A time, beginning about 2 million years ago, when it was common for glaciers to surge down from mountains and form ice sheets. At present, glaciers and ice sheets have shrunk back, but it is likely they will grow again.

Greenland - an area with extensive ice today.

During the last Ice Age, an ice sheet spread down from Canada to sweep across North America as far as the Great Lakes. Ice also spread out from the Rockies, Sierras and the Appalachians as ice caps. The ice flowed along the east coast as far south as New York.

Yosemite valley. Half Dome is in the middle of the picture and My Lyell is in the right distance.

The result of this burial was that there was no chance for the summits of the mountains to be frost shattered into peaks, and instead they are rounded. Those that stood above the ice are sharp peaked. Contrast, for example, Half Dome, Yosemite, which was under the ice with Mt Lyell, much higher, which is sharp peaked because it was above the ice cap.