A glacier is a large mass of ice which is thick enough to deform and flow under its own weight. During the Pleistocene Epoch, conmmonly known as the "Ice Age", great continental-scale masses of ice spread southward into Pennsylvania from Canada. The ice never made it as far south as Pittsburgh, instead ending at Morraine State Park. However, the indirect effects of the glaciers had a profound effect on the landscape of southwestern Pennsylvania.

Evidence of the most recent glacial event (about 20,000 years ago) at Morraine State Park include boulders of igneous rocks which were carried by the glacier from farther north and deposited when the glacier melted.

The ice sheet blocked rivers and streams, causing changes in the patterns of river flow. What is now the Allegheny River used to flow north into the Lake Erie basin. The ice dammed the river, creating Lake Monongahela. The lake eventually became deep enough to overtop higher areas to the south. The lake began to drain to the south, and erosion gradually reversed the slope of the river, so that it continued to flow south after the ice melted.

When the ice sheet melted, it not only dropped all the debris it carried but also put a lot of extra water into area rivers. This large volume of water helped erode and sculpt Pittsburgh's hills and valleys.

Pittsburgh's hilly topography is an indirect product of glaciation, formed when meltwater-swollen rivers cut down into the relatively flat Appalachian Plateau. Note the relatively flat surface formed by the high points in the landscape, a remnant of the old Plateau surface. (Photos taken from site 2 on map.)


Glacier Bay Alaska