When glaciers flow down across the ground, they can break off rocks and pick them up in the ice. As the ice moves and eventually melts, those rocks are deposited. When the large rocks are exposed on the surface, they are termed glacial erratics. Much of Minnesota and the eastern Dakotas are covered under these glacial deposits, and these glacial erratics are relatively common.
Glacial deposits are also interesting because they will have grains or rocks of all sizes, from very fine silt and mud up through large boulders. This can make identifying glacial deposits in the field straightforward in some cases, because there will be many grain sizes all together. When grains settle out of the air or from water, the coarse ones deposit first, and the grains end up becoming finer as you go up the stratigraphic column.