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. [Illustration: _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. [Illustration: _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. There are now two places on the earth where vast glaciers, or ice sheets, cover immense areas of land, one in the Antarctic, a region very little known, the other in Greenland, where there is an ice sheet covering land having an area more than ten times that of the State of New York. Let us study this region to see what is being done there, in order to compare it with what has been done in New York. [Illustration: _Fig. 29. The edge of a part of the great Greenland ice sheet (on the left) resting on the land, over which are strewn many boulders brought by the ice and left there when it melted._] In the interior is a vast plateau of ice, in places over 10,000 feet high, a great icy desert (Fig. 28), where absolutely no life of any kind, either animal or plant, can exist, and where it never rains, but where the storms bring snow even in the middle of summer. Such must have been the condition in northeastern America during the glacial period. [Illustration: _Fig. 30. A scratched pebble taken from the ice of the Greenland glacier._]