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Glacial Geology

About 30 percent of Pennsylvania was covered by glaciers during the Ice Age. It was a time when large sheets of moving ice blanketed the northern half of North America.

These glaciers helped to sculpt the hills and valleys of Pennsylvania and deposited materials such as boulders, sediment and other debris, including pieces of Canada now found in Pennsylvania.

Pennsylvania and the Ice Age

Geologists have pieced together thousands of items of information from all parts of the state that they use to tell the story of the Ice Age in Pennsylvania.

From studying glacial deposits, they know that the northwestern and northeastern corners of Pennsylvania were affected by several glacial advances -- the most recent being approximately 22,000 years ago.

The style of glaciation was quite different in the two corners of Pennsylvania, as are the associated glacial deposits.

In the northwestern corner, depositional processes dominated. The deposits included lots of carbonate minerals from the limestone and dolostone bedrock of the Erie basin, and they included far-traveled clasts of igneous and metamorphic rocks that were plucked from the Canadian Shield.

In the northeastern corner, most likely due to a more rugged land surface, erosional processes dominated. Deposits from older glaciers were eroded away by overriding younger glaciers.

Glacial sediments in this area contain very little in the way of far-traveled clasts; rather, the pieces of rock within the sediments are from local sandstones, siltstones, and shales.

Impacts of Glaciers in Pennsylvania

Here are some of the important impacts of glaciers in Pennsylvania:

  • The Great Lakes were created by glacial erosion, and are important as a natural boundary, a fishery, a transportation medium, and for recreation
  • Most of our natural lakes resulted from glaciation
  • Much of the good soil valued for cultivation is developed on glacial materials
  • Glacial deposits are used extensively as sources of sand and gravel
  • Large quantities of water come from wells in glacial deposits
  • Glacial deposits can be susceptible to slope failure and require consideration if there is development on them

Pennsylvania is a member of the Great Lakes Geologic Mapping Coalition. The states of the coalition have similar geologic conditions as a result of glaciation, and must address common societal issues about land and water resources, the environment, and geologic hazards.