Popular Science Monthly/Volume 44/April 1894/The Ice Age and its Work II
| THE ICE AGE AND ITS WORK. |
By ALFRED R. WALLACE, F. R. S.
ERRATIC BLOCKS AND ICE-SHEETS.—(Continued.)
II.
WE must now consider briefly the distribution of erratics in North America, because they present some peculiar features and teach, us much concerning the possibilities of glacier motion.
An immense area of the Northeastern States, extending south to New York, and then westward in an irregular line to Cincinnati and St. Louis, is almost wholly covered with a deposit of drift material, in which rocks of various sizes are imbedded, while other rocks, often of enormous size, lie upon the surface. These blocks have been carefully studied by the American geologists, and they present us with some very interesting facts. Not only are the distances from which they have been transported very great, but in very many cases they are found at a greater elevation than the place from which they must have come. Prof. G. P. Wright found an enormous accumulation of bowlders on a sandstone plateau in Monroe County, Pennsylvania. Many of these bowlders were granite, and must have come either from the Adirondack Mountains two hundred miles to the north, or from the Canadian Highlands still farther away. This accumulation of bowlders was seventy or eighty feet high, and it extended many miles, descending into a deep valley one thousand feet below the plateau in a nearly continuous line forming part of the southern moraine of the great American ice-sheet.
On the Kentucky hills, about twelve miles south of Cincinnati, conglomerate bowlders containing pebbles of red jasper can be traced to a limited outcrop of the same rock in Canada to the north of Lake Huron, more than six hundred miles distant, and similar bowlders have been found at intervals over the whole intervening country. In both these cases the blocks must have passed over intervening valleys and hills, the latter as high or nearly as high as the source from whence the rocks were derived. Even more remarkable are numerous bowlders of Helderberg limestone on the summit of the Blue Ridge in Pennsylvania, which must have been brought from ledges at least five hundred feet lower than the places upon which they now lie. The Blue Ridge itself shows remarkable signs of glacial abrasion, in a well-defined shoulder marking the southern limit of the ice (as indicated also by heaps of drift and erratics), so that Mr. Wright concludes that several hundred feet of the ridge have been worn away by the ice.
The crowning example of bowlder transportation is, however, afforded by the blocks of light gray gneiss discovered by Prof. Hitchcock on the summit of Mount Washington, over six thousand feet above sea-level, and identified with Bethlehem gneiss, whose nearest outcrop is in Jefferson, several miles to the northwest, and three or four thousand feet lower than Mount Washington.
These varied phenomena of erratic blocks and rock striations, together with the enormous quantity of bowlder clay and glacial drift spread over the whole of the Eastern States, terminating southward in a more or less abrupt line of mounds having all the characteristics of an enormous moraine, have led American geologists to certain definite conclusions in which they all practically agree. It may be well first to give a notion of the enormous amount of the glacial débris under which a large part of the Eastern States is buried. In New England these deposits are of less thickness than farther south, averaging from ten to twenty feet over the whole area. In Pennsylvania and New York east of the Alleghanies the deposits are very irregular, often sixty or seventy feet thick and sometimes more. West of the Alleghanies in New York, Pennsylvania, and Ohio the thickness is much greater, being often one hundred and fifty or two hundred feet in the wide valleys, and forty or fifty feet on many of the uplands. Prof. Newberry calculates that in Ohio it averages sixty feet deep over an area of twenty-five thousand square miles.
The direction of the striæ and of the traveled bowlders together with the form of the great terminal moraines show that there must have been two main centers of outflow for the icesheet, one over Labrador, the other over the Laurentian Highlands north of Lake Superior. The southern margin of the drift may be roughly represented by portions of circles drawn from these two points as centers. The erratics on the summit of Mount Washington show that the ice-sheet must have been a mile thick in its neighborhood, and much thicker at the centers of dispersion, while the masses of drift and erratics on plateaus two thousand feet high near its southern boundary indicate a great thickness at the termination. The Laurentian plateau is now about two thousand feet above the sea-level, but there are numerous indications from buried river channels, filled with drift and far below the sea, which lead to the conclusion that during the Ice age the land was much higher. That snow can accumulate to an enormous extent over land of moderate height when the conditions are favorable for such an accumulation is shown by the case of Greenland, the greater part of whose surface is a vast plateau of ice flowing outward by numerous glaciers into the sea. The center of this plateau where Dr. Nansen crossed it was over nine thousand feet above sea-level, and it may be very much higher farther north. It, therefore, seems probable that the great American ice-sheet was, at least, as high, and perhaps much higher, and this would give sufficient slope for the flow to the southern border. Of course, during the successive stages of the glaciation there may have been numerous local centers from which glaciers radiated, and during the passing away of the Ice Age these local glaciers would have left striae and other indications of their presence. But so much of the area covered by the drift all, in fact, south of the New England mountains and the Great Lakes—is undulating ground, hill, valley, and plateau of moderate height, that here all the phenomena seem to be due to the great confluent ice-sheet during the various phases of its advance and its passing away.
Sir Henry Howorth, in his very instructive work already quoted, denies the existence and even the possibility of such icesheets as those here indicated as having occurred in North America and Europe. He maintains that ice of the requisite thickness could not exist, as it would be crushed or liquefied by its own weight; and further, that if it existed it could not possibly move over hundreds of miles of generally level country, passing over hills and valleys and carrying with it, either on its surface or in its lower strata, the enormous quantity of bowlders, gravel, and clay which we find everywhere overlying the present surface of the ground. No doubt the difficulty does seem an enormous one, but I think that it can be shown to be not so great as it seems; and it is certainly by no means so insuperable as that of the apocryphal floods, or "waves of translation" as they have been called, to which he imputes the phenomena. He asks us to believe in one or more gigantic waves sweeping over Eastern North America, carrying bowlders to the summit of Mount Washington, nearly six thousand feet high, scattering others over an area which is roughly one thousand miles from east to west and six hundred from north to south, and in its course producing those wonderful striae, grooves, and furrows in the rocks photographed in the American reports, and the enormous extent of smooth and rounded rock surfaces that is found over this wide area. Besides these there are two other phenomena absolutely inconsistent with a diluvial agency. One is the enormous deposits of fine compact clay bearing rounded and scratched stones thickly scattered through it, utterly unlike any deposit produced by water, which would necessarily leave the stones hundreds of miles behind the place to which the fine mud would be carried. The other is the existence of well-defined heaps, mounds, and ridges of gravel and bowlders, forming the terminal moraine of the ice-sheet. This is exactly similar in general form and structure to the moraines left by the old Alpine or North British glaciers, and if the former could have been produced by a flood so could the latter. But the American terminal moraine runs across the country almost irrespective of its contour, and is often as well marked on plateaus as in valleys and on the intermediate slopes. Moreover, this moraine often lies on the southern slope of the hills draining toward the Mississippi Valley; and we are asked to believe that a flood vast enough to carry gravel and rocks for hundreds of miles to such a position, left them all stranded on a slope down which it must have been rushing with increased velocity and without hindrance toward the Gulf of Mexico! So far as I know, Sir Henry Howorth is absolutely alone among living writers in his diluvial theories, and I only give this brief statement of their overwhelming impossibilities because his book is so interesting, and his assertions that his theory explains all the facts are so confident and so often repeated that they are likely to confuse the judgment of readers who have not paid special attention to the subject.
Returning to the main question, of the possibility of glaciers or ice-sheets moving over long distances of generally level ground with intervening hills and valleys, there is an important piece of evidence, the bearing of which appears to have been overlooked by objectors. The former existence of the great Rhone glacier carrying erratics to the slopes of the Jura from beyond Geneva on the southwest to Soleure on the northeast, is universally admitted. This glacier passed out of the gorge between the Dent du Midi and the Dent de Morcles, and a little below St. Maurice enters on the alluvial plain which extends to the lake. From this point to Geneva, a distance of about sixty miles, may be considered a level plain, the descent into the lake being balanced by the ascent out of it. Yet it is admitted that the glacier did move over this distance, since erratics which can be traced to their source on the left of the valley below Martigny are found near that city. But the main part of the glacier curved round to the right across the Lake of Neufchâtel, and extended at least as far as Soleure, a distance of about ninety miles. To do this it must have ascended five or six hundred feet to the country around Fribourg, and before reaching Soleure must have passed over a hill three or four hundred feet higher. Yet on the flanks of the Jura above Soleure there are erratics which have been carried on the surface of the glacier from the east side of the valley below Martigny, and close to Soleure itself there are remains of a terminal subglacial moraine of compact bowlder clay. Sir Charles Lyell describes this as— It is evident that, to have produced such effects as are here described, the glacier must have extended much beyond Soleure, and have been very thick even there. It thus proves to demonstration that a glacier can travel for a hundred miles over a generally level country, that it can pass over hills and valleys, and that, even near its termination, it can groove, and grind, and polish rocks, and deposit large masses of hard bowlder clay. And all this was done by a single glacier issuing from a comparatively narrow valley, and then spreading out over an area many times greater than that of its whole previous course. In this case it is clear that such a vast mass of ice, constituting a veritable ice-sheet on a small scale, could not have derived its motion solely from the push given to it by the parent glacier at St. Maurice. Neither could gravitation derived from the slope of the ground have affected it, for it passed mostly over level ground or up slopes, and its termination at Soleure is actually nearly two hundred feet higher than its starting point at the mouth of the valley below St. Moritz! There remains as a cause of motion only the slope of the upper surface of the glacier, the ice slowly flowing downward, and, by means of its tenacity and its viscosity on a large scale, dragging its lower portion still more slowly over the uneven or upward sloping surface. This mode of motion will be discussed later when dealing with the origin of lake-basins.
No doubt at this epoch of maximum glaciation the ice-sheet extended over the whole country between the Bernese Alps and the Jura, and the downward flow of the lateral glaciers along the valley of the Sarine, Aare, and other rivers flowing toward Soleure greatly assisted the general onward motion. But the fact remains, and it can not be too strongly insisted on, that here we have a veritable ice-sheet moving over hill and valley, carrying on its surface quantities of erratic blocks, rounding, striating, and polishing the rocks over which it passed, and with the material thus crushed and ground away forming great deposits of bowlder clay, much of which still remains, although enormous quantities must have been carried away by the rivers to the lowlands of Europe and to the sea. The fact is therefore demonstrated, and is implicitly admitted by the most conservative of glacialists, that in this case an ice-sheet has moved onward over a hilly plateau for nearly a hundred miles, even when its terminal moraine is at a higher level than its exit from the mountain valley where it had its origin.
It will now be well briefly to sketch the distribution of erratic blocks in Great Britain, and the conclusions to be drawn from them as to the former existence of an ice-sheet under which the greater part of our islands was buried.
Every mountain group north of the Bristol Channel was a center from which, in the earlier and later phases of the Ice age, glaciers radiated; but many facts prove that during its maximum development these separate glacier systems became confluent, and formed extensive ice-sheets which overflowed into the Atlantic Ocean on the west, and spread far over the English lowlands on the east and south. This is indicated partly by the great height at which glacial striæ are found, reaching to twenty-five hundred feet in the Lake District and in Ireland, somewhat higher in North Wales, and in Scotland to nearly thirty-five hundred feet; but also by the extraordinary distribution of erratic blocks, many of which can be traced to localities whence they could only have been brought across the sea. The direction of the glacial striæ and of the smoothed side of ice-worn rocks also indicate that the shallow seas were all filled up by ice. The Outer Hebrides, for example, are all ice-ground from the southeast and east, showing that the deep channel of the Minch was filled up, and that the Scotch ice-sheet flowed completely over the islands. On all sides of Ireland, except the southern coast, the ice flowed outward, but on the northeast the flow was diverted southward, and on the extreme north, westward, by the pressure of the overflowing ice-sheet of Scotland which here encountered it. In like manner, the ice-marks on the east coast of Ireland and the west coast of Wales are diverted southward by the mutual pressure of their ice-sheets, which, together with that of the west of Scotland, filled up St. George's Channel. That such was the case is further proved by the fact that the Isle of Man is ice-ground in a general direction from north to south, and to the summit of its loftiest mountains, which rise to a height of over two thousand feet. This could only have been done by an ice-sheet flowing over it, and this view is further supported by some most remarkable facts in the dispersal of local erratics. These are always found to the south of the places where they occur in situ, never to the north; and, what is still more noteworthy, they are often found far above the native rock. Thus, bowlders of the peculiar Foxdale granite are found about fourteen hundred feet higher than the highest point where there is an outcrop of this rock.
The Scotch ice-sheet flowed outward on all sides, but on the east it was met by the southward extension of the great Scandinavian ice-sheet. On the extreme north the meeting of these two ice-sheets resulted in a flow to the northwest which glaciated the Orkney Islands, while the Shetlands, much farther north, received the full impact of the Scandinavian ice alone, and are therefore glaciated from the northeast. The dividing line of the Scotch and Scandinavian ice-sheets was in the North Sea, not far from the east coast of Scotland; but farther south, at Flamborough Head and Holderness, the latter impinged on our coast, bringing with it enormous quantities of Scandinavian rocks. Many years ago Prof. Sedgwick described the cliffs of bowlder clay at Holderness as containing "an incredible number of smooth round blocks of granite, gneiss, greenstone, mica slate, etc., resembling none of the rocks of England, but resembling specimens derived from various parts of the great Scandinavian chain." These are mixed, however, with a number of British rocks from the north and west, indicating the meeting ground of the two conflicting ice-sheets. Similar blocks occur all along the coast as far as the cliffs of Cromer in Norfolk. Across the peninsula of Flamborough about two miles west of the lighthouse there is a moraine ridge containing a few Scandinavian bowlders, but mainly composed of British rocks. These latter consist of numerous carboniferous rocks from the north and northwest, together with many of Shap granite—a peculiar rock found only on Shap Fell in the eastern side of the Lake District, together with a few of Galloway granite. These facts, it will be seen, add further confirmation to the theory of great confluent ice-sheets indicated by the ice-markings upon the various groups of mountains, while it is hopelessly impossible to explain them on any theory of local glaciers, even with the aid of submergence and of floating ice.
The study of our British erratics has been assiduously pursued for many years past by a committee of the British Association; and by means of a map showing the chief facts collected up to this date, kindly furnished me by Mr. Percy F. Kendal, secretary of the committee, I am able to give a brief sketch of the more important of the phenomena, and their bearing on the extent and motion of the British ice-sheet. The general reader may be informed that great numbers of rocks are so local and so characteristic, often being confined to a very limited district or to a single mountain, that the origin of a considerable portion of the erratics can be ascertained with the greatest certainty.
Taking first the Shap granite, which has already been mentioned as occurring at Flamborough Head, we find that it has been carried northward as far as the Solway Frith, and eastward to the Eden Valley in great quantity and over a wide area. Thence can be traced a line of bowlders of this rock over the high plateau of Stainmoor into the valley of the Tees, and onward round the coast by Scarborough to Holderness, while a branch descends southward along the valley of the Ouse to York. Coming back to its source on Shap Fell, a train of bowlders of the same rock has been traced southward in a curving line, passing the east side of Morecambe Bay near Lancaster, and thence sparingly southeastward to near Whalley. Along the same line are found bowlders of peculiar granites from Eskdale and Buttermere, marking the line of junction of the northern ice-sheet with that which filled up the Irish Sea and pressed inward between the glaciers of Cumberland and North Wales. This is indicated by the fact that south of this line are scattered immense quantities of erratics, both from the southwest of Scotland and the Lake District, spreading over the whole of the low country as far as Bridgnorth and Wolverhampton, and eastward to the Derbyshire highlands. These same erratics are found round the north coasts of Wales and part of Anglesea, showing how the ice-flows divided on either side of the mountain mass of North Wales.
The center of the great glacier sheet of North Wales appears to have been over the Arenig Mountains, whence erratics of a peculiar volcanic rock have been traced to the north and east, mingling with the last-described group; while a distinct train of these Welsh erratics stretches southeastward to the country west of Birmingham.
In the Isle of Man are found many erratics from Galloway and a few from the Lake District. But the most remarkable are those of a very peculiar rock found only on Ailsa Craig, a small island in the Frith of Clyde, and a single bowlder of a peculiar pitchstone found only in the Isle of Arran. The Ailsa Craig rock has also been found at Moel Tryfaen, on the west side of Snowdon, and more recently at Killiney, County Dublin, on the seashore.[2]
The case of the bowlders in the Isle of Man, which have been carried nearly eight hundred feet above their source, has already been mentioned, but there are many other examples of this phenomenon in our islands; and as they are of great importance in regard to the general theory of glacial motion a few of them may be noted here. So early as 1818 Mr. Weaver described a granite block on the top of Cronebane, a slate hill in Ireland, and several hundred feet higher than any place where similar granite was to be found in situ; and he also noticed several deposits of limestone gravel in places from three to four hundred feet higher than the beds of limestone rock which are from two to ten miles off. Débris of red sandstone is also found much higher than the parent rock. Bowlders of Shap granite, Mr. Kendal tells us, have passed over Stainmoor by tens of thousands, and in doing so have been carried about two hundred feet above their source; and the curious Permian rock, "Brockram," has been carried in the same direction no less than a thousand feet higher than its highest point of origin.[3] In Scandinavia there are still more striking examples, erratic blocks having been found at an elevation of forty-five hundred feet which could not possibly have come from any place higher than eighteen hundred feet.[4] We thus find clear and absolute demonstration of glacier ice moving up-hill and dragging with it rocks from lower levels to elevations varying from two hundred to twenty-seven hundred feet above their origin. In Switzerland we have proof of the same general fact in the terminal moraine of the northern branch of the Rhone glacier being about two hundred feet higher than the Lake of Geneva, with very much higher intervening ground. As it is universally admitted that the glacier of the Rhone did extend to beyond Soleure, all the a priori objections to the various cases of rocks carried much higher than their origin, in America, the British Isles, and Scandinavia, fall to the ground. We must either deny the existence of the ice-sheet in the great Swiss valley, and find some other means of accounting for the traveled blocks on the Jura between Geneva and Soleure, or admit that the lower strata of a great glacier can travel up-hill and over hill and valley, and that the ice-sheets of the British Isles, of Scandinavia, and of North America merely exhibit the very same characteristics as those of Switzerland, but sometimes on a larger scale. We may not be yet able to explain fully how it thus moves, or what slope of the upper surface is required in order that the bottom of the ice may move up a given ascent, but the fact of such motion can not any longer be denied.
The facts thus established render it more easy for us to accept one of the latest conclusions of British glacialists. A great submergence of a large portion of the British Isles during the Glacial period, or in the interval between successive phases of the Glacial period, has long been accepted by geologists, and maps have been often published showing the small group of islands to which our country was then reduced, the supposed subsidence being about fourteen hundred feet. The evidence for this is the occurrence, at a few spots, of glacial gravels containing marine shells in tolerable abundance, the most celebrated being at Moel Tryfaen, on the west side of Snowdon, at a height of more than thirteen hundred feet. Shell-bearing drifts have also been found near Macclesfield at a height of over eleven hundred feet, and to the east of Manchester at between five and six hundred feet elevation. Others have since been found on Gloppa, a hill near Oswestry. The fact that the shell-bearing gravels of Moel Tryfaen are nearly forty feet thick shows that, if they are due to submergence, the land must have remained stationary at that level for a considerable period of time, and there would probably be other stationary periods at lower levels. Yet nowhere in the valleys or on the hill slopes of Wales, or the Lake District, or in the English lowlands are there any of the old beaches or sea cliffs, ormarine deposits of any kind, that must have been formed during such a subsidence and which can hardly have been everywhere cleared away by subsequent glaciation. Another difficulty is that the shells of these drifts are such as could not have lived together on one spot, some being northern species, others southern, some frequenting sandy others muddy bottoms, some which live only below tidal water while others are shore species. And, lastly, they are very fragmentary, only a small percentage of entire shells being found. In consequence of these various difficulties it was suggested by the late Mr. Belt that the great Irish Sea ice-sheet had carried up a portion of the sea-bottom imbedded in its substance, perhaps containing deposits of shells of various periods and thus explaining the intermixture of species as well as their fragmentary condition. The fact that bowlders and pebbles from Scotland, Ailsa Craig, and Cumberland have been found in the Moel Tryfaen beds almost amounts to a proof that they were so uplifted; and a recent search has shown that in the other localities where marine shells have been found in drift at great elevations similar foreign rocks occur, rendering it almost certain that the same ice-sheets which have distributed foreign erratics so widely over our country, and which in doing so must have passed over the sea-bottom, have in a few cases carried with them a portion of that sea-bottom, and deposited it with the erratics in the places where both are now found. A full discussion of this point, with replies to various objections, by Mr. P. F. Kendal, will be found in the volume already quoted; and he has recently adduced a fresh argument against "the great submergence" in the fact that, if it ever occurred, our lowlands must for a long time have formed the bottom of a sea two hundred fathoms deep, yet not a single shell characteristic of that depth has yet been discovered in the drift.[5] The cumulative