During the Ice Age, when glaciers were all the fashion, they flowed down, and then, as we have seen, melted back a certain distance; then they flowed down again. Sometimes in later visits they flowed further than before, and in so doing, you see, picked up some of the very hills they had previously laid down and set them along somewhere else. Sometimes we find different rows of hills, one right alongside the other. This shows where the glacier melted away toward the mountains, paused, then melted again and so on, each time leaving a group of hills and not coming back there and disturbing them any more.
Such hills as we have been speaking of may be steep or gentle, and from a few feet to more than 1,000 feet high, although they are seldom as high as 1,000 feet.
And there are other kinds of hills made by the glaciers. One of the most curious of these remind you of the serpent mounds left by the mound builders in Ohio. These hills are the deposits left by the streams, the veins inside the glacier's great body. The soil in them is also apt to be in layers like the deposits of other rivers. These hills wind along like serpents, because they reproduce the bends in the streams inside the glacier. Such hills are called "eskers." They are seldom more than a few rods wide and 10 feet or so in height. They run for 10, 20, 40, 50, and sometimes 100 miles.
Around Boston, and all along Cape Cod and in parts of New York and Wisconsin, you will see other hills called "drumlins"; and you will see plenty of them, too. It is estimated that there are 6,000 in western New York and 5,000 in southern Wisconsin, and they are all around Boston. Bunker Hill is a drumlin. You wouldn't have to tell an Irish boy what "drumlin" means, as they have these hills in Ireland, too, and from Ireland came the name. The word means "little hill."
But while Mr. Glacier made the drumlins of the stuff he brought with him, he enjoyed himself (at least let us hope so) tobogganing on hills he found ready made. These hills are real mountains; usually the granite heart of the mountain, because only a very strong rock could stand having one of these playful giants riding over him and live to tell the tale. Such glacier "slides" are referred to as "domes" or "round tops" or "bald mountains."
Mr. Agassiz, the great scientist who spent so many years studying the motion of glaciers, could tell from the height of one of these bald and rounded hills how high the glacier was that rode over it. For instance, the glaciers rode over what is known as Blue Mountain in Pennsylvania, which is 1,500 feet high. "Then," Mr. Agassiz would have said, "the glaciers that did that must have been at least 2,000 feet thick; for a glacier can only flow over a rocky mass when it is half as tall again as the rock."
You see it is the mass of it, the pressure of its own weight, that boosts the glacier up the slide. It seems almost like lifting oneself by one's boot-straps, doesn't it?
III. The Ants and the Volcanoes
Beside all the hills we have mentioned there are several others, well worth looking into; ant-hills, for example, not only because ants are so interesting in themselves but because the ants helped to answer what for a long time was one of the puzzles of science, "How are volcanoes made?"
When your mother's mother went to school—or it may have been back in your mother's mother's mother's time—a little girl, on being asked in the geography class, "What is a volcano?" was expected to say something like this: