Glacial erratics are pieces of rocks that are carried by glacial ice and are erratic because they do not match the bedrock in the area where they are found. They are transported southward from as far away as Michigan and Canada by Pleistocene (Ice Age) glaciers and were left in our area when the glaciers melted.

Because the glacial erratics that we find in southwestern Ohio come from such a large area, they consist of a wide variety of rock types, including igneous, sedimentary, and metamorphic. These erratics can range in size from pebbles to massive pieces that can be found miles away from their original location. Although they match the bedrock of the area from which they came, they look quite different than our local bedrock. A common use of glacial erratics is for landscaping purposes.

Find more information at these sites:
Glacial Erratics at Wikipedia.org
Glacial Erratics at About.com