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. Swarms of drumlins create a landscape which is often described as having a 'basket of eggs topography'. 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. Swarms of drumlins create a landscape which is often described as having a 'basket of eggs topography'. Drumlins occur in various shapes and sizes, including symmetrical (about the long axis), spindle, parabolic forms, together with transverse asymmetrical forms; their long axis is parallel to the direction of movement of the formative flow 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 the questionable assumption that more elongate forms correspond to faster ice motion. That is, since ice flows in laminar flow, the resistance to flow is frictional and depends on area of contact; elongate, subglacial landforms produced by ice would represent relatively slow flow rates. 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 Pleistocene 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 may comprise layers constituting clay, silt, sand, gravel and boulders in various proportions – perhaps, indicating that material was repeatedly added to a core, which may be of rock or glacial till. Alternatively, drumlins may be residual, with the landforms resulting from erosion of material between the landforms. The dilatancy of glacial till was invoked as a major factor in drumlin formation. In other cases, drumlin fields include drumlins made up entirely of hard bedrock (e.g. granite or well-lithified limestone). These drumlins cannot be explained by the addition of soft-sediment to a core. Thus, accretion and erosion of soft-sediment by processes of subglacial deformation do not present unifying theories for all drumlins—some are composed of residual bedrock. Conventional 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 (reference). A hypothesis that catastrophic sub-glacial floods form drumlins by deposition or erosion challenges conventional explanations for drumlins. It includes deposition of glaciofluvial sediment in cavities scoured into a glacier bed by subglacial meltwater and remnant ridges left behind by erosion of soft-sediment or hard-rock by turbulent meltwater. This hypothesis requires huge, subglacial meltwater floods, each of which would raise sea-level by tens of centimetres in a few weeks. Studies of erosional forms in bedrock at French River, Ontario, Canada provide evidence for such floods. The recent retreat of a marginal outlet glacier of Hofsjökull in Iceland exposed a drumlin field with more than 50 drumlins ranging from 90 to 320 m (300–1,050 ft) in length, 30 to 105 m (98–344 ft) in width, and 5 to 10 m (16–33 ft) in height. These 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. The above theory for the formation of these Icelandic drumlins gives the best explanations for one type of drumlin. However, it does not provide a unifying explanation of all drumlins. For example, drumlin fields including drumlins composed entirely of hard bedrock cannot be explained by deposition and erosion of unconsolidated beds. As well, hairpin scours around many drumlins are best explained by the erosive action of horseshoe vortices around obstacles in a turbulent boundary layer. Erosion under a glacier in the immediate vicinity of a 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.