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I

He was a bold man who first conceived the idea of the great continental ice-sheet which in Pleistocene times covered most of the northern part of the continent, and played such a part in shaping the land as we know it. That bold man was Agassiz, who, however, was not bold enough to accept the theory of evolution as propounded by Darwin. The idea of the great glacier did not conflict with Agassiz’s religious predilections, and the theory of evolution did. It was a bold generalization, this of the continental ice-sheet, one of the master-strokes of the scientific imagination. It was about the year 1840 that Agassiz, fresh from the glaciers of the Alps, went to Scotland looking for the tracks of the old glaciers, and he found them at once when he landed near Glasgow. We can all find them now on almost every walk we take to the fields and hills; but until our eyes are opened, how blind we are to them! We are like people who camp on the trail of an army and never suspect an army has passed, though the ruts made by their wagons and artillery and the ruins of their intrenchments are everywhere visible.

When I was a boy on the farm we never asked ourselves questions about the stones and rocks that encumbered the land–whence they came, or what the agency was that brought them. The farmers believed the land was created just as we saw it–stones, boulders, soil, gravel-pits, hills, mountains, and all–and doubtless wished in their hearts that the Creator had not been so particular about the rocks and stones, or had made an exception in favor of their own fields. Rocks and stones were good for fences and foundations, and for various other uses, but they were a great hindrance to the cultivation of the soil. I once heard a farmer boast that he had very strong land–it had to be strong to hold up such a crop of rocks and stones. When the Eastern farmer moved west into the prairie states, or south into the cotton-growing states, he probably never asked himself why the Creator had not cumbered the ground with rocks and stones in those sections, as he had in New York and New England. South of the line that runs irregularly through middle New Jersey, Pennsylvania, Ohio, Indiana, Illinois, Iowa, and so on to the Rockies, he will find few loose stones scattered over the soil, no detached boulders sitting upon the surface, no hills or mounds of gravel and sand, no clay banks packed full of rounded stones, little and big, no rocky floors under the soil which look as if they had been dressed down by a huge but dulled and nicked jack-plane. The reason is that the line I have indicated marks the limit of the old ice-sheet which more than a hundred thousand years ago covered all the northern part of the continent to a depth of from two to four thousand feet, and was the chief instrument in rounding off mountain-tops, scattering rock-fragments, little and big, over our landscapes, grinding down and breaking off the protruding rock strata, building up our banks of mingled clay and stone, changing the courses of streams and rivers, deepening and widening our valleys, transplanting boulders of one formation for hundreds of miles, and dropping them upon the surface of another formation. When it began to melt and retreat, it was the chief agent in building up our river terraces, and our long, low, rounded hills of sand and gravel and clay, called kames and drumlins. In many of our valleys its flowing waters left long, low ridges, gentle in outline, made up entirely of sand and gravel, or of clay. In other places it left moraines made up of earth, gravel, and rock-fragments that make a very rough streak through the farmer’s land. All those high, level terraces along the Hudson, such as that upon which West Point stands, were the work of the old ice-sheet that once filled the river valley. The melting ice was also the chief agent in building up the enormous clay-banks that are found along the shores of the Hudson. The clay formed in very still waters, the sand and gravel in more active waters.