19.7: What is a Glacial Period?

What Is a Glacial Period?

A glaciation Links to an external site. is the modification of the land surface by the action of glaciers. Typically, the dominant glacier will be an ice sheet, but valley glaciers will also advance during this time. At the peak of the Last Glacial Maximum (~21,000 years ago), the Laurentide Ice Sheet Links to an external site. covered most of northern North America and extended as far south as Nebraska and Illinois. During this time, major cities like Chicago, New York City, and Boston were buried beneath one to two miles of ice (Figure 19.12). The ice sheet essentially bulldozed the North American continent and left behind a distinct landscape and landforms. During this time, the valley glaciers in the Sierras and Rockies also grew and heavily altered their landscape.