A drumlin, from the Irish word droimnín ("little ridge"), first recorded in 1833, is an elongated hill in the shape of an inverted spoon or half-buried egg formed by glacial ice acting on underlying unconsolidated till or ground moraine.
Drumlins and drumlin clusters are glacial landforms which have been extensively studied. Geologists have proposed several theoriesabout their origin. They are formed a short distance within the receding glacier ice and record the final direction of ice movement.Drumlins occur in symmetric, spindle, parabolic, and transverse asymmetrical forms. Drumlins are commonly found with other major glacially-formed features and are related on a regional scale to these landforms. The large-scale patterns of these features exhibit spatial organization of the drumlin-forming flows with related tunnel valleys, eskers, scours, and exposed bedrock erosion (scallopingand sichelwannen).
Although one formation theory originally proposed since the 1980s by Bohan-Henry and collaborators suggests that drumlin creation by a catastrophic flooding release of highly pressurized water flowing underneath the glacial ice, the recent retreat of a marginal outlet glacier of Hofsjökull in Iceland provided the opportunity for direct study of a drumlin field with formation of >50 drumlins ranging from 90–320 m (300–1,050 ft) in length, 30–105 m (98–344 ft) in width, and 5–10 m (16–33 ft) in height. This, when combined with drumlin formation identified through imaging beneath the West Antarctica ice, resulted in a significant step in geomorphologic understanding. The Hofsjökull marginal drumlins formed through a progression of subglacial depositional and erosional processes with each horizontal till bed within the drumlin created by an individual surge of the glacier. Erosion under the glacier in the immediate vicinity of the drumlin can be on the order of a meter's depth of sediment per year, with the eroded sediment forming a drumlin as it is repositioned and deposited.