Glacial erosion entrains a boulder of the bedrock. ... As the glacier retreats, the boulder is deposited on a different type of bedrock, forming a glacial erratic. Erratics can range from large boulders to smaller stones and pebbles. All erratics are of a different rock type.

How were glacial erratics formed?

Erratics are formed by glacial ice erosion resulting from the movement of ice. Glaciers erode by multiple processes: abrasion/scouring, plucking, ice thrusting and glacially-induced spalling. Glaciers crack pieces of bedrock off in the process of plucking, producing the larger erratics.

How are erratics created?

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.