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ditions 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?

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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

Berlin.

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 arranged in parallel banks, mounds, and ridges, the longer axes of which coincide with the trend of glaciation. Over wide plains, on the other hand, it rises and falls in long, gentle swellings. This varying configuration is undoubtedly original—it is not the result of subsequent subaerial erosion. In mountain-valleys the ice-flow, subject to no deflection, must have proceeded continuously in one direction, and its groundmoraine, we may suppose, would thus tend to accrete more or less regularly. In the broader lowland tracts, however, as in the lower reaches of Nithsdale, Teviotdale, and Tweeddale, the same uniformity of conditions did not exist. Each of these broad depressions was occupied by mers de glace, formed by the confluence of ice-flows streaming out from various ice-sheds. Under such conditions the movement of the united currents could not be so equable, and in consequence of variations in the pressure of the ice, and in the lines of most rapid motion, the ground-moraine would tend to heap up in banks or ridges, the longer axes of which would necessarily coincide with the direction of ice-flow.1

1 The ''drumlins" and "drums " of Ireland and Scotland appear to be represented in Sweden by certain banks of boulder-clay, which are described by De Geer as a novel kind of radical moraines. He recognises their strong resemblance to the drumlins of New England (Geo/. Fbrcn. Fork., 1895, p. 212). Drumlins occur in the Island of Rttgen, but they would seem to be rare in North Germany. Recently Dr. K. Keilhack has observed them in Neumark^tf/i'*., d. konigl. preuss. geol. Landesanslalt fiir 1893, 1895, p. 190). They have been recognised also in the low grounds of Switzerland by Dr. Frtth CJahresbericht d. St. Gallischen Naturwissensch. Ges., 1894-95). It is probable, however, that the lenticular mounds and banks of till known under the name of drumlins have not all been formed in the same way. Thus the short lenticular drumlins of South Galloway appear to owe their origin to glacial erosion. They are the relics of the sheet of boulder-clay which accumulated under the last general mer de glnce that overwhelmed Scotland. At a later stage the Southern Uplands supported local ice-sheets and large glaciers which, flowing out upon the adjacent low grounds, ploughed into and greatly denuded the old boulder-clay. The drumlins of this region are, in short, simply rochts moutonne■es, composed sometimes entirely of boulder-clay, at other times partly of boulder-clay and partly of solid rock.

Once more, over the peripheral areas of the inland ice, as in the great plains of Germany, the influence exerted by the confluence of ice-flows just referred to would no longer be felt, at least to the same extent. When the ice had fairly escaped from uplands and hilly ground all minor movements would merge in one continuous broad outflow, the ground-moraine, as a result, being spread out more or less uniformly.

Looked at broadly, Northern Europe displays a central region of glacial erosion and a peripheral area of glacial accumulation. In the former, as in the Scandinavian peninsula, Finland, and the more elevated portions of the British Islands, bare rock is conspicuous over wide districts, while glacial accumulations, confined for the most part to hollows and depressions, attain as a rule no great thickness. Outside of such areas of special erosion, on the other hand, as in the low grounds of England and the plains of Northern Europe, naked rock appears only at intervals, while morainic materials and fluvio-glacial deposits reach their greatest development.

Under the ice-sheet rock-grinding and rock-shattering were carried on side by side. No doubt the boulder-clays frequently rest upon a smoothed and

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