Drumlins are elongated hills of glacial deposits. They can be 1 km long and 500 m wide, often occurring in groups. A group of drumlins is called a drumlin swarm or a basket of eggs, eg Vale of Eden.[UK]."

Source : BBC Glacial Depostion

They are formed from loose debris (clay, sill, boulders etc) that would have been dragged along by a previously existing glacier or ice sheet. They are clearly directional, with their axes showing the direction of flow of the glacier.

The means of formation, is however, unclear. There are three main theories, which have been discussed since the 1830s : the Constuctional Theory, the Sheer Stress Theory and the Sub-Glacial Flood Theory.

Details of the theories can be found at Wikipedia

Also see : Mima moundsplugin-autotooltip__plain plugin-autotooltip_bigMima mounds

Mima Mounds are low, dome-like, roughly symmetrical natural mounds on terrestrial terrain - mainly composed of loose, unstratified, often gravelly sediment. They have a diameter of 3 to 50 m, and a height of 0.2 to 2 m.

They are found mainly in the northwestern United States - though similar structures have also been found on every continent except Antarctica. They sometimes occur in groups of thousands.