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 Wikioedia
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