Drumlins in upper New York State. These features tell us that ice sheets grew across this region in the past. There is much discussion between geologists about how drumlins form. They may form behind obstacles, just like fluted ground moraine, and may have a bedrock hill at their head. Or, big bursts of meltwater may scour away the landscape, leaving piles of material sculpted in drumlins.
Some drumlins are erosional features, carved by glacial processes out of the underlying basement.
Illustration of one way to make a drumlin. Meltwater in the glacier can form a network of tunnels that eventually reach the glacier front. Usually, the tunnels are sub-parallel to the direction of ice movement. At the glacier nose, the meltwater and its sediment load pours out and spreads into the proglacial zone as an esker fan. Tunnel at the edge of Athabasca Glacier in Alberta, Canada.