Glacial Erratics are rocks that have been transported by glaciers and left behind in a new place when the glacier retreats. Almost always the erratics show a different geology than the place they are left.
Some are moved a very short distance, but some have been moved many many miles.
Eastern Washington is one place glacial erratics can be found. The results of erratics can be found on the rocks in New York’s Central Park. Many of the boulders there have scatches on their tops – grooves left by other boulders that were bounced along above them in the ice tha covered the northeast 12,000 years ago.
This erratic is 20 ft high gneiss, left behind by Steller Glacier, in Alaska. The ground is covered by sediment also deposited by the glacier.
USGS photo