What kind of rock is deposited by a glacier?

Large rocks deposited by melting glaciers are called "glacial erratics," after the Latin word "errare" or "to wander." Erratics can be carried hundreds or thousands of miles by a glacier before they are deposited.

Because it is not difficult to determine the origin of most rocks, glacial erratics are very useful for determining the prehistoric flow of glaciers. Another type of erratic, a drop rock, is deposited on the ocean bed by a floating glacier; these erratics are helpful for determining prehistoric ocean conditions such as water temperature and currents. Smaller rocks, gravel and clay sediments dropped by glaciers are also useful in determining facts such as the farthest extent and the height of prehistoric glaciation.