Page:Popular Science Monthly Volume 9.djvu/203

Rh bodies of ice not only flow like a heavy lava-stream, conforming themselves to all inequalities of the surface, but they also scrape along in solid mass, as if pushed by some irresistible force from behind. Mountain-glaciers show both motions. But the great polar glacier, extending over comparatively level surfaces, seems to have been pushed bodily outward from its fixed polar base, and to have moved almost entirely under the mighty impulse of expansion. The parallel scratches and furrows which, in our hemisphere, mount straight up the north sides of mountains; the worn and rounded appearance of those sides and of the summits, as compared with the rough, unsmoothed southern slopes; the erratic blocks, or some peculiar specimens like the native copper of Lake Superior, carried almost directly south for scores or hundreds of miles, over heights, and even over arms of the sea—all show conclusively that the great glacier pushed its meridional course over all obstacles and to long distances.

Imbedding in its under surface the grit and gravel on which it froze, this mountain grindstone grated and ground the solid rocks over which it passed into the various materials of soil. Sand and gravel were the products from granitic rocks and sandstones, clay from the slates and shales, and loam from the softer lime-rocks. But the most striking effects which the polar glacier produced were the long ridges of gravel and bowlder-clay hills which it scraped up as it advanced, and left at the end of its journey, or at each halting-place of its retreat. For it must be borne in mind that the glacier was still pushing southward all the time that it was, on the whole, retreating. These terminal moraines are either the promiscuous gatherings of clay and bowlders and earths of all kinds, or, if they have been subjected to the sorting influence of moving waters, they are gravel hills with sandy bases, and clay flats extending usually to the southward of them. They run in somewhat parallel courses easterly and westerly, sometimes hundreds of miles. Great numbers of these concentric ridges may be counted in Western New York, between the long Lake Ontario ridge and the lake hills of the south part of the State. Several cross the New England States, one running along the coast of Maine, and westerly through the White Mountains. In addition to these are the lateral moraines, running in an opposite direction. These were, some of them, pushed out at the sides by outstretching arms of the glacier; others were formed by streams running down through breaks or fiords in the melting ice-sheet. So extensive and so marked are the traces of the great polar glacier over all middle latitudes, both north and south, that it may truly be called the great landscape-gardener of the temperate zones.

But it is natural to conclude that, if there has been one glacial era caused by astronomical cycles, there must also have been others in earlier geological times. And, as we turn back the pages of the great earth-book, we find therein recorded the evidences of the vicissitudes