Drumlins are long, streamline shaped mounds which parallel the direction of previous ice flow. Drumlins are steep in the direction from which the ice advanced and flatten in the direction of flow. Although the shape varies from rounded hills to long narrow shapes, they are usually tear-drop in plan and long section, but trihedral in cross section. Many drumlins vary in length from 1000 to 2000 m, in width from 400 to 600 m, and in height from 15 to 30 m. Drumlins rarely occur singly, but rather in groups which create distinct fields.
The majority of drumlins are composed of till similar to the till of the surrounding area; this till is normally packed hard so that it is extremely compacted. Masses of sorted and stratified sediment often occur as pockets or lenses.
Some evidence suggests that drumlins form as a result of direct deposition of till, and that the glacier tends to mold these features into shape. Other evidence indicates that drumlins are previously deposited till (possibly a terminal or recessional moraine of a previous glacial advance) which is simply reshaped by the new glaciation.