base of the main slope, provided the slope is steep. The colder the air, the lower the altitude at which this occurs. For example, it is much lower in winter than in summer. Indeed, the higher altitudes in the Alps are sunny in winter even where there are abundant clouds lower down.
IV. The presence of extensive lakes and other evidences of a pluvial climate during glacial periods in non-glaciated regions which are normally dry is another of the facts which most glacial hypotheses fail to explain satisfactorily.Beyond the ice sheets many regions appear to have enjoyed an unusually heavy precipitation during the glacial epochs. The evidence of this is abundant, including numerous abandoned strand lines of salt lakes and an abundance of coarse material in deltas and flood plains.J. D. Whitney,[51]in an interesting but neglected volume, was one of the first to marshal the evidence of this sort. More recently Free[52]has amplified this. According to him in the Great Basin region of the United States sixty-two basins either contain unmistakable evidence of lakes, or belong to one of the three great lake groups named below. Two of these, the Lake Lahontan and the Lake Bonneville groups, comprise twenty-nine present basins, while the third, the Owens-Searles chain, contained at least five large lakes, the lowest being in Death Valley. In western and central Asia a far greater series of salt lakes is found and most of these are surrounded by strands at high levels. Many of these are described inExplorations in Turkestan,The Pulse of Asia, andPalestine and Its Transformation. There has been a good deal of debate as to whether these lakes actually date from the glacial period, as is claimed by C. E. P. Brooks, for example, or from some other period. The evidence, however, seems to be convincing that the lakes expanded when the ice also expanded.
According to the older glacial hypotheses the lower temperature which is postulated as the cause of glaciation would almost certainly mean less evaporation over the oceans and hence less precipitation during glacial periods. To counteract this the only way in which thelevel of the lakes could be raised would be because the lower temperature would cause less evaporation from their surfaces. It seems quite impossible, however, that the lowering of temperature, which is commonly taken to have been not more than 10°C., could counteract the lessened precipitation and also cause an enormous expansion of most of the lakes. For example, ancient Lake Bonneville was more than ten times as large as its modern remnant, Great Salt Lake, and its average depth more than forty times as great.[53]Many small lakes in the Old World expanded still more.[54]For example, in eastern Persia many basins which now contain no lake whatever are floored with vast deposits of lacustrine salt and are surrounded by old lake bluffs and beaches. In northern Africa similar conditions prevail.[55]Other, but less obvious, evidence of more abundant rainfall in regions that are now dry is found in thick strata of gravel, sand, and fine silt in the alluvial deposits of flood plains and deltas.[56]
The cyclonic hypothesis supposes that increased storminess accounts for pluvial climates in regions that are now dry just as it accounts for glaciation in the regions of the ice sheets. Figs. 2 and 3, it will be remembered, illustrate what happens when the sun is active. Solar activity is accompanied by an increase in storminess in the southwestern United States in exactly the region where elevated strands of diminished salt lakes are most numerous. In Fig. 3, the same condition is seen inthe region of salt lakes in the Old World. Judging by these maps, which illustrate what has happened since careful meteorological records were kept, an increase in solar activity is accompanied by increased rainfall in large parts of what are now semi-arid and desert regions. Such precipitation would at once cause the level of the lakes to rise. Later, when ice sheets had developed in Europe and America, the high-pressure areas thus caused might force the main storm belt so far south that it would lie over these same arid regions. The increase in tropical hurricanes at times of abundant sunspots may also have a bearing on the climate of regions that are now arid. During the glacial period some of the hurricanes probably swept far over the lands. The numerous tropical cyclones of Australia, for example, are the chief source of precipitation for that continent.[57]Some of the stronger cyclones locally yield more rain in a day or two than other sources yield in a year.
V. The occurrence of widespread glaciation near the tropics during the Permian, as shown in Fig. 7, has given rise to much discussion. The recent discovery of glaciation in latitudes as low as 30° in the Proterozoic is correspondingly significant. In all cases the occurrence of glaciation in low and middle latitudes is probably due to the same general causes. Doubtless the position and altitude of the mountains had something to do with the matter. Yet taken by itself this seems insufficient. Today the loftiest range in the world, the Himalayas, is almost unglaciated, although its southern slope may seem at first thought to be almost ideally located in this respect. Some parts rise over 20,000 feet and certain lower slopes receive 400 inches of rain per year. The small size of the Himalayan glaciers in spite of these favorable conditionsis apparently due largely to the seasonal character of the monsoon winds. The strong outblowing monsoons of winter cause about half the year to be very dry with clear skies and dry winds from the interior of Asia. In all low latitudes the sun rides high in the heavens at midday, even in winter, and thus melts snow fairly effectively in clear weather. This is highly unfavorable to glaciation. The inblowing southern monsoons bring all their moisture in midsummer at just the time when it is least effective in producing snow. Conditions similar to those now prevailing in the Himalayas must accompany any great uplift of the lands which produces high mountains and large continents in subtropical and middle latitudes. Hence, uplift alone cannot account for extensive glaciation in subtropical latitudes during the Permian and Proterozoic.
Fig. 7. Permian geography and glaciation.(After Schuchert.)
The assumption of a great general lowering of temperature is also not adequate to explain glaciation in subtropical latitudes. In the first place this would require a lowering of many degrees,—far more than in the Pleistocene glacial period. The marine fossils of the Permian, however, do not indicate any such condition. In the second place, if the lands were widespread as they appear to have been in the Permian, a general lowering of temperature would diminish rather than increase the present slight efficiency of the monsoons in producing glaciation. Monsoons depend upon the difference between the temperatures of land and water. If the general temperature were lowered, the reduction would be much less pronounced on the oceans than on the lands, for water tends to preserve a uniform temperature, not only because of its mobility, but because of the large amount of heat given out when freezing takes place, or consumed in evaporation. Hence the general lowering of temperaturewould make the contrast between continents and oceans less than at present in summer, for the land temperature would be brought toward that of the ocean. This would diminish the strength of the inblowing summer monsoons and thus cut off part of the supply of moisture. Evidence that this actually happened in the cold fourteenth century has already been given in Chapter VI. On the other hand, in winter the lands would be much colder than now and the oceans only a little colder, so that the dry outblowing monsoons of the cold season would increase in strength and would also last longer than at present. In addition to all this, the mere fact of low temperature would mean a general reduction in the amount of water vapor in the air. Thus, from almost every point of view a mere lowering of temperature seems to be ruled out as a cause of Permian glaciation. Moreover, if the Permian or Proterozoic glacial periods were so cold that the lands above latitude 30° were snow-covered most of the time, the normal surface winds in subtropical latitudes would be largely equatorward, just as the winter monsoons now are. Hence little or no moisture would be available to feed the snowfields which give rise to the glaciers.
It has been assumed by Marsden Manson and others that increased general cloudiness would account for the subtropical glaciation of the Permian and Proterozoic. Granting for the moment that there could be universal persistent cloudiness, this would not prevent or counteract the outblowing anti-cyclonic winds so characteristic of great snowfields. Therefore, under the hypothesis of general cloudiness there would be no supply of moisture to cause glaciation in low latitudes. Indeed, persistent cloudiness in all higher latitudes would apparently deprive the Himalayas of most of their present moisture,for the interior of Asia would not become hot in summer and no inblowing monsoons would develop. In fact, winds of all kinds would seemingly be scarce, for they arise almost wholly from contrasts of temperature and hence of atmospheric pressure. The only way to get winds and hence precipitation would be to invoke some other agency, such as cyclonic storms, but that would be a departure from the supposition that glaciation arose from cloudiness.
Let us now inquire how the cyclonic hypothesis accounts for glaciation in low latitudes. We will first consider the terrestrial conditions in the early Permian, the last period of glaciation in such latitudes. Geologists are almost universally agreed that the lands were exceptionally extensive and also high, especially in low latitudes. One evidence of this is the presence of abundant conglomerates composed of great boulders. It is also probable that the carbon dioxide in the air during the early Permian had been reduced to a minimum by the extraordinary amount of coal formed during the preceding period. This would tend to produce low temperature and thus make the conditions favorable for glaciation as soon as an accentuation of solar activity caused unusual storminess. If the storminess became extreme when terrestrial conditions were thus universally favorable to glaciation, it would presumably produce glaciation in low latitudes. Numerous and intense tropical cyclones would carry a vast amount of moisture out of the tropics, just as now happens when the sun is active, but on a far larger scale. The moisture would be precipitated on the equatorward slopes of the subtropical mountain ranges. At high elevations this precipitation would be in the form of snow even in summer. Tropical cyclones, however, as is shown inEarth and Sun, occur in the autumn andwinter as well as in summer. For example, in the Bay of Bengal the number recorded in October is fifty, the largest for any month; while in November it is thirty-four, and December fourteen as compared with an average of forty-two for the months of July to September. From January to March, when sunspot numbers averaged more than forty, the number of tropical hurricanes was 143 per cent greater than when the sunspot numbers averaged below forty. During the months from April to June, which also would be times of considerable snowy precipitation, tropical hurricanes averaged 58 per cent more numerous with sunspot numbers above forty than with numbers below forty, while from July to September the difference amounted to 23 per cent. Even at this season some snow falls on the higher slopes, while the increased cloudiness due to numerous storms also tends to preserve the snow. Thus a great increase in the frequency of sunspots is accompanied by increased intensity of tropical hurricanes, especially in the cooler autumn and spring months, and results not only in a greater accumulation of snow but in a decrease in the melting of the snow because of more abundant clouds. At such times as the Permian, the general low temperature due to rapid convection and to the scarcity of carbon dioxide presumably joined with the extension of the lands in producing great high-pressure areas over the lands in middle latitudes during the winters, and thus caused the more northern, or mid-latitude type of cyclonic storms to be shifted to the equatorward side of the continents at that season. This would cause an increase of precipitation in winter as well as during the months when tropical hurricanes abound. Many other circumstances would coöperate to produce a similar result. For example, the general low temperature would cause the sea to be covered withice in lower latitudes than now, and would help to create high-pressure areas in middle latitudes, thus driving the storms far south. If the sea water were fresher than now, as it probably was to a notable extent in the Proterozoic and perhaps to some slight extent in the Permian, the higher freezing point would also further the extension of the ice and help to keep the storms away from high latitudes. If to this there is added a distribution of land and sea such that the volume of the warm ocean currents flowing from low to high latitudes was diminished, as appears to have been the case, there seems to be no difficulty in explaining the subtropical location of the main glaciation in both the Permian and the Proterozoic. An increase of storminess seems to be the key to the whole situation.
One other possibility may be mentioned, although little stress should be laid on it. InEarth and Sunit has been shown that the main storm track in both the northern and southern hemispheres is not concentric with the geographical poles. Both tracks are roughly concentric with the corresponding magnetic poles, a fact which may be important in connection with the hypothesis of an electrical effect of the sun upon terrestrial storminess. The magnetic poles are known to wander considerably. Such wandering gives rise to variations in the direction of the magnetic needle from year to year. In 1815 the compass in England pointed 24-½° W. of N. and in 1906 17° 45' W. Such a variation seems to mean a change of many miles in the location of the north magnetic pole. Certain changes in the daily march of electromagnetic phenomena over the oceans have led Bauer and his associates to suggest that the magnetic poles may even be subject to a slight daily movement in response to the changes in the relative positions of the earth and sun.Thus there seems to be a possibility that a pronounced change in the location of the magnetic pole in Permian times, for example, may have had some connection with a shifting in the location of the belt of storms. It must be clearly understood that there is as yet no evidence of any such change, and the matter is introduced merely to call attention to a possible line of investigation.
Any hypothesis of Permian and Proterozoic glaciation must explain not only the glaciation of low latitudes but the lack of glaciation and the accumulation of red desert beds in high latitudes. The facts already presented seem to explain this. Glaciation could not occur extensively in high latitudes partly because during most of the year the air was too cold to hold much moisture, but still more because the winds for the most part must have blown outward from the cold northern areas and the cyclonic storm belt was pushed out of high latitudes. Because of these conditions precipitation was apparently limited to a relatively small number of storms during the summer. Hence great desert areas must have prevailed at high latitudes. Great aridity now prevails north of the Himalayas and related ranges, and red beds are accumulating in the centers of the great deserts, such as those of the Tarim Basin and the Transcaspian. The redness is not due to the original character of the rock, but to intense oxidation, as appears from the fact that along the edges of the desert and wherever occasional floods carry sediment far out into the midst of the sand, the material has the ordinary brownish shades. As soon as one goes out into the places where the sand has been exposed to the air for a long time, however, it becomes pink, and then red. Such conditions may have given rise to the high degree of oxidation in the famous Permian red beds. If the air of the early Permian contained an unusual percentage ofoxygen because of the release of that gas by the great plant beds which formed coal in the preceding era, as Chamberlin has thought probable, the tendency to produce red beds would be still further increased.
It must not be supposed, however, that these conditions would absolutely limit glaciation to subtropical latitudes. The presence of early Permian glaciation in North America at Boston and in Alaska and in the Falkland Islands of the South Atlantic Ocean proves that at least locally there was sufficient moisture to form glaciers near the coast in relatively high latitudes. The possibility of this would depend entirely upon the form of the lands and the consequent course of ocean currents. Even in those high latitudes cyclonic storms would occur unless they were kept out by conditions of pressure such as have been described above.
The marine faunas of Permian age in high latitudes have been interpreted as indicating mild oceanic temperatures. This is a point which requires further investigation. Warm oceans during times of slight solar activity are a necessary consequence of the cyclonic hypothesis, as will appear later. The present cold oceans seem to be the expectable result of the Pleistocene glaciation and of the present relatively disturbed condition of the sun. If a sudden disturbance threw the solar atmosphere into violent commotion within a few thousand years during Permian times, glaciation might occur as described above, while the oceans were still warm. In fact their warmth would increase evaporation while the violent cyclonic storms and high winds would cause heavy rain and keep the air cool by constantly raising it to high levels where it would rapidly radiate its heat into space.
Nevertheless it is not yet possible to determine how warm the oceans were at the actual time of the Permianglaciation. Some faunas formerly reported as Permian are now known to be considerably older. Moreover, others of undoubted Permian age are probably not strictly contemporaneous with the glaciation. So far back in the geological record it is very doubtful whether we can date fossils within the limits of say 100,000 years. Yet a difference of 100,000 years would be more than enough to allow the fossils to have lived either before or after the glaciation, or in an inter-glacial epoch. One such epoch is known to have occurred and nine others are suggested by the inter-stratification of glacial till and marine sediments in eastern Australia. The warm currents which would flow poleward in inter-glacial epochs must have favored a prompt reintroduction of marine faunas driven out during times of glaciation. Taken all and all, the Permian glaciation seems to be accounted for by the cyclonic hypothesis quite as well as does the Pleistocene. In both these cases, as well as in the various pulsations of historic times, it seems to be necessary merely to magnify what is happening today in order to reproduce the conditions which prevailed in the past. If the conditions which now prevail at times of sunspot minima were magnified, they would give the mild conditions of inter-glacial epochs and similar periods. If the conditions which now prevail at times of sunspot maxima are magnified a little they seem to produce periods of climatic stress such as those of the fourteenth century. If they are magnified still more the result is apparently glacial epochs like those of the Pleistocene, and if they are magnified to a still greater extent, the result is Permian or Proterozoic glaciation. Other factors must indeed be favorable, for climatic changes are highly complex and are unquestionably due to a combination of circumstances. The point which is chiefly emphasized in this book is that amongthose several circumstances, changes in cyclonic storms due apparently to activity of the sun's atmosphere must always be reckoned.
One of the most remarkable formations associated with glacial deposits consists of vast sheets of the fine-grained, yellowish, wind-blown material called loess. Somewhat peculiar climatic conditions evidently prevailed when it was formed. At present similar deposits are being laid down only near the leeward margin of great deserts. The famous loess deposits of China in the lee of the Desert of Gobi are examples. During the Pleistocene period, however, loess accumulated in a broad zone along the margin of the ice sheet at its maximum extent. In the Old World it extended from France across Germany and through the Black Earth region of Russia into Siberia. In the New World a still larger area is loess-covered. In the Mississippi Valley, tens of thousands of square miles are mantled by a layer exceeding twenty feet in thickness and in many places approaching a hundred feet. Neither the North American nor the European deposits are associated with a desert. Indeed, loess is lacking in the western and drier parts of the great plains and is best developed in the well-watered states of Iowa, Illinois, and Missouri. Part of the loess overlies the non-glacial materials of the great central plain, but the northern portions overlie the drift deposits of the first three glaciations. A few traces of loess are associated with the Kansan and Illinoian, the second and third glaciations, but most of the Americanloess appears to have been formed at approximately the time of the Iowan or fourth glaciation, while only a little overlies the drift sheets of the Wisconsin age. The loess is thickest near the margin of the Iowan till sheet and thins progressively both north and south. The thinning southward is abrupt along the stream divides, but very gradual along the larger valleys. Indeed, loess is abundant along the bluffs of the Mississippi, especially the east bluff, almost to the Gulf of Mexico.[58]
It is now generally agreed that all typical loess is wind blown. There is still much question, however, as to its time of origin, and thus indirectly as to its climatic implications. Several American and European students have thought that the loess dates from inter-glacial times. On the other hand, Penck has concluded that the loess was formed shortly before the commencement of the glacial epochs; while many American geologists hold that the loess accumulated while the ice sheets were at approximately their maximum size. W. J. McGee, Chamberlin and Salisbury, Keyes, and others lean toward this view. In this chapter the hypothesis is advanced that it was formed at the one other possible time, namely, immediately following the retreat of the ice.
These four hypotheses as to the time of origin of loess imply the following differences in its climatic relations. If loess was formed during typical inter-glacial epochs, or toward the close of such epochs, profound general aridity must seemingly have prevailed in order to kill off the vegetation and thus enable the wind to pick up sufficient dust. If the loess was formed during times of extreme glaciation when the glaciers were supplying large quantities of fine material to outflowing streams, less aridity would be required, but there must have beensharp contrasts between wet seasons in summer when the snow was melting and dry seasons in winter when the storms were forced far south by the glacial high pressure. Alternate floods and droughts would thus affect broad areas along the streams. Hence arises the hypothesis that the wind obtained the loess from the flood plains of streams at times of maximum glaciation. If the loess was formed during the rapid retreat of the ice, alternate summer floods and winter droughts would still prevail, but much material could also be obtained by the winds not only from flood plains, but also from the deposits exposed by the melting of the ice and not yet covered by vegetation.
The evidence for and against the several hypotheses may be stated briefly. In support of the hypothesis of the inter-glacial origin of loess, Shimek and others state that the glacial drift which lies beneath the loess commonly gives evidence that some time elapsed between the disappearance of the ice and the deposition of the loess. For example, abundant shells of land snails in the loess are not of the sort now found in colder regions, but resemble those found in the drier regions. It is probable that if they represented a glacial epoch they would be depauperated by the cold as are the snails of far northern regions. The gravel pavement discussed below seems to be strong evidence of erosion between the retreat of the ice and the deposition of the loess.
Turning to the second hypothesis, namely, that the loess accumulated near the close of the inter-glacial epoch rather than in the midst of it, we may follow Penck. The mammalian fossils seem to him to prove that the loess was formed while boreal animals occupied the region, for they include remains of the hairy mammoth, woolly rhinoceros, and reindeer. On the other hand, the typicalinter-glacial beds not far away yield remains of species characteristic of milder climates, such as the elephant, the smaller rhinoceros, and the deer. In connection with these facts it should be noted that occasional remains of tundra vegetation and of trees are found beneath the loess, while in the loess itself certain steppe animals, such as the common gopher or spermaphyl, are found. Penck interprets this as indicating a progressive desiccation culminating just before the oncoming of the next ice sheet.
The evidence advanced in favor of the hypothesis that the loess was formed when glaciation was near its maximum includes the fact that if the loess does not represent the outwash from the Iowan ice, there is little else that does, and presumably there must have been outwash. Also the distribution of loess along the margins of streams suggests that much of the material came from the flood plains of overloaded streams flowing from the melting ice.
Although there are some points in favor of the hypothesis that the loess originated (1) in strictly inter-glacial times, (2) at the end of inter-glacial epochs, and (3) at times of full glaciation, each hypothesis is much weakened by evidence that supports the others. The evidence of boreal animals seems to disprove the hypothesis that the loess was formed in the middle of a mild inter-glacial epoch. On the other hand, Penck's hypothesis as to loess at the end of inter-glacial times fails to account for certain characteristics of the lowest part of the loess deposits and of the underlying topography. Instead of normal valleys and consequent prompt drainage such as ought to have developed before the end of a long inter-glacial epoch, the surface on which the loess lies shows many undrained depressions. Some of these can be seenin exposed banks, while many more are inferred from the presence of shells of pond snails here and there in the overlying loess. The pond snails presumably lived in shallow pools occupying depressions in the uneven surface left by the ice. Another reason for questioning whether the loess was formed at the end of an inter-glacial epoch is that this hypothesis does not provide a reasonable origin for the material which composes the loess. Near the Alps where the loess deposits are small and where glaciers probably persisted in the inter-glacial epochs and thus supplied flood plain material in large quantities, this does not appear important. In the broad upper Mississippi Basin, however, and also in the Black Earth region of Russia there seems to be no way to get the large body of material composing the loess except by assuming the existence of great deserts to windward. But there seems to be little or no evidence of such deserts where they could be effective. The mineralogical character of the loess of Iowan age proves that the material came from granitic rocks, such as formed a large part of the drift. The nearest extensive outcrops of granite are in the southwestern part of the United States, nearly a thousand miles from Iowa and Illinois. But the loess is thickest near the ice margin and thins toward the southwest and in other directions, whereas if its source were the southwestern desert, its maximum thickness would probably be near the margin of the desert.
The evidence cited above seems inconsistent not only with the hypothesis that the loess was formed at the end of an inter-glacial epoch, but also with the idea that it originated at times of maximum glaciation either from river-borne sediments or from any other source. A further and more convincing reason for this last conclusion is the probability and almost the certainty thatwhen the ice advanced, its front lay close to areas where the vegetation was not much thinner than that which today prevails under similar climatic conditions. If the average temperature of glacial maxima was only 6°C. lower than that of today, the conditions just beyond the ice front when it was in the loess region from southern Illinois to Minnesota would have been like those now prevailing in Canada from New Brunswick to Winnipeg. The vegetation there is quite different from the grassy, semi-arid vegetation of which evidence is found in the loess. The roots and stalks of such grassy vegetation are generally agreed to have helped produce the columnar structure which enables the loess to stand with almost vertical surfaces.