Judy Amick, age 13, of  for her question;

How do we know that America was once covered with glaciers?

Imagine a sheet of ice two miles thick. This weighty monster would certainly leave its footprints on the ground. It would press the earth below it down and out of shape. Boulders and hard rocks would be crushed underneath. Such a glacier would end in a sheer wall of ice like a high Cliff. There would be tell‑tale marks on the ground to show where this massive mountain of ice came to an end.

Many Ice Age glaciers were this big. We can trace where they crushed with heavy footprints and clawed hard rocks with their icy fingers. Like all glaciers, they were constantly creeping forward. On the way, they rubbed jagged peaks into rounded hills. They tore off huge boulders and carried them southward. They moved soil from the hilltops down into the valleys.

Still more changes were made when the glaciers melted. As they formed, they gathered tons of debris. Stonos, pebbles, silt and great boulders wore frozen in the solid ice. Much of this material had been toted southward as the glaciers advanced from the north. When the ice melted, they were dumped in new homes. Experts can recognize the kinds of rock and the kinds of mound left by this glacial drift. They can also trace the scruffs and scratches made  by the icy feet and fingers of the terrible glaciers.