We have said that glaciers generally carry large fragments of rock, which they deposit in confused heaps at their lower extremities. It sometimes happens, however, that a glacier descends into a lake, or into the sea, before it melts, and large masses of it, or icebergs, are floated off with their freight of rock fragments. These loaded icebergs are sometimes carried great distances before they entirely dissolve, and in this manner large unworn angular blocks of rock may be dropped on the bed of the sea hundreds of miles from their original site.
In many parts of Great Britain the geologist finds heaps of gravel and sand containing large fragments of rock which exactly resemble the terminal heaps or moraines of modern glaciers. He also finds huge blocks of rock or boulders resting upon the bare surface of rocks of quite a different character. One of the largest of the boulders is situated at the head of the Devil's Glen, in the county of Wicklow, its dimensions being twenty-seven feet long, by eighteen wide, and fifteen high. It consists of granite, and rests upon a bed of slate six or eight miles from the granite district, a wide shallow valley intervening. Another large boulder of granite has recently been discovered in the chalk near Croydon, and geologists have come to the conclusion that this mass of rock must have wandered hither from the North of Europe.
These curious heaps and boulders prove that
