Drumlin

A drumlin, from the Irish word droimnín ("littlest ridge"), first recorded in 1833, and in the classical sense 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 occur in various shapes and sizes, including symmetric (about the long axis), spindle, parabolic, and transversely asymmetric forms; their long axis is parallel to the direction of ice movement at the time of formation.

Drumlins are typically 1 to 2 km (0.62–1.24 mi) long, less than 50 m (160 ft) high and between 300 to 600 metres (980–1,970 ft) wide. Drumlins generally have a length:width ratio of between 1:2 and 1:3.5, with more elongate forms corresponding to faster ice motion.

Drumlins and drumlin clusters are glacial landforms composed primarily of glacial till. They form near the margin of glacial systems, and within zones of fast flow deep within ice sheets, and are commonly found with other major glacially-formed features (including tunnel valleys, eskers, scours, and exposed bedrock erosion).

Drumlins are often in drumlin fields of similarly shaped, sized and oriented hills. Many drumlin fields are observed to occur in a fan-like distribution. The Múlajökull drumlins of Hofsjökull are also arrayed in a splayed fan distribution around an arc of 180°.

Drumlins usually comprise layers constituting till, sand and gravel in various proportions – indicating that material was repeatedly added to a core, which may be of rock or glacial till.The dilatancy of glacial till was invoked as a major factor in drumlin formation.

Models of drumlin formation fall into two camps:constructional, in which they form as sediment is manipulated into shape, for example via subglacial deformation; and Remnant/erosional, which proposes that drumlins form by erosion of material from an unconsolidated bed. Catastrophic sub-glacial flooding has also been proposed as a theory of drumlin formation, but is now discredited. This theory includes deposition of glaciofluvial sediment in cavities scoured into the glacier bed by subglacial meltwater, and remnant ridges left behind by erosion of soft sediment or hard rock by subglacial meltwater.

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