MR. BOULDER ON HIS PERCH
This is what is called a "perched boulder." Being a harder kind of rock than that on which it was left by the glaciers, it has held out against the winds and weather, while the stone under it has been worn away.
In eastern Wisconsin, along with these stones, have been found pieces of copper, although there are no copper deposits near by. To the northeast of where the fragments of copper were found are the great copper deposits of what is now Michigan, and from this region the glaciers brought the copper and scattered it about as they moved south and southwest. So these mysterious stones and other things kept pointing toward the north, in a kind of dumb show.
In mountain rain storms you can see the torrents driving great stones before them, so one of the first theories about the stranded boulders was that, at some time in the earth's history, there had been great floods covering whole continents, sweeping away rocks from the mountains and carrying them here, there, and everywhere. That theory also accounted for the rounded shape of the boulders, for if you have a volume of water big enough and swift enough you can roll boulders wherever you like.
WHAT A QUEER HOBBY-HORSE!
But why should the boulder trains all lead to the north? And how could water carry boulders right across a deep mountain valley and pile them high up on the mountains on the other side? How could water perch one boulder on another or on a flat ledge of rock or on the summits of the cliffs? Boulders so perched are very common, and often they are so nicely balanced that a man can set them rocking; and sometimes a small boy can do it. Every young man who goes to Dartmouth College knows about the rocking stone some half mile east of the college. In the town of Barre is a big boulder with a small boulder on its back, and the small boulder can be set rocking like a child's hobby-horse.
HOW THE MOUNTAIN TORRENTS HELP SHAPE THE BOULDERS]
The only thing that could handle boulders in this way, so it turned out, were the glaciers. By following up the boulders to their homes in the mountains they found on the backs of the glaciers of to-day stones just like those in our fields, and they found them thickly scattered over the ground where the glaciers melted back during the summer months. The glaciers not only pick up boulders from the mountain torrent beds, as they move along, but themselves pluck rocks from mountain sides. Huge blocks of rock, dislodged when water freezes in the cracks of the mountain walls, also fall upon the glacier. It was the boulders held underneath the ice that left their autographs, deep grooves on the native bed-rock in the regions into which the glaciers of the Ice Age came.