A glacial erratic is a piece of rock that differs from the size and type of rock native to the area in which it rests. "Erratics" take their name from the Latin word errare (to wander), and are carried by glacial ice, often over distances of hundreds of kilometres. They can be transported by ice rafting.
Subsequently, one may also ask, how are erratics deposited?
Glacial erratics. Definition: An erratic is a boulder transported and deposited by a glacier having a lithology different than the bedrock upon which it is sitting. Erratics are useful indicators of patterns of former ice flow. Of rock or sand reposeth, there to sun itself."
How drumlins are formed?
Drumlins. DRUMLIN An oval-shaped hill, largely composed of glacial drift, formed beneath a glacier or ice sheet and aligned in the direction of ice flow. Their formation remains controversial (see below) but in spite of this they are extremely useful for reconstructing former ice sheets.