Glacial Geology in Pennsylvania
"Have you heard the story of the Ice Age, a time when large sheets of moving ice (glaciers) blanketed the northern half of North America? Unbelievable though it may seem, half of our continent was once buried beneath thousands of feet of ice."
The northwestern and northeastern corners of Pennsylvania, separated by the Salamanca Reentrant in New York, have been glaciated in the geologically recent past. Northwestern Pennsylvania was glaciated by the Erie lobe and Grand River sublobe coming out of the north (see sketch on the right), while northeastern Pennsylvania felt the effects of the Ontario lobe glacier coming from the northeast. The style of glaciation was quite different in the two corners, as are the resulting deposits.
Glaciation in northwestern Pennsylvania was similar to that in the Midwest, although the extent of glaciation on the Allegheny Plateau was less extensive and the parallel moraines closer together than in the Midwest. However, similar to the Midwest, there are multiple, extensive till sheets preserved, suggesting that depositional processes dominated glaciation in northwestern Pennsylvania. Sections frequently expose multiple layers of deposits preserved from several glacial advances- a vertical stratigraphy. Many far-traveled clasts of igneous and metamorphic rock from the Canadian Shield, foreign to the bedrock of the area, are found in the tills and glaciofluvial deposits. The glacier crossed limestone and dolostone (carbonate) bedrock in the Erie basin, as well as on the Plateau, resulting in the deposition of sediments containing a relatively large proportion of carbonate minerals. Multiple parallel end moraines created by numerous minor retreats and readvances extend across northwestern Pennsylvania.
In contrast, glaciation in northeastern Pennsylvania was more similar to glaciation in New England. Presumably, due to the more rugged topography in northeastern than in northwestern Pennsylvania, erosion was a more dominant process. Each glaciation effectively eroded existing glacial deposits from earlier glaciations. As a result, a vertical stratigraphy is rarely, if ever found. The erosive nature of the glaciers also resulted in deposits with a very high percentage of locally-derived clasts and a paucity of far-traveled clasts from the Canadian Shield. The lack of carbonate bedrock crossed by the glacier also resulted in the deposition of sediments with a very low carbonate content. A single end moraine at the margin of the late Wisconsinan deposits is discontinuous, except for where it extends across the Pocono Plateau.
To learn more about glaciers in Pennsylvania, see our Education Series 6 publication, Pennsylvania and the Ice Age. For a page-sized map of how far glaciers advanced into Pennsylvania, see Map 59, Glacial Deposits of Pennsylvania. For detailed information about glaciation in northwestern Pennsylvania, see our General Geology Report 32.
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. By integrating their expertise and resources, the geological surveys of the coalition states are addressing these issues more effectively than could any one agency.
Geologic mapping of surficial glacial deposits is ongoing in Pennsylvania. For more information, contact Gary Fleeger at 717–702–2045.