THE POLAR GLACIERS. 183 



bodies of ice not only flow like a heavy lava-stream, conforming them- 

 selves to all inequalities of the surface, hut 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 a il 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. Sev- 

 eral 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 



