Chapter 4

Fig. 14.—By the courtesy of the National Geographical Society.

Leaving Yakutat Bay, and following the route indicated upon the accompanying map, they travelled on glacial ice almost the entire distance to the foot of Mount St. Elias. The numerous glaciers coming down from the summit of the mountain-ridge become confluent nearer the shore, and spread out over an area of about a thousand square miles. This is fitly named the Malaspina Glacier, after the Spanish explorer who discovered it in 1792.

It is not necessary to add further particulars concerningthe results of this expedition, since they are so similar to those already detailed in connection with the Muir Glacier. A feature, however, of special interest, pertains to the glacial lakes which are held in place by the glacial ice at an elevation of thousands of feet above the sea. One of considerable size is indicated upon the map just south of what was called Blossom Island, which, however, is not an island, but simply anunatak, the ice here surrounding a considerable area of fertile land, which is covered with dense forests and beautified by a brilliant assemblage of flowering plants. In other places considerable vegetation was found upon the surface of moraines, which were probably still in motion with the underlying ice.

Greenland.—The continental proportions of Greenland, and the extent to which its area is covered by glacial ice, make it by far the most important accessible field for glacial observations. The total area of Greenland can not be less than five hundred thousand square miles—equal in extent to the portion of the United States east of the Mississippi and north of the Ohio. It is now pretty evident that the whole of this area, except a narrow border about the southern end, is covered by one continuous sheet of moving ice, pressing outward on every side towards the open water of the surrounding seas.

For a long time it was the belief of many that a large region in the interior of Greenland was free from ice, and was perhaps inhabited. It was in part to solve this problem that Baron Nordenskiöld set out upon his expedition of 1883. Ascending the ice-sheet from Disco Bay, in latitude 69°, he proceeded eastward for eighteen days across a continuous ice-field. Rivers were flowing in channels upon the surface like those cut on land in horizontal strata of shale or sandstone, only that the pure deep blue of the ice-walls was, by comparison, infinitely more beautiful. These rivers were not, however, perfectly continuous. After flowing for a distance in channels on the surface, they, one and all, plunged with deafening roar into some yawning crevasse, to find their way to the sea through subglacial channels. Numerous lakes with shores of ice were also encountered.

Fig. 15.—Map of Greenland. The arrow-points mark the margin of the ice-field.

“On bending down the ear to the ice,” says this explorer, “we could hear on every side a peculiar subterranean hum, proceeding from rivers flowing within the ice; and occasionally a loud, single report, like that of a cannon, gave notice of the formation of a new glacier-cleft.... In the afternoon we saw at some distance from us a well-defined pillar of mist, which, when we approached it, appeared to rise from a bottomless abyss, into which a mighty glacier-river fell. The vast, roaring water-mass had bored for itself a vertical hole, probably down to the rock, certainly more than two thousand feet beneath, on which the glacier rested.”[AJ]

[AJ]Geological Magazine, vol. ix, pp. 393, 399.

At the end of the eighteen days Nordenskiöld found himself about a hundred and fifty miles from his starting-point, and about five thousand feet above the sea. Here the party rested, and sent two Eskimos forward onskidor—a kind of long wooden skate, with which they could move rapidly over the ice, notwithstanding the numerous small, circular holes which everywhere pitted the surface. These Eskimos were gone fifty-seven hours, having slept only four hours of the period. It is estimated that they made about a hundred and fifty miles, and attained an altitude of six thousand feet. The ice is reported as rising in distinct terraces, and as seemingly boundless beyond. If this is the case, two hundred miles from Disco Bay, there would seem little hope of finding in Greenland an interior freed from ice. So we may pretty confidently speak of that continental body of land as still enveloped in an ice-sheet. Up to about latitude 75°, however, the continent is fringed by a border of islands, over whichthere is no continuous covering of ice. In south Greenland the continuous ice-sheet is reached about thirty miles back from the shore.

A summary of the results of Greenland exploration was given by Dr. Kink in 1886, from which it appears that since 1876 one thousand miles of the coast-line have been carefully explored by entering every fiord and attempting to reach the inland ice. According to this authority—

We are now able to demonstrate that a movement of ice from the central regions of Greenland to the coast continually goes on, and must be supposed to act upon the ground over which it is pushed so as to detach and transport fragments of it for such a distance.... The plainest idea of the ice-formation here in question is given by comparing it with an inundation.... Only the marginal parts show irregularity; towards the interior the surface grows more and more level and passes into a plain very slightly rising in the same direction. It has been proved that, ascending its extreme verge, where it has spread like a lava-stream over the lower ground in front of it, the irregularities are chiefly met with up to a height of 2,000 feet, but the distance from the margin in which the height is reached varies much. While under 6812° north latitude it took twenty-four miles before this elevation was attained, in 7212° the same height was arrived at in half the distance....

A general movement of the whole mass from the central regions towards the sea is still continued, but it concentrates its force to comparatively few points in the most extraordinary degree. These points are represented by the ice-fiords, through which the annual surplus ice is carried off in the shape of bergs.... In Danish Greenland are found five of the first, four of the second, and eight of the third (or least productive) class, besides a number of inlets which only receive insignificant fragments.Direct measurements of the velocity have now been applied on three first-rate and one second-rate fiords, all situated between 69° and 71° north latitude. The measurements have been repeated during the coldest and the warmest season, and connected with surveying and other investigations of the inlets and their environs. It is now proved that the glacier branches which produce the bergs proceed incessantly at a rate of thirty to fifty feet per diem, this movement being not at all influenced by the seasons. . . .

In the ice-fiord of Jakobshavn, which spreads its enormous bergs over Disco Bay and probably far into the Atlantic, the productive part of the glacier is 4,500 metres (about 212miles) broad. The movement along its middle line, which is quicker than on the sides nearer the shores, can be rated at fifty feet per diem. The bulk of ice here annually forced into the sea would, if taken on the shore, make a mountain two miles long, two miles broad, and 1,000 feet high. The ice-fiord of Torsukatak receives four or five branches of the glacier; the most productive of them is about 9,000 metres broad (five miles), and moves between sixteen and thirty-two feet per diem. The large Karajak Glacier, about 7,000 metres (four miles) broad, proceeds at a rate of from twenty-two to thirty-eight feet per diem. Finally, a glacier branch dipping into the fiord of Jtivdliarsuk, 5,800 metres broad (three miles), moved between twenty-four and forty-six feet per diem.[AK]

[AK]See Transactions of the Edinburgh Geological Society for February 18, 1886, vol. v, part ii, pp. 286-293.

The principal part of our information concerning the glaciers of Greenland north of Melville Bay was obtained by Drs. Kane and Hayes, in 1853 and 1854, while conducting an expedition in search of Sir John Franklin and his unfortunate crew. Dr. Hayes conducted another expeditionto the same desolate region in 1860, while other explorers have to some extent supplemented their observations. The largest glacier which they saw enters the sea between latitude 79° and 80°, where it presents a precipitous discharging front more than sixty miles in width and hundreds of feet in perpendicular height.

Dr. Kane gives his first impressions of this grand glacier in the following vivid description:

“I will not attempt to do better by florid description. Men only rhapsodize about Niagara and the ocean. My notes speak simply of the ‘long, ever-shining line of cliff diminished to a well-pointed wedge in the perspective’; and, again, of ‘the face of glistening ice, sweeping in a long curve from the low interior, the facets in front intensely illuminated by the sun.’ But this line of cliff rose in a solid, glassy wall three hundred feet above the water-level, with an unknown, unfathomable depth below it; and its curved face, sixty miles in length from Cape Agassiz to Cape Forbes, vanished into unknown space at not more than a single day’s railroad-travel from the pole. The interior, with which it communicated and from which it issued, was an unsurveyedmer de glace—an ice-ocean to the eye, of boundless dimensions.

“It was in full sight—the mighty crystal bridge which connects the two continents of America and Greenland. I say continents, for Greenland, however insulated it may ultimately prove to be, is in mass strictly continental. Its least possible axis, measured from Cape Farewell to the line of this glacier, in the neighbourhood of the eightieth parallel, gives a length of more than 1,200 miles, not materially less than that of Australia from its northern to its southern cape.

“Imagine, now, the centre of such a continent, occupied through nearly its whole extent by a deep, unbroken sea of ice that gathers perennial increase from the water-shed of vast snow-covered mountains and all the precipitationsof its atmosphere upon its own surface. Imagine this, moving onwards like a great glacial river, seeking outlets at every fiord and valley, rolling icy cataracts into the Atlantic and Greenland seas; and, having at last reached the northern limit of the land that has borne it up, pouring out a mighty frozen torrent into unknown arctic space!

“It is thus, and only thus, that we must form a just conception of a phenomenon like this great glacier. I had looked in my own mind for such an appearance, should I ever be fortunate enough to reach the northern coast of Greenland; but, now that it was before me, I could hardly realize it. I had recognized, in my quiet library at home, the beautiful analogies which Forbes and Studer have developed between the glacier and the river. But I could not comprehend at first this complete substitution of ice for water.

“It was slowly that the conviction dawned on me that I was looking upon the counterpart of the great river-system of Arctic Asia and America. Yet here were no water-feeders from the south. Every particle of moisture had its origin within the polar circle and had been converted into ice. There were no vast alluvions, no forest or animal traces borne down by liquid torrents. Here was a plastic, moving, semi-solid mass, obliterating life, swallowing rocks and islands, and ploughing its way with irresistible march through the crust of an investing sea.”[AL]

[AL]Arctic Explorations in the Years 1853, 1854, and 1855, vol. i, pp. 225-228.

Much less is known concerning the eastern coast of Greenland than about the western coast. For a long time it was supposed that there might be a considerable population in the lower latitudes along the eastern side. But that is now proved to be a mistake. The whole coast is very inhospitable and difficult of approach. From latitude65° to latitude 69° little or nothing is known of it. In 1822-’23 Scoresby, Cleavering, and Sabine hastily explored the coast from latitude 69° to 76°, and reported numerous glaciers descending to the sea-level through extensive fiords, from which immense icebergs float out and render navigation dangerous. In 1869 and 1870 the second North-German Expedition partly explored the coast between latitude 73° and 77°. Mr. Payer, an experienced Alpine explorer, who accompanied the expedition, reports the country as much broken, and the glaciers as “subordinated in position to the higher peaks, and having their moraines, both lateral and terminal, like those of the Alpine ranges, and on a still grander scale.” Petermann Peak, in latitude 73°, is reported as 13,000 feet high. Captain Koldewey, chief of the expedition, found extensive plateaus on the mainland, in latitude 75°, to be “entirely clear of snow, although only sparsely covered with vegetation.” The mountains in this vicinity, also, rising to a height of more than 2,000 feet, were free from snow in the summer. Some of the fiords in this vicinity penetrate the continent through several degrees of longitude.

An interesting episode of this expedition was the experience of the crew of the ship Hansa, which was caught in the ice and destroyed. The crew, however, escaped by encamping on the ice-floe which had crushed the ship. From this, as it slowly floated towards the south through several degrees of latitude, they had opportunity to make many important observations upon the continent itself. As viewed from this unique position the coast had the appearance everywhere of being precipitous, with mountains of considerable height rising in the background, from which numerous small glaciers descended to the sea-level.

In 1888 Dr. F. Nansen, with Lieutenant Sverdrup and four others, was left by a whaler on the ice-pack bordering the east of Greenland about latitude 65°, and in sight of the coast. For twelve days the party was on the ice-packfloating south, and so actually reached the coast only about latitude 64°. From this point they attempted to cross the inland ice in a northwesterly direction towards Christianshaab. They soon reached a height of 7,000 feet, and were compelled by severe northerly storms to diverge from their course, taking a direction more to the west. The greatest height attained was 9,500 feet, and the party arrived on the western coast at Ameralik Fiord, a little south of Godhaab, about the same latitude at which they entered.

It thus appears that subsequent investigations have confirmed in a remarkable manner the sagacious conclusions made by the eminent Scotch geologist and glacialist Robert Brown in 1875, soon after his own expedition to the country. “I look upon Greenland and its interior ice-field,” he writes, "in the light of a broad-lipped, shallow vessel, but with chinks in the lips here and there, and the glacier like viscous matter in it. As more is poured in, the viscous matter will run over the edges, naturally taking the line of the chinks as its line of outflow. The broad lips of the vessel are the outlying islands or ‘outskirts’; the viscous matter in the vessel the inland ice, the additional matter continually being poured in in the form of the enormous snow covering, which, winter after winter, for seven or eight months in the year, falls almost continuously on it; the chinks are the fiords or valleys down which the glaciers, representing the outflowing viscous matter, empty the surplus of the vessel—in other words, the ice floats out in glaciers, overflows the land in fact, down the valleys and fiords of Greenland by force of the superincumbent weight of snow, just as does the grain on the floor of a barn (as admirably described by Mr. Jamieson) when another sackful is emptied on the top of the mound already on the floor. ‘The floor is flat, and therefore does not conduct the grain in any direction; the outward motion is due to the pressureof the particles of grain on one another; and, given a floor of infinite extension and a pile of sufficient amount, the mass would move outward to any distance, and with a very slight pitch or slope it would slide forward along the incline.’ To this let me add that if the floor on the margin of the heap of grain was undulating the stream of grain would take the course of such undulations. The want, therefore, of much slope in a country and the absence of any great mountain-range are of very little moment to the movement of land-ice,provided we have snow enough" On another page Dr. Brown had well said that “the country seems only a circlet of islands separated from one another by deep fiords or straits, and bound together on the landward side by the great ice covering which overlies the whole interior.... No doubt under this ice there lies land, just as it lies under the sea; but nowadays none can be seen, and as an insulating medium it might as well be water.”

In his recently published volumes descriptive of the journey across the Greenland ice-sheet, alluded to onpage 39, Dr. Nansen sums up his inferences in very much the same way: “The ice-sheet rises comparatively abruptly from the sea on both sides, but more especially on the east coast, while its central portion is tolerably flat. On the whole, the gradient decreases the farther one gets into the interior, and the mass thus presents the form of a shield with a surface corrugated by gentle, almost imperceptible, undulations lying more or less north and south, and with its highest point not placed symmetrically, but very decidedly nearer the east coast than the west.”

From this rapid glance at the existing glaciers of the world we see that a great ice age is not altogether a strange thing in the world. The lands about the south pole and Greenland are each continental in dimensions, and present at the present time accumulations of land-ice so extensive, so deep, and so alive with motion as to prepareour minds for almost anything that may be suggested concerning the glaciated condition of other portions of the earth’s surface. Thevera causais sufficient to accomplish anything of which glacialists have ever dreamed. It only remains to enquire what the facts really are and over how great an extent of territory the actual results of glacial action may be found. But we will first direct more particular attention to some of the facts and theories concerning glacial motion.

CHAPTER III.

GLACIAL MOTION.

That glacial ice actually moves after the analogy of a semi-fluid has been abundantly demonstrated by observation. In the year 1827 Professor Hugi, of Soleure, built a hut far up upon the Aar Glacier in Switzerland, in order to determine the rate of its motion. After three years he found that it had moved 330 feet; after nine years, 2,354 feet; and after fourteen years Louis Agassiz found that its motion had been 4,712 feet. In 1841 Agassiz began a more accurate series of observation upon the same glacier. Boring holes in the ice, he set across it a row of stakes which, on visiting in 1842, he found to be no longer in a straight line. All had moved downwards with varying velocity, those near the centre having moved farther than the others. The displacements of the stakes were in order, from side to side, as follows: 160 feet, 225 feet, 269 feet, 245 feet, 210 feet, and 125 feet. Agassiz followed up his observations for six years, and in 1847 published the results in his celebrated work System Glacière.

Fig. 16.

But in August, 1841, the distinguished Swiss investigator had invited Professor J. D. Forbes, of Edinburgh, to interest himself in solving the problem of glacial motion. In response to this request, Professor Forbes spent three weeks with Agassiz upon the Aar Glacier. Stimulatedby the interest of this visit, Forbes returned to Switzerland in 1842 and began a series of independent investigations upon the Mer de Glace. After a week’s observations with accurate instruments, Forbes wrote to Professor Jameson, editor of the Edinburgh New Philosophical Journal, that he had already made it certain that “the central part of the glacier moves faster than the edges in a very considerable proportion, quite contrary to the opinion generally maintained.” This letter was dated July 4, 1842, but was not published until the October following, Agassiz’s results, so far as then determined, were, however, published in Comptes Rendus of the 29th of August, 1842, two months before the publication of Forbes’s letter. But Agassiz’s letter was dated twenty-seven days later than that of Forbes. It becomes certain, therefore, that both Agassiz and Forbes, independently and about the same time, discovered the fact that the central portion of a glacier moves more rapidly than the sides.

In 1857 Professor Tyndall began his systematic and fruitful observations upon the Mer de Glace and other Alpine glaciers. Professor Forbes had already demonstrated that, with an accurate instrument of observation, the motion of a line of stakes might be observed after the lapse of a single clay, or even of a few hours. As a result of Tyndall’s observations, it was found that the most rapid daily motion in the Mer de Glace in 1857 was about thirty-seven inches. This amount of motion was near the lower end of the glacier On ascending the glacier, the rate was found in general to be diminished; but the diminution was not uniform throughout the whole distance, being affected both by the size and by the contour of the valley. The motion in the tributary glaciers was also much less than that of the main glacier.

This diminution of movement in the tributary glaciers was somewhat proportionate to their increase in width.For example, the combined width of the three tributaries uniting to form the Mer de Glace is 2,597 yards; but a short distance below the junction of these tributaries the total width of the Mer de Glace itself is only 893 yards, or one-third that of the tributaries combined. Yet, though the depth of the ice is probably here much greater than in the tributaries, the rapidity of movement is between two and three times as great as that of any one of the branches.[AM]

[AM]See Tyndall’s Forms of Water, pp. 78-82.

From Tyndall’s observations it appears also that the line of most rapid motion is not exactly in the middle of the channel, but is pushed by its own momentum from one side to the other of the middle, so as always to be nearer the concave side; in this respect conforming, as far as its nature will permit, to the motion of water in a tortuous channel.

Fig. 17.

It is easy to account for this differential motion upon the surface of a glacier, since it is clear that the friction of the sides of the channel must retard the motion of ice as it does that of water. It is clear also that the friction of the bottom must retard the motion of ice even more than it is known to do in the case of water. In the formation of breakers, when the waves roll in upon a shallowing beach, every one is familiar with the effect of the bottom upon the moving mass. Here friction retards the lower strata of water, and the upper strata slide over the lower, and, where the water is of sufficient depth and the motion is sufficiently great, the crest breaks down in foam before the ever-advancing tide. A similar phenomenon occurs when dams give way and reservoirs suddenly pour their contents into the restricted channelsbelow. At such times the advancing water rolls onwards like the surf with a perpendicular front, varying in height according to the extent of the flood.

Seasoning from these phenomena connected with moving water, it was naturally suggested to Professor Tyndall that an analogous movement must take place in a glacier. Choosing, therefore, a favourable place for observation on the Mer de Glace where the ice emerged from a gorge, he found a perpendicular side about one hundred and fifty feet in height from bottom to top. In this face he drove stakes in a perpendicular line from top to bottom. Upon subsequently observing them, Tyndall found, as he expected, that there was a differential motion among them as in the stakes upon the surface. The retarding effect of friction upon the bottom was evident. The stake near the top moved forwards about three times as fast as the one which was only four feet from the bottom.

Fig. 18.

The most rapid motion (thirty-seven inches per day) observed by Professor Tyndall upon the Alpine glaciers occurred in midsummer. In winter the rate was only about one-half as great; but in the year 1875 the Norwegian geologist, Helland, reported a movement of twenty metres (about sixty-five feet) per day in the Jakobshavn Glacier which enters Disco Bay, Greenland, about latitude 70°. For some time there was a disposition on the part of many scientific men to doubt the correctness of Holland’s calculations. Subsequent observations have shown, however, that from the comparatively insignificant glaciers of the Alps they were not justified in drawing inferences with respect to the motion of the vastly larger masses which come down to the sea through the fiords of Greenland.The Jakobshavn Glacier was about two and a half miles in width and its depth very likely more than a thousand feet, making a cross-section of more than 1,400,000 square yards, whereas the cross-section of the Mer de Glace at Montanvert is estimated to be but 190,000 square yards or only about one-seventh the above estimate for the Greenland glacier. As the friction of the sides would be no greater upon a large stream than upon a small one, while upon the bottom it would be only in proportion to the area, it is evident that we cannot tell beforehand how rapidly an increase in the volume of the ice might augment the velocity of the glacier.

At any rate, all reasonable grounds for distrusting the accuracy of Helland’s estimates seem to have been removed by later investigations. According to my own observations in the summer of 1886 upon the Muir Glacier, Alaska, the central portions, a mile back from the front of that vast ice-current, were moving from sixty-five to seventy feet per day. These observations were taken with a sextant upon pinnacles of ice recognizable from a baseline established upon the shore. It is fair to add, however, that during the summer of 1890 Professor H. F. Reid attempted to measure the motion of the same glacier by methods promising greater accuracy than could be obtained by mine. He endeavoured to plant, after the method of Tyndall, a line of stakes across the ice-current. But with his utmost efforts, working inwards from both sides, he was unable to accomplish his purpose, and so left unmeasured a quarter of a mile or more of the most rapidly-moving portion of the glacier. His results, therefore, of ten feet per day in the most rapidly-moving portion observed cannot discredit my own observations on a portion of the stream inaccessible by his method. A quarter of a mile in width near the centre of so vast a glacier gives ample opportunity for a much greater rate of motion than that observed by Professor Reid. Especially may this betrue in view of Tyndall’s suggestion that the contour of the bottom over which the ice flows may greatly affect the rate in certain places. A sudden deepening of the channel may affect the motion of ice in a glacier as much as it does that of water in a river.

Other observations also amply sustain the conclusions of Helland. As already stated, the Danish surveying party under Steenstrup, after several years’ work upon the southwestern coast of Greenland, have ascertained that the numerous glaciers coming down to the sea in that region and furnishing the icebergs incessantly floating down Baffin’s Bay, move at a rate of from thirty to fifty feet per day, while Lieutenants Ryder and Bloch, of the Danish Navy, who spent the year 1887 in exploring the coast in the vicinity of Upernavik, about latitude 73°, found that the great glacier entering the fiord east of the village had a velocity of ninety-nine feet per day during the month of August.[AN]

[AN]Nature, December 29, 1887.

It is easier to establish the fact of glacial motion than to explain how the motion takes place, for ice seems to be as brittle as glass. This, however, is true of it only when compelled suddenly to change its form. When subjected to slow and long-continued pressure it gradually yet readily yields, and takes on new forms. From this capacity of ice, it has come to be regarded by some as a really viscous substance, like tar or cooling lava, and upon that theory Professor Forbes endeavours to explain all glacial movement.

The theory, however, seems to be contradicted by familiar facts; for the iceman, after sawing a shallow groove across a piece of ice, can then split it as easily as he would a piece of sandstone or wood. On the glaciers themselves, likewise, the existence of innumerable crevasses would seem to contradict the plastic theory of glacier motion;for, wherever the slope of the glacier’s bed increases, crevasses are formed by the increased strain to which the ice is subjected. Crevasses are also formed in rapidly-moving glaciers by the slight strain occasioned by the more rapid motion of the middle portion. Still, in the words of Tyndall, “it is undoubted that the glacier moves like a viscous body. The centre flows past the sides, the top flows over the bottom, and the motion through a curved valley corresponds to fluid motion.”[AO]

[AO]Forms of Water, p. 163.

To explain this combination of the seemingly contradictory qualities of brittleness and viscosity in ice, physicists have directed attention to the remarkable transformations which take place in water at the freezing-point. Faraday discovered in 1850 that "when two pieces of thawing ice are placed together they freeze together at the point of contact.[AP]

[AP]Ibid., p. 164.

“Place a number of fragments of ice in a basin of water and cause them to touch each other; they freeze together where they touch. You can form a chain of such fragments; and then, by taking hold of one end of the chain, you can draw the whole series after it. Chains of icebergs are sometimes formed in this way in the arctic seas.”[AQ]

[AQ]Ibid., pp. 164, 165.

This is really what takes place when a hard snow-ball is made by pressure in the hand. So, by subjecting fragments of ice to pressure it is first crumbled to powder, and then, as the particles are pressed together in close contact, it resumes the nature of ice again, though in a different form, taking now the shape of the mould in which it has been pressed.

Thus it is supposed that, when the temperature of ice is near the melting-point, the pressure of the superincumbent mass may produce at certain points insensible disintegration, while, upon the removal of the pressure bychange of position, regulation instantly takes place, and thus the phenomena which simulate plasticity are produced. As the freezing-point of water is, within a narrow range, determined by the amount of pressure to which it is subjected, it is not difficult to see how these changes may occur. Pressure slightly lowers the freezing-point, and so would liquefy the portions of ice subjected to greatest pressure, wherever that might be in the mass of the glacier, and thus permit a momentary movement of the particles, until they should recongeal in adjusting themselves to spaces of less pressure.[AR]This is the theory by which Professor James Thompson would account for the apparent plasticity of glacial ice.

[AR]Forms of Water, p. 168.

CHAPTER IV.

SIGNS OF PAST GLACIATION.

The facts from which we draw the inference that vast areas of the earth’s surface which are now free from glaciers were, at a comparatively recent time, covered with them, are fourfold, and are everywhere open to inspection. These facts are: 1. Scratches upon the rocks. 2. Extensive unstratified deposits of clay and sand intermingled with scratched stones and loose fragments of rock. 3. Transported boulders left in such positions and of such size as to preclude the sufficiency of water-carriage to account for them. 4. Extensive gravel terraces bordering the valleys which emerge from the glaciated areas. We will consider these in their order:

1. The scratches upon the rocks.

Almost anywhere in the region designated as having been covered with ice during the Glacial period, the surface of the rocks when freshly uncovered will be found to be peculiarly marked by grooves and scratches more or less fine, and such as could not be produced by the action of water. But, when we consider the nature of a glacier, these marks seem to be just what would be produced by the pushing or dragging along of boulders, pebbles, gravel, and particles of sand underneath a moving mass of ice.

Running water does indeed move gravel, pebbles, and boulders along with the current, but these objects are not held by it in a firm grasp, such as is required to make a groove or scratch in the rock. If, also, there are inequalitiesin the compactness or hardness of the rock, the natural action of running water is to hollow out the soft parts, and leave the harder parts projecting. But, in the phenomena which we are attributing to glacial action, there has been a movement which has steadily planed down the surface of the underlying rock; polishing it, indeed, but also grooving it and scratching it in a manner which could be accomplished only by firmly held graving-tools.

Fig. 19.—Bed-rock scored with glacial marks, near Amherst, Ohio. (From a photograph by Chamberlin.)

This polishing and scratching can indeed be producedby various agencies; as, for example, by the forces which fracture the earth’s crust, and shove one portion past another, producing what is called aslicken-side. Or, again, avalanches or land-slides might be competent to produce the results over limited and peculiarly situated areas. Icebergs, also, and shore ice which is moved backwards and forwards by the waves, would produce a certain amount of such grooving and scratching. But the phenomena to which we refer are so extensive, and occur in such a variety of situations, that the movement of glacial ice is alone sufficient to afford a satisfactory explanation. Moreover, in Alaska, Greenland, Norway, and Switzerland, and wherever else there are living glaciers, it is possible to follow up these grooved and striated surfaces till they disappear underneath the existing glaciers which are now producing the phenomena. Thus by its tracks we can, as it were, follow this monster to its lair with as great certainty as we could any animal with whose footprints we had become familiar.

2. The till, or boulder-clay.

A second sign of the former existence of glaciers over any area consists of an unstratified deposit of earthy material, of greater or less depth, in which scratched pebbles and fragments of rock occur without any definite arrangement.

Moving water is a most perfect sieve. During floods, a river shoves along over its bed gravel and pebbles of considerable size, whereas in time of low-water the current may be so gentle as to transport nothing but fine sand, and the clay will be carried still farther onwards, to settle in the still water and form a delta about the river’s mouth. The transporting capacity of running water is in direct ratio to the sixth power of its velocity. Other things being equal, if the velocity be doubled, the size of the grains of sand or gravel which it transports is increasedsixty-four fold.[AS]So frequent are the changes in the velocity of running water, that the stratification of its deposits is almost necessary and universal. If large fragments of rocks or boulders are found embedded in stratified clay, it is pretty surely a sign that they have been carried to their position by floating ice. A small mountain stream with great velocity may move a good-sized boulder, while the Amazon, with its mighty but slow-moving current, would pass by it forever withoutstirring it from its position. But the vast area which is marked in our map as having been covered with ice during the Glacial period is characterised by deep and extensivedeposits of loose material devoid of stratification, and composed of soil and rock gathered in considerable part from other localities, and mixed in an indiscriminate mass with material which has originated in the disintegration of the underlying local strata.

[AS]Le Conte’s Geology, p. 19.

Fig. 20.—Scratched stone from the till of Boston. Natural size about one foot and a half long by ten inches wide. (From photograph.)

Fig. 21.—Typical section of till in Seattle. Washington State, about two hundred feet above Puget Sound. This is on the height between the sound and Lake Washington.

Fig. 22.—Ideal section, showing how the till overlies the stratified rocks.

Fig. 23.—Vessel Rock, a glacial boulder in Gilsum. N. H. (C. H. Hitchcock.)

3. Transported boulders.

Where there is a current of water deep enough to float large masses of ice, there is scarcely any limit to the size of boulders which may be transported upon them, or to the distance to which the boulders may be carried and dropped upon the bottom. The icebergs which break off from the glaciers of Greenland may bear their burdens of rock far down into the Atlantic, depositing them finally amidst the calcareous ooze and the fine sediment from theGulf Stream which is slowly covering the area between Northern America and Europe. Northern streams like the St. Lawrence, which are deeply frozen over with ice in the winter, and are heavily flooded as the ice breaks up in the spring, afford opportunity for much transportation of boulders in the direction of their current. In attributing the transportation of a boulder to glacial ice, it is necessary, therefore, to examine the contour of the country, so as to eliminate from the problem the possibility of the effects having been produced by floating ice.

Another source of error against which one has to be on his guard arises from the close resemblance of boulders resulting from disintegration to those which have been transported by ice from distant places. Owing to the fact that large masses of rocks, especially those which are crystalline, are seldom homogeneous in their structure, it results that, under the slow action of disintegrating and erosive agencies, the softer parts often are completely removed before the harder nodules are sensibly affected, and these may remain as a collection of boulders dotting the surface. Such boulders are frequent in the granitic regions of North Carolina and vicinity, where there has been no glacial transportation. Several localities in Pennsylvania, also, south of the line of glacial action as delineated by Professor Lewis and myself, had previously been supposed to contain transported boulders of large size, but on examination they proved in all cases to be resting upon undisturbed strata of the parent rock, and were evidently the harder portions of the rock left in loco by the processes of erosion spoken of. In New England, also, it is possible that some boulders heretofore attributed to ice-action may be simply the results of these processes of disintegration and erosion. Whether they are or not can usually be determined by their likeness or unlikeness to the rocks on which they rest; but oftentimes, where a particular variety of rock is exposed over a broad area, itis difficult to tell whether a boulder has suffered any extensive transportation or not.

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