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land, after conferring with geologists to whom the diluvial accumulations of the great plains are familiar, comes to the conclusion that the deposits probably attain an average thickness of 150 feet. The materials being partly of local and partly of foreign origin, he deducts the former (estimated at 50 feet), and thus obtains a thickness of 100 feet for the detritus derived from Sweden and Finland, and spread over the low grounds of North Germany, etc. According to this geologist, the glaciated areas of Sweden and Finland, which supplied the detritus, are some 800,000 square kilometres in extent (497,120 square miles), while the area in Russia and North Germany over which Swedish and Finnish erratic materials are spread is estimated at 2,040,000 square kilometres (1,267,656 square miles). Were those materials therefore transferred to the lands from which they have been derived, they would raise the general surface by 255 feet. This estimate, it need hardly be said, is a mere rough approximation, and is probably excessive. But even if it be supposed that Helland has exaggerated both the amount of foreign erratic materials and the extent of the area over which it is distributed, we shall still be compelled to admit that the surface of Scandinavia must have been greatly modified by glacial erosion. If we deduct two-thirds from Helland■s result we have still left sufficient material to raise the general surface of Finland and Sweden by 85 feet.
In the following chapter reference is made to the
loss as being primarily a flood-loam of glacial times. Much of that occurring in the river-valleys of Central Europe has, no doubt, been derived from the Alpine lands; but the vast accumulations of loss in Southern and South-eastern Russia doubtless owe their oriirin chietly to the flood-waters escaping from the margins of the old "inland ice." All these deposits, as we shall see, have been more or less rearranged and modified by subaerial action, but the materials themselves would seem to have resulted, in largest measure at least, from the washing and weathering of glacial accumulations. In short, they are additional evidence of the effective erosive action of flowing ice.
The researches of geologists in North America art■ on all fours with those carried on in Europe. They tell precisely the same tale. The American boulderclays, fluvio-glacial gravels, and loss present us with similar phenomena. As in Europe so in North America, broken and ruptured rocks are of common occurrence under the overlying ground-moraines. The ice-sheet, as Dana remarks, "carried ddbris for the most part, not from the slopes and summits of emerged ridges, but from those underneath it. . . . It obtained its load by abrading, ploughing, crushing, and tearing from those underlying slopes and summits. . . . The ice-mass was a coarse tool; but through the facility with which it broke and adapted itself to uneven surfaces, it was well fitted for all kinds of shoving, tearing, and abrading work. Moreover it was a tool urged on by enormous pressure.
A. thickness of 1000 feet corresponds to at least 50,000 pounds to the square foot. The ice that was forced into the openings and crevices in the rocks had thereby enormous power in breaking down ledges, prizing off boulders, and in abrading and corroding."
3. Alodifications of the surface produced by glacial action. Having now learned that glacier-ice is a most effective eroding agent, we have next to consider the modifications of the land-surface brought about by glacial action. Looked at broadly, as we have seen, each glaciated region shows a central area of erosion and a peripheral area of accumulation. Not that erosion and accumulation are confined in this way each to a separate tract, but simply that in the central area erosion is in excess of accumulation, while in the surrounding region the reverse is the case. It will conduce to clearness, therefore, if we consider first the characteristic features which are the direct result of glacial erosion. Thereafter we shall glance at the aspect presented by a land more or less covered with glacial and fluvio-glacial detritus.
Unquestionably the most notable features7 of a well glaciated country is its rounded and flowing configuration, a configuration which is always most striking when viewed in the direction of glaciation. Tors, peaks, buttresses, and ridges have been smoothed down, escarpments bevelled off, and asperities in general softened. This is the direct result of glacial abrasion, but accumulation also has helped in the
production of a flowing contour, for many of the 16
dimples and smooth depressions upon hill-tops and hill-slopes are more or less due to glacial depositior.. While projecting rock-masses have been abraded ariu removed, irregular hollows, gullies, ravines, and other rough depressions have often been partially or completely obliterated by the deposition in them of morainic materials, abrasion and accumulation together having thus resulted in the production of a more or less undulating surface. In the phenomena of "crag and tail" we see another effect of the same twofold action. Isolated stacks and bastions of rock, which faced the direction of ice-flow, have been rounded and bevelled-off, and frequently a hollow dug out in front, while morainic dt'bris has been heaped up behind to form the so-called " tail" of the hill. There are endless modifications of this structure. Thus in many hilly tracts which have been completely overwhelmed by an ice-flow we may often trace series of parallel ridges and intervening hollows of various width, height, and depth, which obviously extend in the direction of former glaciation. These are the result partly of erosion and partly of accumulation. The hollows show where the rock has most readily yielded to glacial erosion, while the ridges consist of irregular-shaped masses and ledges of more durable rocks, and of morainic material which has gathered in their rear. Into these and other details of glacial action, however, it is not necessary to go. For our purpose it is enough to recognise the general fact that glaciation tends to obscure and obliterate the
features which result from the action of the ordinary agents of erosion and denudation. Hence all wellglaciated areas show a somewhat monotonous outline —round-backed rocks, smoothed and undulating hillslopes and hill-tops,—in a word, undulating contours are everywhere conspicuous.
The effect produced by glacial action is perhaps most strikingly displayed in regions the more elevated portions of which have risen above the surface of the ice, and so escaped abrasion. In the great valleys of the Alps, for example, how strongly contrasted are the glaciated and non-glaciated areas! In the Upper Engadine the valley slopes up to a height of 2000 feet or thereabout are conspicuously abraded, while above that level all is harsh and rugged. It is the same in our own islands, as, for example, in the Outer Hebrides, where the whole area is smoothed and rounded up to a height of 1500 or 1600 feet, above which level the rocks present quite a different aspect.
But glacier-ice does not only abrade and bevel-off prominent rock-ledges, peaks, tors, bastions, and buttresses, it also excavates hollows, which may vary in extent from a few feet or yards in depth and width to great depressions measuring many fathoms deep and not a few miles in extent. Here, however, we come upon the vexed question of the origin of rock-basins, the consideration of which may be conveniently deferred for the present.
The transfer of detritus from the area of dominant glacial erosion, and its distribution over the peripheral