Glacial period

“ And on the scarred and grooved rocks was the final evidence; for these deep scratches had surely been ground by the advancing glacier as it slowly and silently moved onward. “The idea that such phenomena were not restricted to regions where glaciers now are found, but that traces of glacial action could be seen over enormous tracts of the earth’s surface, perhaps including regions in the tropics, and that in countries now temperate there might be discovered, not only the remains of tropical fauna and flora, but also distinct indications of a period of arctic cold—this was as new as startling.” ”

“ When we remember that an arctic-alpine flora formerly flourished on the low grounds of central Europe, it seems extremely probable that the tundra epoch must fall within the glacial period. But the glacial period embraced a complex series of geographical and climatic changes, and it is necessary, therefore, to come somewhat closer to the question. Among the most conspicuous deposits of the Ice age are moraines of all kinds and fluvio-glacial gravels, while the löss, as we have seen, is the most prominent accumulation of the tundra and steppe epochs. ”

“ Let us now see what the fauna and flora, as far as we know it, tell us of the climate of the Glacial period. At the very outset of our inquiry we are confronted with one very serious difficulty in the problem, and that is the supposed occurrence of inter-glacial mild phases alternating with colder ones during the Ice Age. At first, when traces of a temperate flora and fauna were discovered intercalated between two layers of boulder clay, their presence was explained by the supposition of a mild inter-glacial period. ”

“ The glacial period is not only closely related to the topography of North America and parts of Europe in the changing of river beds, the formation of lakes, the transportation of rock, the grinding down of mountains and spreading the débris over thousands of miles in extent, but it is related in an intimate way to many of the sciences, such as botany and zoölogy. A study of the flight of animals and plants in front of the great advancing ice sheet is a subject of intense interest. ”

“ Evidence was led to show that the latest great development of glacier-ice in Europe took place after the southern pachyderms and palæolithic man had vacated England—that during this last stage of the Glacial period man lived contemporaneously with a northern and alpine fauna in such regions as southern France—and lastly, that palæolithic man and the southern mammalia never revisited north-western Europe after extreme glacial conditions had disappeared. ”

“ They are, therefore, of no small importance in reference to the chronology of the glacial period, since they afford evidence of an assemblage of fossil shells with a proportion of between eight and nine in a hundred of extinct species occurring so far south as latitude 53 degrees north, and indicating so cold a climate as to imply that the glacial period commenced before the close of the Pliocene era. ”

“ His special field of work, however, was the history of the glacial period in all its aspects, and as time went on he came to be recognised as the most thorough-going advocate of repeated glacial and interglacial epochs. ”

“ Could we have been here to observe during the glacial period, we should have overlooked a wrinkled ocean of ice as continuous as that now covering the landscapes of Greenland; filling every valley and cañon with only the tops of the fountain peaks rising darkly above the rock-encumbered ice-waves like islets in a stormy sea—those islets the only hints of the glorious landscapes now smiling in the sun. ”

“ Probably all these ends had been to some extent, and in some localities, attained in the earliest human period, when man was contemporary with many large animals now extinct. But a serious change was to occur in human prospects. There is the best geological evidence that in the northern hemisphere the mild climate of the earlier Post-glacial period relapsed into comparative coldness, though not so extreme as that of the preceding Glacial age. ”

“ In the first place, there is a larger part of America which has been subjected to glacial action than what we find in Europe. In Europe, only the northern half of Great Britain, the Scandinavian peninsulas, a part of Northern Germany, and the region of Switzerland were under the surface of the glaciers during the last glacial period. In America, practically all the country north of the Susquehanna, and more than half of the States north of the Ohio, had their soils influenced by this ice period. The effects of glaciation on the soils of the region where it has acted are important. ”

“ The woolly mammoth lived in Spain and Italy. In short, the climate and conditions of life were vastly different in the two stages.
We must now turn our attention to the proofs of glaciers in Europe, the phenomena from which this age derives its name. Descriptions of Alpine glaciers are common enough, but as glaciers and the Glacial Age have a great deal to do with the antiquity of man, we can not do better than to learn what we can of their formation, and their wonderful extension during this period. The school-boy knows that by pressure he gives his snowball nearly the hardness of ice. ”

“ Throughout earlier Tertiary and Secondary times an equally high excentricity often occurred, but it never produced a glacial epoch, because the north temperate and polar areas had less high land, and were more freely open to the influx of warm oceanic currents. But wherever great plateaux with lofty mountains occurred in the temperate zone a considerable local glaciation might be produced, which would be specially intense during periods of high excentricity; and it is to such causes we must impute the indications of ice-action in the vicinity of the Alps during the Tertiary period. ”

“ T. C. Chamberlin, W. Upham, G. F. Wright in North America, have all tended to confirm the view that it is to the movement of glaciers and ice-sheets that we must look as the predominant agent of transport and abrasion in this period. The three stages through which our knowledge of glacial work has advanced may thus be summarized: (1) the diluvial hypothesis, deposits formed by floods; (2) the drift hypothesis, deposits formed mainly by icebergs and floating ice; (3) the ice-sheet hypothesis, deposits formed directly or indirectly through the agency of flowing ice. ”

“ ANCIENT moraines, striations denoting the action of vanished glaciers, the lost rocks, clay-beds evidently of glacial origin—all these are evidences testifying that at some period not very remote, as we count geological periods, the whole northern hemisphere down to the southern limit of 40° was submerged and covered with vast glaciers and ice-floes. There is not a living scientist of any eminence who questions the truth of this assertion. ”

“ To this we may add the fact, that rude stone implements have been found in the ‘glacial drift’ in New Jersey, United States, and that some geologists regard this as proof that man lived on the{272} earth during that far-back, dreary, and cold glacial period. ”

“ The retreat of the first Alpine glaciers, caused partly by a lowering of that chain, may have been synchronous with the period of great submergence and floating ice in England. The second advance of the glaciers may have coincided in date with the re-elevation of the Alps, as well as of the Scotch and Welsh mountains; and lastly, the final retreat of the Swiss and Italian glaciers may have taken place when man and the extinct mammalia were colonising the north-west of Europe, and beginning to inhabit areas which had formed the bed of the glacial sea during the era of chief submergence. ”

“ But when the action of glaciers was studied and the recent occurrence of a glacial epoch demonstrated as a fact, many phenomena—such as moraines and other gravel deposits, boulder clay, erratic boulders, grooved and rounded rocks, and Alpine lake basins—were seen to be due to this altogether distinct cause. There was no breach of continuity, no sudden catastrophe; the cold period came on and passed away in the most gradual manner, and its effects often passed insensibly into those produced by denudation or upheaval ”

“ Nor are such interglacial beds confined to northern and north-western Europe. In the Alpine Lands of the central and southern regions of our Continent they are equally well developed. Impressed by the growing strength of the evidence, it is no wonder that geologists, after a season of doubt, should at last agree in the conclusion that the glacial conditions of the Pleistocene period were interrupted by at least one protracted interglacial epoch. ”

“ To cause extensive glaciation we must have, in addition to favorable eccentricity, a large precipitation. With these two factors combined we get an ice age, but not otherwise. The result has an important bearing on geologic glacial periods and their explanation.
Once formed, an ice-sheet cools everything about it and chills the climate of its hemisphere. ”

“ This great diversity of opinion is eloquent of the difficulties of the subject; it is impossible not to see that the discovery of interglacial epochs bears a close relationship to the origin of certain hypotheses of the cause of glaciation; while it is significant that those who have had to do the actual mapping of glacial deposits have usually greater difficulty in finding good evidence of such definite ameliorations of climate, than those who have founded their views upon the examination of numerous but isolated areas. ”

“ If the cold of the glacial epoch came on slowly, if it was long before it reached its greatest intensity, and again if it abated gradually, we may expect to find the earliest and latest formed drift less barren of organic remains than that deposited during the coldest period. ”

“ For several hundred years all the great glaciers have receded, but we should not expect such recession to continue indefinitely. Geology furnishes abundant evidence that great changes took place in the climate of the earth during the prehistoric ages; that there were several glacial periods, the last occurring during pleistocene times, somewhere between twenty and fifty thousand years ago, and that there were intervals between the culminations of the Ice Ages of probably fifty thousand to one hundred thousand years. ”

“ It can be demonstrated, however, that the glaciation of the Alps and of northern Europe were contemporaneous, and the facts go far to prove that the glaciers of the Rocky Mountains and the inland-ice of north-east America likewise co-existed. At all events all the old glacial accumulations of our hemisphere are of Pleistocene age, and it is for the advocates of the hypothesis under review to prove that they are not — 279 — contemporaneous. ”

“ Benefits of glaciation. Like the driftless area, the preglacial surface over which the ice advanced seems to have been well dissected after the late Tertiary uplifts, and to have been carved in many places to steep valley slopes and rugged hills. The retreating ice sheets, which left smooth plains and gently rolling country over the wide belt where glacial deposition exceeded glacial erosion, have made travel and transportation easier than they otherwise would have been. ”

“ So long as ocean-currents from the tropics found ready entrance to polar regions across vast tracts of what is now dry land, extreme and widespread glacial conditions were impossible. Any lowering of temperature due to cosmical causes might indeed induce new snow-fields and glaciers to appear, or existing ones to extend themselves in northern regions and the most elevated lands of lower latitudes; but such local glaciation need not have seriously affected any of the areas in which coal-seams were being formed. ”

“ Those, on the other hand, who restrict the action of land ice to local glaciers, and call in the aid of floating ice and the arctic current, maintain that the process of glaciation is limited rather by place than by time. Ever since the conditions of the earth have been such as to give rise to the formation of polar ice, the shores and the shallow seas to which the arctic current has borne it must have been subject to glacial action such as we have endeavored to describe. ”

“ The best authorities also suppose that the reign of snow and ice was broken by at least one return (possibly more) of genial climate, when animals and plants from the south again visited the countries of Northern Europe—only, however, to be once more driven forth by a return of arctic cold. But finally, before the increasing warmth of a genial Climate, the glaciers vanished, not to return again, and the Glacial Age became merged in that of the present.
It is no longer a question that man lived in Europe during the largest portion of this age, if not from the beginning. ”

“ Then, the sun still cooling, the amount of evaporation fell off, till it ceased to afford the excessive supply, and the glaciers became stationary, then retrograde till they were reduced to the relatively insignificant proportions in which they now appear. This theory explains the deposition of glacial moraines in the midst of tree-ferns and a Mediterranean vegetation; for, while the glaciers were extensive in consequence of the immense precipitation everywhere but immediately upon them, a warmer climate prevailed than is now enjoyed in the same regions. ”

“ In both North America and Europe the high mountains, well south of the great glacier limits, especially the Sierras, Rockies, Alps, Pyrenees, and Caucasus, supported many large local glaciers in valleys which now contain none at all or only relatively small ones.
Records of glaciation, such as glacial scratches, bowlders, lakes, etc., occur high up in the White and Green Mountains, Adirondacks, Catskills, and the Berkshire Hills, thus proving that the ice must have been at least some thousands of feet thick over New England and New York. ”