Chapter 7

[BH]American Journal of Science, vol. cxxxii, 1886, p. 77.

When, now, we attentively consider the combination of causes necessary to produce the climatic conditions of the great Ice age of North America, we shall be prepared to find far more extensive variations in the progress of the continental glacier, both during its advance and during its retreat, than are to be observed in any existing local glaciers.

With respect to the arguments adduced in favor of a succession of glacial epochs in America the following criticisms are pertinent:

1. So far as we can estimate, a temporary retreat of the front, lasting a few centuries, would be sufficient to account for the vegetable accumulations that are found buried beneath the glacial deposits in southern Ohio, Indiana, central Illinois, and Iowa, while a temporary readvance of the ice would be sufficient to bury the vegetable remains beneath a freshly accumulated mass of till. Thus, as Dr. Bell suggested, the interglacial vegetal deposits do not necessarily indicate anything more than a temporary oscillation of the ice-front, and do not carry with them the necessity of supposing a disappearance of the ice from the whole glaciated area. Thus the introduction of a whole Glacial period to account for such limited phenomena is a violation of the well-known law of parsimony, which requires us in our explanations of phenomena to be content with the least cause which is sufficient to produce them. In the present instance a series of comparatively slight oscillations of the ice-front during a single glacial period would seem to be sufficient to account for all the buried forests and masses of vegetaldébristhat occur either in the United States or in the Dominion of Canada.

2. Another argument for the existence of two absolutely distinct glacial periods in North America has been drawn from the greater oxidation of the clays and the more extensive disintegration of certain classes of the boulders found over the southern part of the glaciated area of the Mississippi Valley, than has taken place in the more northerly regions. Without questioning this statement of fact (which, however, I believe to be somewhat exaggerated), it is not difficult to see that the effects probably are just what would result from a single long glacial period brought about by such causes as we have seen to be probably in operation in America. For if one reflects upon the conditions existing when the Glacial period began, he will see that, during the long ages of warm climatewhich characterised the preceding period, the rocks must have been extensively disintegrated through the action of subaërial agencies. The extent to which this disintegration takes place can be appreciated now only by those who reside outside of the glaciated area, where these agencies have been in uninterrupted action. In the Appalachian range south of the glaciated region the granitic masses and strata of gneiss are sometimes found to be completely disintegrated to a depth of fifty or sixty feet; and what seem to be beds of gravel often prove in fact to be horizontal strata of gneiss from which the cementing material has been removed by the slow action of acids brought down by the percolating water.

Now, there can be no question that this process of disintegration had proceeded to a vast extent before the Glacial period, so that, when the ice began to advance, there was an enormous amount of partially oxidised and disintegrated material ready to be scraped off with the first advance of ice, and this is the material which would naturally be transported farthest to the south; and thus, on the theory of a single glacial period, we can readily account for the greater apparent age of the glacialdébrisnear the margin. Thisdébriswas old when the Glacial period began.

3. With reference to the argument for two distinct glacial periods drawn from the smaller apparent amount of glacial erosion over the southern part of the glaciated area, we have to remark that that would occur in case of a single ice-invasion as well as in case of two distinct ice-invasions, in which the later did not extend so far as the former.

From the very necessity of the case, glacial erosion diminishes as the limit of the extent of the glaciation is approached. At the very margin of the glacier, motion has ceased altogether. Back one mile from the margin only one mile of ice-motion has been active in erosion,while ten miles back from its front there has been ten times as much moving ice actually engaged in erosion, and in the extreme north several hundred times as much ice, Thus it is evident that we do not need to resort to two glacial periods to account for the relatively small amount of erosion exhibited over the southern portion of our glaciated area.

At the same time, it should be said that the indications of active glacial erosion near the margin are by no means few or small. In Lawrence County, Pennsylvania, on the very margin of the glaciated area, Mr. Max Foshay[BI]has discovered very extensive glacial grooves, indicating much vigour of ice-action even beyond the more extensive glacial deposits which Professor Lewis and myself had fixed upon as the terminal moraine. In Highland and Butler Counties, Ohio, and in southwestern Indiana and southern Illinois, near the glacial margin, glacial grooves and striæ are as clear and distinct in many cases as can anywhere be found; while upon the surface of the limestone rocks within the limits of the city of St. Louis, where the glacial covering is thin, and where disintegrating agencies had had special opportunities to work, I found very clear evidences of a powerful ice-movement, which had planed and scratched the rock surface; and at Du Quoin, Illinois, as already related, the fragments thrown up from the surface of the rock, fifty or sixty feet below the top of the soil, were most beautifully planed and striated. It should be observed, also, that this whole area is so deeply covered withdébristhat the extent of glacial erosion underneath is pretty generally hid from view.

[BI]Bulletin of the Geological Society, vol. ii, pp. 457-464.

4. The uniformity of the distribution of the glacial deposits over the southern portion of the glaciated area in the Mississippi Valley is partly an illusion, due to thefact that there was a vast amount of deposition by water over that area during the earlier stages of the ice-retreat. This has been due partly to the gentler slope which would naturally characterise the borders of an area of elevation, and partly to an extensive subsidence which seems to have begun soon after the ice had reached its farthest extent of motion.

It should be borne in mind that at all times a glacier is accompanied by the issue of vast streams of water from its front, and that these of course increase in volume when the climax has been reached and the ameliorating influences begin to melt away the accumulated mass of ice and to add the volume of its water to that produced by ordinary agencies. As these subglacial streams of water poured out upon the more gentle slopes of the area in front of the ice, they would distribute a vast amount of fine material, which would settle into the hollow places and tend to obscure the irregularities of the previous direct glacial deposit.

Such an instance came clearly under my own observation in the vicinity of Yankton, in South Dakota, where, upon visiting a locality some miles from any river, and to which workmen were resorting for sand, I found that the deposit occupied a kettle-hole, filling it to its brim, and had evidently been superimposed by a temporary stream of water flowing over the region while the ice was still in partial occupation of it. Thus, no doubt, in many cases, the original irregularities of the direct glacial deposits have been obliterated, even where there has been no general subsidence.

But, in the area under consideration, the loess, or loam, is so extensive that it is perhaps necessary to suppose that the central portions of the Mississippi Valley were subjected to a subsidence amounting to about five hundred feet; so that the glacial streams from the retreating ice-front met the waters of the ocean in southernIllinois and Indiana; thus accounting for the extensive fine silt which has done so much over that region to obscure the glacial phenomena.

West of the Rocky Mountains.

The glacial phenomena in the United States west of the Rocky Mountains must be treated separately, since American geologists have ceased to speak of an all-pervading ice-cap extending from the north pole. But, as already said, the glaciation of North America has proceeded from two definite centres of ice-accumulation, one of which we have been considering in the pages immediately preceding. The great centre of glacial dispersion east of the Rocky Mountains is the region south of Hudson Bay, and the vast ice-field spreading out from that centre is appropriately named the Laurentide Glacier. The movement of ice in this glacial system was outward in all directions from the Laurentian hills, and extended west several hundred miles, well on towards the eastern foot of the Rocky Mountains.

The second great centre of glacial dispersion occupies the vast Cordilleran region of British Columbia, reaching from the Rocky Mountains on the northeast to the Coast Range of the Pacific on the southwest, a width of four hundred miles. The length is estimated by Dr. Dawson to be twelve hundred miles. The principal centre of ice-accumulation lies between the fifty-fifth and the fifty-ninth parallel. From this centre the movement was in all directions, but chiefly to the northwest and to the south. The movement of the Cordilleran glaciers extended northwest to a distance of three hundred and fifty miles, leaving their moraines far down in the Yukon Valley on the Lewes and Pelly Rivers.[BJ]Southward theCordilleran Glacier moved to a distance of six hundred miles, extending to the Columbia River, in the eastern part of the State of Washington.

[BJ]See George M. Dawson, in Science, vol. xi, 1888, p. 186, and American Geologist, September, 1890, pp. 153-162.

From this centre, also, the ice descended to the sea-level upon the west, and filled all the channels between Vancouver’s Island and the mainland, as well as those in the Alexander Archipelago of Alaska. South of Vancouver’s Island a glacier pushed out through the straits of Juan de Fuca to an unknown distance. All the islands in Puget Sound are composed of glacialdébris, resembling in every respect the terminal moraines which have been described as constituting many of the islands south of the New England coast. The ice-movement in Puget Sound, however, was probably northward, resulting from glaciers which are now represented by their diminutive descendants on the flanks of Mount Rainier.

South of the Columbia River the country was never completely enveloped by the ice, but glaciers extended far down in the valleys from all the lofty mountain-peaks. In Idaho there are glacial signs from the summit of the Rocky Mountains down to the westward of Lake Pend d’Oreille. In the Yellowstone Park there are clear indications that the whole area was enveloped in glacial ice. An immense boulder of granite, resting upon volcanic deposits, may be found a little west of Inspiration Point, on the Yellowstone Cañon. Abundant evidences of glacial action are also visible down the Yellowstone River to the vicinity of Livingston, showing that that valley must have been filled with glacial ice to a depth of sixteen hundred feet. To the west the glaciers from the Yellowstone Park extended to the border of Idaho, where a clearly marked terminal moraine is to be found in the Tyghee Pass, leading over from the western fork of the Madison River into Lewis Fork of the Snake River. South of Yellowstone Park the Teton Mountains were an important centre for the dispersion of local glaciers, but they did not descendupon the western side much below the 6,000-foot level, and only barely came to the edge of the great Snake River lava plains. To the east the movement from the Teton Mountains joined that from various other lofty mountains, where altogether they have left a most intricate system of glacial deposits, in whose reticulations Jackson’s Lake is held in place.

Fig. 37.—Moraines of Grape Creek, Sangre del Cristo Mountains, Colorado (after Stevenson).

In Utah extensive glaciers filled all the northern valleys of the Uintah Mountains, and extended westward in the Wahsatch range to the vicinity of Salt Lake City. The mountain region of Colorado, also, had its glaciers, occupying the head-waters of the Arkansas, the Platte, the Gunnison, and the Grand Rivers. The most southern point in the Rocky Mountains at which signs of local glaciers have been noted is near the summits of the SanJuan range, in southwestern Colorado. Here a surface of about twenty-five square miles, extending from an elevation of 12,000 feet down to 8,000 feet, shows every sign of the former presence of moving ice. The greater part of the glaciation in Colorado is confined to elevations above 10,000 feet.

The whole range of the Sierra Nevada through Oregon, and as far south as the Yosemite Valley in California, formerly sustained glaciers of far greater size than any which are now found in those mountains. In general these glaciers were much longer on the western side of the Sierra Nevada than on the eastern. On the eastern side glaciers barely came down to Lake Tahoe and Lake Mono in California. The State of Nevada seems to have been entirely free from glaciers, although it contains numerous mountain-peaks more than ten thousand feet high. In the Yosemite Cañon glaciers extended down the Merced River to the mouth of the cañon; while in the Tuolumne River, a few miles to the north, the glaciers which still linger about the peaks of Mount Dana filled the valley for a distance of forty miles.

It is a question among geologists whether or not the glaciation west of the Rocky Mountains was contemporaneous with that of the eastern part of the continent. The more prevalent opinion among those who have made special study of the phenomena is that the development of the Cordilleran glaciers was independent of that of the Laurentide system. At any rate, the intense glaciation of the Pacific coast seems to have been considerably later than that of the Atlantic region. Of this we will speak more particularly in discussing the questions of the date and the cause of the Glacial period. It is sufficient for us here simply to say that, from his extensive field observations throughout the Cordilleran region, Dr. George M. Dawson infers that there have been several successive alternations of level on the Pacific coast corresponding tosuccessive glacial and interglacial epochs, and that when there was a period of elevation west of the Rocky Mountains there was a period of subsidence to the east, andvice versa. In short, he supposes that the east and west for a long time played a game of seesaw, with the Rocky Mountains as the fulcrum. We give his scheme in tabulated form.

Scheme of Correlation of the Phenomena of the Glacial Period in the Cordilleran Region and in the Region of the Great Plains.

CORDILLERAN REGION.REGION OF THE GREAT PLAINS.Cordilleran zone at a high elevation. Period of most severe glaciation and maximum development of the great Cordilleran Glacier.Correlative subsidence and submergence of the great plains, with possible contemporaneous increased elevation of the Laurentian axis and maximum development of ice upon it. Deposition of the lower boulder-clay of the plains.Gradual subsidence of the Cordilleran region and decay of the great glacier, with deposition of the boulder-clay of the interior plateau and the Yukon basin, of the lower boulder-clay of the littoral and probably also, at a later stage (and with greater submergence), of the interglacial silts of the same region.Correlative elevation of the western part, at least, of the great plains, which was probably more or less irregular and led to the production of extensive lakes in which interglacial deposits, including peat, were formed.Re-elevation of the Cordilleran region to a level probably as high as or somewhat higher than the present. Maximum of second period of glaciation.Correlative subsidence of the plains, which (at least in the western part of the region) exceeded the first subsidence and extended submergence to the base of the Rocky Mountains near the forty-ninth parallel. Formation of second boulder-clay, and (at a later stage) dispersion of large erratics.Partial subsidence of the Cordilleran region, to a level about 2,500 feet lower than the present. Long stage of stability. Glaciers of the second period considerably reduced. Upper boulder-clay of the coast probably formed at this time, though perhaps in part during the second maximum of glaciation.Correlative elevation of the plains, or at least of their western portion, resulting in a condition of equilibrium as between the plains and the Cordillera, theirrelativelevels becoming nearly as at present. Probable formation of the Missouri coteau along a shore-line during this period of rest.Renewed elevation of the Cordilleran region, with one well-marked pause, during which the littoral stood about 200 feet lower than at present. Glaciers much reduced, and diminishing in consequence of general amelioration of climate towards the close of the Glacial period.Simultaneous elevation of the great plains to about their present level, with final exclusion of waters in connection with the sea. Lake Agassiz formed and eventually drained towards the close of this period. This simultaneous movement in elevation of both great areas may probably have been connected with a more general northern elevation of land at the close of the Glacial period.

Cordilleran zone at a high elevation. Period of most severe glaciation and maximum development of the great Cordilleran Glacier.

Correlative subsidence and submergence of the great plains, with possible contemporaneous increased elevation of the Laurentian axis and maximum development of ice upon it. Deposition of the lower boulder-clay of the plains.

Gradual subsidence of the Cordilleran region and decay of the great glacier, with deposition of the boulder-clay of the interior plateau and the Yukon basin, of the lower boulder-clay of the littoral and probably also, at a later stage (and with greater submergence), of the interglacial silts of the same region.

Correlative elevation of the western part, at least, of the great plains, which was probably more or less irregular and led to the production of extensive lakes in which interglacial deposits, including peat, were formed.

Re-elevation of the Cordilleran region to a level probably as high as or somewhat higher than the present. Maximum of second period of glaciation.

Correlative subsidence of the plains, which (at least in the western part of the region) exceeded the first subsidence and extended submergence to the base of the Rocky Mountains near the forty-ninth parallel. Formation of second boulder-clay, and (at a later stage) dispersion of large erratics.

Partial subsidence of the Cordilleran region, to a level about 2,500 feet lower than the present. Long stage of stability. Glaciers of the second period considerably reduced. Upper boulder-clay of the coast probably formed at this time, though perhaps in part during the second maximum of glaciation.

Correlative elevation of the plains, or at least of their western portion, resulting in a condition of equilibrium as between the plains and the Cordillera, theirrelativelevels becoming nearly as at present. Probable formation of the Missouri coteau along a shore-line during this period of rest.

Renewed elevation of the Cordilleran region, with one well-marked pause, during which the littoral stood about 200 feet lower than at present. Glaciers much reduced, and diminishing in consequence of general amelioration of climate towards the close of the Glacial period.

Simultaneous elevation of the great plains to about their present level, with final exclusion of waters in connection with the sea. Lake Agassiz formed and eventually drained towards the close of this period. This simultaneous movement in elevation of both great areas may probably have been connected with a more general northern elevation of land at the close of the Glacial period.

In New Zealand the marks of the Glacial period are unequivocal The glaciers which now come down from the lofty mountains upon the South Island of New Zealand to within a few hundred feet of the sea then descended to the sea-level. The longest existing glacier in New Zealand is sixteen miles, but formerly one of them had a length of seventy-eight miles. One of the ancient moraines contains a boulder from thirty to forty feet in diameter, and the amount of glacialdébriscovering the mountain-sides is said to be enormous. Reports have also been recently brought of signs of ancient glaciers in Australia.

Fig. 38.—Generalised view of the whole glaciated region of North America. The area of motionless ground-ice is shown by the white lines in northern part of Alaska.

According to Darwin, there are distinct signs of glaciation upon the plains of Patagonia sixty or seventy miles east of the foot of the mountains, and in the Straits of Magellan he found great masses of unstratified glacial material containing boulders which were at least one hundred and thirty miles away from their parent rock; while upon theisland of Chiloe he found embedded in “hardened mud” boulders which must have come from the mountain-chains of the continent. Agassiz also observed unquestionable glacial phenomena on various parts of the Fuegian coast, and indeed everywhere on the continent south of latitude 37°. Between Concepcion and Arauco, in latitude 37°, Agassiz observed, near the sea-level, a glacial surface well marked with furrows and scratches, and as well preserved, he says, “as any he had seen under the glaciers of the present day.”

Fig. 39.—Quartzite boulder of 45 cubic metres, on Mont Lachat, 800 metres above the valley of the Belley, in Ain, France (Falsan).

CHAPTER VI.

ANCIENT GLACIERS IN THE EASTERN HEMISPHERE.

About two million square miles of northern Europe were covered with perennial ice during the Glacial period. From the scratches upon the rocks, and from the direction in which material has been transported, it is evident that the main centre of radiation is to be found in the mountains of Scandinavia, and that the glaciers still existing in Norway are the lineal descendants of those of the great Ice age.

So shallow are the Baltic Sea and the German Ocean, that their basins were easily filled with ice, upon which Scandinavian boulders could be transported westward to the east shore of England, southward into the plains of Germany, and eastward far out upon the steppes of Russia. The islands north of Scotland bear marks also of an ice-movement from the direction of Norway. If Scotland itself was not overrun with Scandinavian glaciers, the reason was that it had ice enough of its own, and from its highlands set up a counter-movement, which successfully resisted the invasion from the Scandinavian Peninsula. But, elsewhere in Europe, Scandinavian ice moved freely outward to the extent of its capacity. Then, as now also, the Alps furnished centres for ice-movement, but the glaciers were limited to the upper portions of the valleys of the Rhône, the Rhine, and the Danube upon the west and north, and to a still smaller area upon the southern side.

MAP showing GLACIATED AREAS in North America and Europe.Fig. 40.

Fig. 40.

Central and Southern Europe.

The main centres of ice-movement in the Alps during the Glacial period are the same as those which furnish the lingering glaciers of the present time. From the water-shed between the Rhine, the Rhône, and the Aar, glaciers of immense size descended all the valleys now occupied by those streams. The valley of the Rhône between the Bernese and the Pennine Alps was filled with a glacier of immense depth, which was maintained by fresh supplies from tributaries upon either side as far down as Martigny. Glacial markings at the head of the Rhône Valley are found upon the Schneestock,[BK]at an elevation above the sea of about 11,500 feet (3,550 metres), or about 1,500 feet above the present surface of the Rhône Glacier. At Fiesch, about twenty miles below, where tributaries from the Bernese Oberland snow-fields were received, the thickness of the glacier was upwards of 5,000 feet (1,680 metres). Near Martigny, about fifty miles farther down the valley, where the glacier was abruptly deflected to the north, the depth of the ice was still upwards of 1,600 metres. From Martigny northward the thickness of the ice decreased rapidly for a few miles, where, at the enlargement of the valley above the head of Lake Geneva, it was less than 1,200 metres in thickness, and spread out over the intervening plain as far as Chasseron, with a nearly level surface, transporting, as we have before said, Alpine boulders to the flanks of the Juras, and landing them about 3,000 feet (1,275 metres) above the level of Lake Geneva. The width of the main valley is here about fifty miles, making the slope of the surface of the ice about twenty feet to the mile.

[BK]A. Falsan’s La Période Grlaciaire étudiée principalement en France et en Suisse, chapitre xv.

From its “vomitory,” at the head of Lake Geneva, theice of the ancient Rhône Glacier spread to the right and to the left, while its northern boundary was abruptly terminated by the line of the Jura Mountains. The law of glacial motion was, however, admirably illustrated in the height to which the ice rose upon the flanks of the Jura. At Chasseron, in the direct line of its onward motion, it rose to its highest point, while both to the southwest and to the northeast, along the line of the Juras, the ice-action was limited to constantly decreasing levels.

Down the valley of the Rhône the direction of motion was determined by the depression of Lake Geneva, at the lower end of which it received its main tributary from Mont Blanc, which had come down from Chamouni through the valley of the river Arve. From this point it was deflected by a spur of the Jura Mountains more and more southward to the vicinity of Culoz, near the mouth of Lake Bourget. Here the glacier coming down from the western flanks of the Alps, through the upper valley of the Isère, past Chambéry, became predominant, and deflected the motion to the west and north, whither the ice extended to a line passing through Bourg, Lyons, and Vienne, leaving upon one of the eminences on which Lyons is built a boulder several feet in diameter, which is duly preserved and labelled in the public park in that portion of the city. Farther south, glaciers of less extent marked the Alps most of the way to the Mediterranean, but they were not at all comparable in size to those from the central region.

To the right of Lake Geneva the movement started by the Rhône Glacier spread eastward, being joined in the vicinity of Berne by the confluent ice-stream which descended from the north flank of the Bernese Oberland, through the valley of the Aar. These united streams filled the whole valley with ice as far down as Soleure.[BL]

[BL]See mapof Rhône Glacier, onp. 58.

MAP OF GLACIAL MOVEMENTS IN FRANCE AND SWITZERLAND.Click on map to view larger sized.

Farther eastward, other ice-streams from the Alps became predominant, one of which, moving down the Reuss, deployed out upon the country lying north of Lucerne and Zug. Still farther down, the ancient glacier which descended the Limmatt spread itself out over the hills and lowlands about Zürich, one of its moraines of retrocession nearly dividing the lake into two portions.

Guyot and others have shown that the superficial deposits of this portion of Switzerland are just such as would be distributed by glaciers coming down from the above-mentioned Alpine valleys. Uniting together north of Zürich, these glaciers pushed onward as far as the Rhine below Schaffhausen. In Frickthal the glacial ice was still 1,200 feet thick, and at Kaisterberg between 400 and 500 feet.

At Lucerne there is a remarkable exposure of pot-holes, and a glaciated surface such as could be produced only by the combined action of moving ice and running water; thus furnishing to tourists an instructive object-lesson. Among the remarkable instances of boulders transported a long distance in Switzerland, is that of a block of granite carried from the Valais to the vicinity of Soleure, a distance of one hundred and fifteen miles, which weighs about 4,100 tons. “The celebrated Pierre-à-Bot, above Neufchâtel, measures 50’ × 20’ × 40’, and contains about 40,000 cubic feet of stone; while the Pierre-des-Marmettes, near Monthey, contains no less than 60,840 cubic feet.”

The ancient glacier of the Rhine, receiving its initial impulse in the same centre as that of the Rhône, fully equalled it in all its dimensions. Descending eastward from its source near the Schneestock to Chur, a distance of fifty miles, it turned northward and continued forty-five miles farther to the head of Lake Constance, where it spread out in fan-shape, extending northwest to Thiengen, below Schaffhausen, and covering a considerable area northand northeastward of the lake, reaching in the latter direction Ulm, upon the Danube—the whole distance of the movement being more than one hundred and fifty miles. Through other valleys tributary to the Danube, glaciers descended upon the upper plains of Bavaria, from the Tyrolese Alps to the vicinity of Munich. From Gross Glockner as a centre in the Noric Alps, vast rivers of ice, of which the Pasterzen Glacier is the remnant, poured far down into the valleys of the Inn and the Enns on the north and into that of the Drave on the southeast. Farther eastward in this part of Europe the mountains seem to have been too low to have furnished centres for any general dispersion of glacial ice.

Fig. 41.—Map showing the Lines ofDébrisextending from the Alps into the Plains of the Po (after Lyell).A.Crest of the Alpine water-shed;B.Névé-fields of the ancient glaciers;C.Moraines of ancient glaciers.Click on image to view larger sized.

Upon the south side of the Alps the ancient glaciers spread far out upon the plains of Lombardy, where moraines of vast extent and of every description enable the student to determine the exact limits of the ancient ice-action. From the southern flanks of Mont Blanc and Monte Rosa, and from the snow-fields of the western Alps, glaciers of great volume descended into the valley of Dora Baltea (vale of Aosta), and on emerging from the mountain valley Spread Out over the plains around Ivrea, leaving moraine hills in some instances 1,500 feet in height. The total length of this glacier was as much asone hundred and twenty miles. From the snow-fields in the vicinity of Mont Cenis, also, glaciers extended down the Dora Ripera to the vicinity of Turin, and down other valleys to a less extent. The lateral moraines of the Diore, on the south side of Mont Blanc, at the head of the Dora Baltea, are 2,000 feet above the present river, and extend upon the left bank for a distance of twenty miles.

From the eastern Alps, glaciers descended through all the valleys of the Italian lakes and deposited vast terminal moraines, which still obstruct the drainage, and produce the charming lakes of that region. A special historic interest pertains to the series of concentric moraines south of Lake Garda, since it was in the reticulations of this glacial deposit that the last great battle for Italian liberty was fought on June 24, 1859. Defeated in the engagements farther up the valley of the Po, the Austrian general Benedek took his final stand to resist the united forces of France and Italy behind an outer semicircle of the moraine hills south of this lake (some of which are 500 or 600 feet above the surrounding country), with his centre at Solferino, about ten miles from Peschera. Here, behind this natural fortification, he awaited the enemy, who was compelled to perform his manœuvres on the open plain which spread out on every side. But the natural fortifications furnished by the moraine hills were too extensive to be defended by an army of moderate size. The troops of Napoleon and Victor Immanuel concentrated at Solferino and broke through the line. Thus the day was lost to the Austrians, and they retired from Lombardy, leaving to Italy both the artificial and the natural fortifications that guard the southern end of this important entrance to the Tyrolese Alps. When once his attention is called to the subject, the traveller upon the railroad cannot fail to notice this series of moraines, as he enters it through a tunnel at Lonato on the west, and emerges from it at Soma Campagna, eighteen or twenty miles distantto the east. A monument celebrating the victory stands upon a moraine hill about half-way between, at Martino della Battaglie.

In other portions of central and southern Europe the mountains were too low to furnish important centres for glacial movements. Still, to a limited extent, the signs of ancient glaciers are seen in the mountains of the Black Forest, in the Harz and Erzgebirge, and in the Carpathians on the east and among the Apennines on the south. In Spain, also, there were limited ice-fields on the higher portions of the Sierra Nevada and in the mountains of Estremadura, and perhaps in some other places. In France, small glaciers were to be found in the higher portions of the Auvergne, of the Morvan, of the Vosges, and of the Cevennes; while, from the Pyrenees, glaciers extended northward throughout nearly their whole extent. The ice-stream descending from the central mass of Maladetta through the upper valley of the Garonne, was joined by several tributaries, and attained a length of about forty-five miles.

The British Isles.

During the climax of the Glacial period the Hebrides to the north of Scotland were covered with ice to a depth of 1,600 feet. How far westward of this it moved out to the sea, it is of course impossible to tell. But in the channels between the Hebrides and Scotland it is evident that the water was completely expelled by the ice, and that, from a height of 1,600 feet above the Hebrides to the northern shores of Scotland, there was a continuous ice-field sloping southward at the rate of about twenty-five feet a mile.

Scotland itself was completely enveloped in glacial ice. Prevented by the Scandinavian Glacier from moving eastward, the Scotch movement was compelled to be westward and southward. On the southwest the ice-stream reachedthe shores of Ireland, and became confluent with the glaciers that enveloped that island, completely filling the Irish Sea.

There are so many controverted points respecting the glacial geology of England, and they have such an important bearing upon the main question of this volume, that a pretty full discussion of them will be necessary. I have recently been over enough of the ground myself to become satisfied of the general correctness of the views entertained by my late colleague, the lamented Professor Henry Carvill Lewis, whose death in 1888 took place before the publication of his most mature conclusions. But the lines of investigation to which he gave so powerful an impulse have since been followed out by an active body of scientific observers. To give the statement of facts greater precision and authority, I have committed the preparation of it to the Secretary of the Northwest of England Boulder Committee, Percy F. Kendall, F. G. S., Lecturer on Geology at the Yorkshire College, Leeds, and at the Stockport Technical School, England.[BM]

[BM]Mr. Kendall’s contribution extends topage 181.

“All the characteristic evidences of the action of land-ice can be found in the greatest perfection in many parts of England and Wales. Drumlins, kames,roches moutonnées, far-travelled erratics, terminal moraines, and perched blocks, all occur. There are, besides, in the wide-spread deposits of boulder-clay which cover so many thousands of square miles on the low grounds lying on either side of the Pennine chain, evidences of the operation of ice-masses of a size far exceeding that of the grandest of existing European glaciers. But, while the proofs of protracted and severe glaciation are thus patent, there are, nevertheless, many apparently anomalous circumstances which arrest the attention when the whole country is surveyed. The glacial phenomena appear to be strictly limited to the country lying to the northward of a line extendingfrom the Bristol Channel to the mouth of the Thames; and within the glaciated area there are many extensive tracts of land devoid of ‘drift’ or other indications of ice-action.

“By comparison with the phenomena displayed in the North American continent, English glacial geology must seem puny and insignificant; but, just as with the features of the ‘Solid Geology,’ we have compressed within the narrow limits of our isles an epitome of the features which across the Atlantic require a continent for their exposition. It has resulted from this concentration that English geology requires a much closer and more minute investigation. And the difficulty which has been experienced by glacial geologists of dealing with an involved series of facts has, in the absence of any clue leading to the co-ordination of a vast series of more or less disconnected observations, resulted in the adoption, to meet certain local anomalies, of explanations which were very difficult if not impossible of reconciliation with facts observed in adjacent areas. Thus, to account for shell-bearing drift extending up to the water-shed on one side of a lofty range of hills, a submergence of the land to a depth of 1,400 feet has been postulated; leaving for independent explanation the fact, that the opposite slopes of the hills and the low ground beyond were absolutely destitute of drift or of any evidence of marine action.

“In the following pages I must adopt a somewhat dogmatic tone, in order to confine myself within the limits of space which are imposed; and trust rather to the cohesion and consistency of the explanations offered and to a few pregnant facts than to the weighing and contrasting of rival theories.

“The facts point conclusively to the action in the British Isles of a series of glaciers radiating outward from the great hill chains or clusters, and, as the refrigeration progressed, becoming confluent and moving though in thesame general direction, yet with less regard to the minor inequalities of the ground. During these two stages many glaciers must have debouched upon the sea-coast, with the consequent production of icebergs, which floated off with loads of boulders and dispersed them in the random fashion which is a necessary characteristic of transport by floating ice.

“With a further accentuation of the cold conditions the discharge of bergs from terminal fronts which advanced into the extremely shallow seas surrounding the British shores would be quite inadequate to relieve the great press of ice, and a further coalescence of separate elements must have resulted. In the case of enclosed seas—as, for example, the Irish Sea—the continued inthrust of glacier-ice would expel the water completely; and the conjoined ice-masses would take a direction of flow the resultant of the momentum and direction of the constituent elements. In other cases—as, for example, in the North Sea—extraneous ice approaching the shores might cause a deflection of the flow of the native glaciers, even though the foreign ice might never actually reach the shore.

“To such a system of confluent glaciers, and to the separate elements out of which they grew, and into which, after the culmination, they were resolved, I attribute the whole of the phenomena of the English and Welsh drift. And only at one or two points upon the coast, and raised but little above the sea-level, can I recognise any signs of marine action.

“The Preglacial Level of the Land.—There is very little direct evidence bearing upon this point. In Norfolk the famous forest bed, with its associated deposits, stands at almost precisely the level which it occupied in preglacial times. At Sewerby, near Flamborough Head, there is an ancient beach and ‘buried cliff’ which the sea is now denuding of its swathing of drift-deposits, and its level can be seen to be almost absolutely coincident with the presentbeach. Mr. Lamplugh, whose description of the ‘Drifts of Flamborough Head,’[BN]constitutes one of the gems of glacial literature, considers that there is clear evidence that the land stood at this level for a long period. The beach is covered by a rain-wash of small extent, and that in turn by an ancient deposit of blown sand, while the lowest member of the drift series of Yorkshire covers the whole. Mr. Lamplugh thinks that the blown sand may indicate a slight elevation of the land; but the beach appears to me to be the storm beach, and the reduction in the force of the waves such as would result from the approach of an ice-front a few miles to the seaward would probably produce the necessary conditions.

[BN]Quarterly Journal of the Geological Society, vol. xlvii.

“Six miles to the northward of Flamborough, at Speeton, a bed of estuarine silt containing the remains of mollusca in the position of life occurs at an altitude of ninety feet above high-water mark. Mr. Lamplugh inclines to the opinion that this bed is of earlier date than the ‘buried cliff’; he also admits the possibility that its superior altitude may be due to a purely local upward bulging of the soft Lower Cretaceous clays upon which the estuarine bed rests by the weight of the adjacent lofty chalk escarpment.

“The evidence obtained from inland sections and borings in different parts of England has been taken to indicate a greater altitude in preglacial times. Thus, in Essex, deep-borings have revealed the existence of deep drift-filled valleys, having their floors below sea-level. The valley of the Mersey is a still better example. Numerous borings have been made in the neighbourhood of Widnes and at other places in the lower reaches of the river, making it clear that there is a channel filled with drift and extending to 146 feet below mean sea-level. This, with several other instances, has been taken to indicatea greater altitude for the land in preglacial times, since a river could not erode its channel to such a depth below sea-level. The argument appears inconclusive for one principal reason: no mention is made of any river gravels or other alluvium in the borings. Indeed, there is an explicit statement that the deposits are all glacial, showing that the channel must have been cleared out by ice. This, therefore, leaves open the vital question, whether the deposits removed were marine or fluviatile. It may be remarked that the great estuary of the Mersey has undoubtedly been produced by a post-glacial (and probably post-Roman) movement of depression.

“The Preglacial Climate.—In all speculations regarding the cause of the Glacial epoch, due account must be taken of the undoubted fact that it came on with extreme slowness and departed with comparative suddenness. In the east of England an almost perfect and uninterrupted sequence of deposits is preserved, extending from the early part of the Pliocene period down to the present day.

“These in descending order are:

“1. Post-glacial sands, gravels, etc.

“2. Glacial series.

“3. The ‘Forest Bed’ and associated marine deposits.

“4. Chillesford clay and sand.

“5. The many successive stages of the Red Crag. (The Norwich Crag is a local variation of the upper part of the Red Crag.)

“6. The Coralline Crag.

“The fossils preserved in these deposits, apart from the physical indications, exhibit the climatal changes which accompanied their deposition. The Coralline Crag contains a fauna consisting mainly of species which now range to the Mediterranean, many of them being restricted to the warm southern waters. Associated with these are a few boreal forms, but they are represented in general by few individuals. Here and there in the deposits ofthis age far-travelled stones are to be found, but they are always accounted great rarities.

“The Red Crag consists of an irregular assemblage of beaches and sand-banks of widely different ages, but their sequence can be made out with ease by a study of the fauna. In the oldest deposits, Mediterranean species are very numerous, while the boreal forms are comparatively rare; but in successive later deposits the proportions are very gradually reversed, and from the overlying Chillesford series the Mediterranean species are practically absent. The physical indications runpari passuwith the paleontological, and in the newer beds of the Red Crag far-travelled stones are common.

“In the Forest Bed series there is a marine band—theLeda myalisbed—which contains an almost arctic assemblage of shells; while at about the same horizon plant remains have been found, including such high northern species asSalix polarisandBetula nana.

“The glacial deposits do not, in my opinion, contain anywhere in England or Wales a genuine intrinsic fauna, such shells as occur in the East Anglian glacial deposits having been derived in part from a contemporary sea-bed, and, for the rest, from the older formations, down perhaps to the Coralline Crag. In the post-glacial deposits we have hardly any trace of a survival of the boreal forms, and I consider that the whole marine fauna of the North Sea was entirely obliterated at the culmination of the Glacial epoch, and that the repeopling in post-glacial times proceeded mainly from the English Channel, into which the northern forms never penetrated.

"The Great Glacial Centres.

“Where such complex interactions have to be described as were produced by the conflicting glaciers of the British Isles it is difficult to deal consecutively with the phenomena of any one area, but with short digressions in explanationof special points it may be possible to accomplish a clear presentation of the facts.

“Wales.—The phenomena of South Wales are comparatively simple. Great glaciers travelled due southward from the lofty Brecknock Beacons, and left the characteristicmoutonnéeappearance upon the rocky bed over which they moved. The boulder-transport is in entire agreement with the other indications, and there are no shells in the drift. The facts awaiting explanation are the occurrence in the boulder-clays of Glamorganshire, at altitudes up to four hundred feet, of flints, and of igneous rocks somewhat resembling those of the Archæan series of the Wrekin. At Clun, in Shropshire, a train of erratics (see map) has been traced back to its source to the westward. On the west coast, in Cardigan Bay, the boulders are all such as might have been derived from the interior of Wales. At St. David’s Peninsula, Pembrokeshire, striæ occur coming in from the northwest, and, taken with the discovery of boulders of northern rocks, may point to a southward extension of a great glacier produced by confluent sheets that choked the Irish Sea. Information is very scanty regarding large areas in mid-Wales, but such as can be gathered seems to point to ice-shedding having taken place from a north and south parting line. In North Wales, much admirable work has been done which clearly indicates the neighbourhood of Great Arenig (Arenig Mawr) as the radiant point for a great dispersal of blocks of volcanic rock of a characteristic Welsh type.

“Ireland.—A brief reference must be made to Ireland, as the ice which took origin there played an important part in bringing about some strange effects in English glaciation, which would be inexplicable without a recognition of the causes in operation across the Irish Sea. Ireland is a great basin, surrounded by an almost continuous girdle of hills. The rainfall is excessive, and the snow-fall was probably more than proportionately great;therefore we might expect that an ice-sheet of very large dimensions would result from this combination of favouring conditions. The Irish ice-sheet appears to have moved outward from about the centre of the island, but the main flow was probably concentrated through the gaps in the encircling mountains.

“Galloway.—The great range of granite mountains in the southwestern corner of Scotland seems to have given origin to an immense mass of ice which moved in the main to the southward, and there are good grounds for the belief that the whole ice-drainage of the area, even that which gathered on the northern side of the water-shed, ultimately found its way into the Irish Sea basin and came down coastwise and across the low grounds of the Rinns of Galloway, being pushed down by the press of Highland ice which entered the Firth of Clyde. It is a noteworthy fact that marine shells occur in the drift in the course taken by the ice coming on to the extremity of Galloway from the Clyde.

“The Lake District.—A radial flow of ice took place down the valleys from about the centre of the Cumbrian hill-plexus, but movement to the eastward was at first forbidden by the great rampart of the Cross Fell escarpment, which stretches like a wall along the eastern side of the Vale of Eden.

“During the time when the Cumbrian glaciers had unobstructed access to the Solway Frith, to the Irish Sea, and to Morecambe Bay, the dispersal of boulders of characteristic local rocks would follow the ordinary drainage-lines; but, as will be shown later, a state of affairs supervened in the Irish Sea which resulted, in many cases, in a complete reversal of the ice-flow.

“The Pennine Chainwas the source of glaciers of majestic dimensions upon both its flanks in the region north of Skipton, but to the southward of that breach in the chain (see map) no evidence is obtainable of any local glaciers.

"The Confluent Glaciers.

“With the growth of ice-caps upon the great centres a condition of affairs was brought about in the Irish Sea productive of results which will readily be foreseen. The enormous volumes of ice poured into the shallow sea from north, south, east, and west, resulted in such a congestion as to necessitate the initiation of some new systems of drainage.

“The Irish Sea Glacier.—The ice from Galloway, Cumbria, and Ireland became confluent, forming what the late Professor Carvill Lewis termed ‘the Irish Sea Glacier,’ and took a direction to the southward. Here it came in diametrical conflict with the northward-flowing element of the Welsh sheet, which it arrested and mastered; and the Irish Sea Glacier bifurcated, probably close upon the precipitous Welsh coast to the eastward of the Little Orme’s Head, and the two branches flowed coastwise to eastward and westward, keeping near the shore-line.

“The westerly branch swept round close to the coast in a southwesterly direction, and completely overrode Anglesea; striating the rock-surfaces from northeast to southwest (see map), and strewing the country with its bottom-moraine, containing characteristic northern rocks, such as the Galloway granites, the lavas and granites of the central and western portions of the Lake District, and fragments of shells derived from shell-banks in the Irish Sea. One episode of this phase of the ice-movement was the invasion of the first line of hills between the Menai Straits and Snowdon. The gravels and sands of Fridd-bryn-mawr, Moel Tryfaen, and Moel-y-Cilgwyn, are the coarser washings of the bottom-moraine, and consequently contain such rock-fragments and shells as characterise it. From Moel-y-Cilgwyn southward, evidence is lacking regarding the course taken by the glacier, but it probably passed over or between the Rivals Mountains (Yr Eifl), and downCardigan Bay at some distance from the coast in confluence with the ice from mid-Wales; and, as I have suggested, may have passed over St. David’s Head.

“Returning now towards the head of the glacier we may follow with advantage its left bank downward. The ice-flow on the Cumberland coast appears to have resembled very much that in North Wales. A great press of ice from the northward (Galloway) seems to have had a powerful ‘easting’ imparted to it by the conjoint influences of the thrust of the Irish ice and the inflow of ice from the Clyde. Whatever may have been the cause, the effect is clear: about Ravenglass cleavage took place, and a flow to northward and to southward, each bending easterly. By far the larger mass took a southerly course and bent round Black Combe, over Walney, and a strip of the mainland about Barrow in Furness, and out into and across Morecambe Bay. Its limits are marked in the field by the occurrence of the same rocks which characterise it in Anglesea, viz., the granites of Galloway and of west and central Cumbria.

“The continued thrust shouldered in the glacier upon the mainland of Lancashire, but the precise point of emergence has not yet been traced, though it cannot be more than a few miles from the position indicated on the map. I should here remark, that all along the boundaries the Irish Sea Glacier was confluent with local ice, except, probably, in that part of the Pennine chain to the southward of Skipton. Down to Skipton there was a great mass of Pennine ice which was compelled to take an almost due southerly course, and thus to run directly athwart the direction of the main hills and valleys. A sharp easterly inflection of the Irish Sea Glacier carried it up the valley of the Ribble, and thence, under the shoulder of Pendle, to Burnley, where Scottish granites are found in the boulder-clay.

“On the summit of the Pennine water-shed, at HealdMoor, near Todmorden (1,419 feet), boulder-clay has been found containing erratics belonging to this dispersion; while in the gorge of the Yorkshire Calder, which flows along the eastern side of the same hill, not a vestige of such a deposit is to be found, saving a few erratic pebbles at a distance of eight or ten miles, which were probably carried down by flood-wash from the edge of the ice.

“From this point the limits of the ice may be traced along the flanks of the Pennine chain at an average altitude of about 1,100 feet.

“At one place the erratics can be traced to a position which would indicate the formation of an extra-morainic lake having its head at a col about 1,000 feet above sea-level, separating it from the valley of an eastward-flowing stream, the Wye, about twelve miles down which a few granite blocks have been found. Other extra-morainic lakes must have been formed, but very little information has been collected regarding them. The Irish Sea Glacier can be shown to have spread across the whole country to the westward of the line I have traced, and beyond the estuary of the Dee.

“I may now follow its boundaries on the Welsh coast, and pursue the line to the final melting-place of the glacier. From the Little Orme’s Head the line of confluence with the native ice is pretty clearly defined. It runs in, perhaps, half a mile from the shore, until the broad low tract of the Vale of Clwyd is reached. Here the northern ice obtained a more complete mastery, and pushed in even as far as Denbigh. This extreme limit was probably attained as a mere temporary episode. Horizontal striæ on a vertical face of limestone on the crags dominating the mouth of the vale on the eastern side attest beyond dispute the action of a mass of land-ice moving in from the north.

“I may here remark, that in this district the deposits furnish a very complete record of the events of the Glacialperiod. In the cliffs on the eastern side of the Little Orme’s Head, and at several other points along the coast towards the east, a sequence may be observed as follows:

“4. Boulder-clay with northern erratics and shells.

“3. Sands and gravels with northern erratics and shells.

“2. Boulder-clay with northern erratics and shells.

“1. Boulder-clay with Welsh erratics and no shells.

“A similar succession is to be seen in the Vale of Clwyd. The interpretation is clear: In the early stages of glaciation the Welsh ice spread without hindrance to, and laid down, bed No. 1; then the northern ice came down, bringing its typical erratics and the scourings of the sea-bottom, and laid down the variable series of clays, sands, and gravels which constitute Nos. 2, 3, and 4 of the section.

Back to Index Next