Glaciers can pick up chunks of rocks and transport them over long distances. When they drop these rocks, they are often far from their origin—the outcrop or bedrock from which they were plucked. These rocks are known as glacial erratics.
Erratics record the story of a glacier's travels. Looking for the bedrock units that correspond with erratics can reveal complex flow patterns.
An excellent example of how erratics can tell the story of past glaciations comes from Yosemite National Park: a field of erratics there contain rocks that were plucked from an outcrop east of Tioga Pass, over the current watershed divide. These erratics reveal that the glaciers there actually flowed uphill, over Tioga Pass.