All over northeastern North America and northwestern Europe the soil is of the same nature as that just described. In our own country this kind of soil reaches down as far as the edge of the shaded area in the map ([Fig. 27]), and it will be noticed that all of New York is within that area excepting the extreme southwestern part near the southern end of Chautauqua lake.
Not only is the soil peculiar within this district, but there are many small hills of clay or sand, or sometimes of both together ([Figs. 33] and [34]). They rise in hummocky form and often have deep pits or kettle-shaped basins between, sometimes, when the soil is clayey enough to hold water, containing tiny pools. These hills extend in somewhat irregular ranges stretching across the country from the east toward the west. The position of some of these ranges is indicated on the map ([Fig. 27]).
For a long time people wondered how this soil with its foreign pebbles and boulders, altogether called "drift," came to be placed where it is; they were especially puzzled to tell how the large boulders, called erratics ([Fig. 21]), should have been carried from one place to another. It was suggested that they came from the bursting of planets, from comets, from the explosion of mountains, from floods, and in other ways equally unlikely; but Louis Agassiz, studying the glaciers of the Alps and the country round about, was impressed by the resemblance between the "drift" and the materials carried by living glaciers.
Agassiz, therefore, proposed the hypothesis that glaciers had carried the drift and left it where we now find it; but for many years his glacial hypothesis met with a great deal of opposition because it seemed impossible that the climate could have changed so greatly as to cover what is now a temperate land with a great sheet of ice. Indeed, even now, although all who have especially studied the subject are convinced, many people have not accepted Agassiz's explanation, just as years ago, long after it was proved that the earth rotated each day, many people still believed that it was the sun, not the earth, that was moving.
Fig. 27. Map showing the extent of the ice sheet in the United States. Position of some of the moraines indicated by the heavily shaded lines. (After Chamberlain.)
The glacial explanation is as certain as that the earth rotates. For some reason, which we do not know, the climate changed and allowed ice to cover temperate lands, as before that time the climate had changed so as to allow plants like those now growing as far south as Virginia to live in Greenland, now ice covered. When the ice of the glacier melted away it left many signs of its presence; and when the temperate latitude plants grew in Greenland they left seeds, leaves and tree trunks which have been imbedded in the rocks as fossils. One may now pick the leaves of temperate climate trees from the rocks beneath a great icecap.
Fig. 28. A view over the great ice plateau of Greenland, with a mountain peak projecting above it.
To one who studies them, the signs left by the glacier are as clear proof as the leaves and seeds. From these signs we know that the climate has changed slowly, but we have not yet learned why it changed.