Drumlins are long, low hills made of glacial till which were deposited beneath glaciers during the last ice age (before about 20 000 years ago). they are elongated features which may be approx 1km long, 500m wide and 50m high. Despite the ingrained belief that they are asymmetric in profile with a steeper stoss (upflow) and gentle, tapered lee (downflow) slope, recent research shows they are actually largely symmetrical in profile.
They are made up of glacial till, or ground moraine, which is derived from ground-up clay and boulders eroded by abrasion (scraping), plucking (block removal) and freeze-thaw weathering. The till is carried by the glacier as it flows and slides downslope, lubricated by meltwater.
The reasons for drumlin formation are not fully understood. They sometimes form where glaciers slow down or become overloaded with sediment, e.g. where it leaves the mountains and enters a lowland region. The continued movement of the glacier over the till shapes it into streamlined drumlins.
Drumlins are often found in clusters, all oriented in the same direction. Dense drumlin fields are sometimes called 'basket of eggs' landscapes because of their lumpy form. Good examples are found in Eden Valley in the Lake District.