At times in the geologic past, up to a third of the earth's surface was covered by thick sheets of glacial ice. The last glacial ice sheet in North America had melted by about 10,000 years ago. The movement of glaciers across the continents profoundly changed the landscape through extensive erosion, transportation, and deposition of rock and sediment. Glaciers sculpted some of the spectacular erosion features in the high mountain ranges and also eroded large areas down to flat bedrock surfaces. The deposition of glacial sediments formed some of the extensive, gently rolling landscapes we see today, such as in southern Wisconsin.
Glaciers are thick, sprawling masses of ice that form on land during cooler climatic periods. Glaciers begin to grow when more snow accumulates than is lost through melting during the year. The older snow compacts, recrystallizes, and turns to ice from the increasing weight of the new snow above it. When a glacier reaches a sufficient size and mass, the force of gravity begins to move it downslope.
Glaciation is the movement of an ice sheet over a land surface. Two types of glaciation are recognized: continental and alpine. Continental glaciation affects a broad section of a continental land mass, such as Antarctica. Alpine glaciation is usually restricted to deep valleys in high mountainous terrain.
The idea that, compared to what we see today, a much larger portion of the earth was covered by glaciers in the geologic past was first proposed in Europe in the early 1800s by Swiss scientist Louis Agassiz. Known as the theory of glacial ages, it also meant that colder climates had existed for thousands of years. A series of glacial episodes, separated by periods of warmer climate, have occurred in the last few million years. Glaciers during this “Ice Age” stripped the soil from the northern part of North America (Canada) and deposited it farther south, creating the rich farmland in the midwestern United States.