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o supervene at present, each individual glacier would )egin to advance, and as it progressed the zone of nost active rock-shattering by frost would descend ivith it to lower and lower levels. But at each step in its advance the glacier would encounter no greater accumulations of rock-rubbish than had all along gathered in its neighbourhood. In short, as Dr. Bohm remarks, weathering would proceed no more rapidly in front of one of the enormous glaciers of the Ice Age than it does now in the vicinity of existing glaciers. "When the Inn Glacier," he says, "had advanced as far as Innsbruck, it would enter no zone of more active rock-shattering than is met with to-day in front of the glaciers of the Oetzthal." It is obvious, therefore, that if the glaciers of the Ice Age derived their subglacial detritus either from above or from frost-riven ddbris and superficial deposits lying in their path, their ground-moraines could not at any one place have attained a greater thickness than those of existing Alpine glaciers; and yet, as is well known, the old ground-moraines reach an astonishing thickness, their bulk being in direct proportion to the size of the former ice-flows.
One may readily exaggerate the importance of the rock-rubbish which is almost everywhere conspicuous in the Alps. The enormous screes of angular blocks and dibris which shoot down from cliff and buttress contain prodigious quantities of materials. Here, we are apt to think, is sufficient loose material wherewith to form ground-moraines as thick and extensive as those of the Glacial Period. But is this actually the case? If all the ddbris in question could be lifted and equally distributed over the Alpine lands it would certainly not suffice to raise the general surface of those lands by more than a few feet or yards. The old morainic accumulations, on the other hand, could they be replaced, would add considerably to the average height of the surface. Professor Penck has shown, for example, that the morainic accumulations of the Isar Glacier average a thickness of 20 metres, and cover an area of some 1800 square kilometres. They have been derived from an area 2800 square kilometres in extent. Could they be restored, therefore, they would raise the general surface by about 13 metres. In other words, an area of 1081 square miles has been lowered by some 41 feet. In Dr. Penck■s estimate only the morainic matter has been considered, the equally great mass of fluvio-glacial gravels (consisting almost exclusively of remodified infraglacial detritus) has been entirely neglected. Further, we must remember that during the formation of the moraines and fluvioglacial gravel, enormous quantities of the fine flour of rocks—the result of glacial grinding—must have been carried away in suspension, and deposited in regions far beyond the glaciated areas.
Such considerations as these show that the old morainic accumulations cannot consist merely of the superficial rock-rubbish which the old glaciers found ready to hand, and swept out as they advanced. All such loose accumulations, after excessive glacial conditions had supervened, must erelong have become exhausted, and can form only a small proportion of the ancient ground-moraines. Whence, then, was the great bulk of the material derived? Surely from infraglacial sources, as the direct result of glacial erosion. The immense ice-flows of the Glacial Period must at an early stage have completed the removal of preglacial detritus—none of that detritus can now linger underneath any existing glacier, either in the Alps or in Norway. Yet, as we have seen, groundmoraines are forming at present in both regions. In the Alps, according to Professor Heim and others, the ground-moraines are fed from the surface, but this can be true to only a very limited extent. The plateau ice-sheets of Norway carry no superficial detritus, and their ground-moraines are, therefore, supposed by some to represent the rock-rubbish which gathered over the Scandinavian heights in preglacial times! A vast ice-sheet, as we know, overflowed those regions during the Glacial Period, and buried the low grounds to great depths under the detritus which it carried outwards from the mountains, and yet we are to believe that much loose rock-rubbish of preglacial age still remains to be removed from the continuously ice-covered plateaux of Norway! Must we likewise believe that the "inland ice" of Greenland, which has probably persisted since Pliocene times, has not yet succeeded in removing the products of subaerial weathering, which came into existence before glacial conditions had supervened in Arctic regions? CHAPTER XI
LAND-FORMS MODIFIED BY GLACIAL ACTION (Continued}
FORMER GLACIAL CONDITIONS OF NORTHERN EUROPE—EXTENT OF THE OLD INLAND ICE—GENERAL CHARACTER OF BOULDERCLAY—CENTRAL REGION OF GLACIAL EROSION AND PERIPHERAL AREA OF GLACIAL ACCUMULATION — FLUVIO-GLACIAL DEPOSITS—LOESS, ORIGIN OF ITS MATERIALS—GLACIATION OF NORTH AMERICA—MODIFICATIONS OF SURFACE PRODUCED BY GLACIAL ACTION.
IF a study of the glacial and fluvio-glacial deposits of the Alpine lands leaves us in no doubt as to the efficiency of glacial erosion, an investigation of the similar accumulations of Northern Europe and North America is even more convincing. The boulder-clays of those wide regions are true groundmoraines, recalling in every particular the groundmoraines of the Alpine lands. At the climax of the Glacial Period a great ice-sheet covered all Northern and North-western Europe, extending east from the British area to the Timan mountains, and south to the German ranges. The ice-sheet thus occupied an area of 2,500,000 square miles or thereabout in extent. Above the surface of this inland ice peered some of the loftier mountain-tops of Scandinavia, and a few
JSTunatakkr in the British Islands. In the low grounds
of Scotland the sheet could hardly have averaged less
than 2500 to 3000 feet in thickness. In some of the
Norwegian fiords it exceeded 5500 feet. Taking the
elevation of the ice-shed in Scandinavia as 7000 feet,
and the height reached by the ice-front upon the
northern slopes of the mountains of Germany as 1350
feet, we get a thickness for the ice-sheet in South
Sweden of 2900 feet, of 2500 feet in Denmark, and
of 1306 feet or thereabout in the neighbourhood of
It is obviously impossible that the ground-moraines of an ice-sheet of such dimensions could have been derived or even supplemented to any extent from superficial sources. The boulder-clays are the direct products of glacial erosion. They consist essentially of unweathered material. Boulders, smaller stones, grit, sand, and the finer-grained rock-meal or flour are all alike fresh; they have not been altered chemically as they would have been had they come from superficial sources. They could not have been derived from above, and they cannot represent the weathered rock-ddbrt■s of preglacial times.
The external configuration assumed by boulderclay seems likewise to point to the infraglacial origin of the deposit. In relatively narrow mountain-valleys it forms broad terraces or platforms—now trenched and furrowed by streams and rivers. In broad lowland tracts, as in Tweeddale and Nithsdale, it is ar