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Page:Popular Science Monthly Volume 44.djvu/801

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THE ICE AGE AND ITS WORK.

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