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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〔Menzies(1979) quoted in Benn, D.I. & Evans, D.J.A. 2003 ''Glaciers & Glaciation'' , Arnold, London (p431) ISBN 0-340-58431-9〕 formed by glacial ice acting on underlying unconsolidated till or ground moraine. ==Drumlin formation== Drumlins and drumlin clusters are glacial landforms, composed primarily of glacial till, which have been extensively studied. Geologists have proposed several theories about their origin. They can form both near the margin of glacial systems and within zones of fast flow deep within the ice sheet. They record the direction of ice movement at the time of formation. Drumlins occur in symmetric, spindle, parabolic, and transverse asymmetrical forms. However it is increasingly being recognised that there is no true 'drumlin' shape, and that rather subglacial bedforms can take on a wide range of shapes and sizes.〔(【引用サイトリンク】title=The planar shape of drumlins )〕 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 (scalloping and sichelwannen). One formation theory originally proposed in the 1980s by John Shaw and collaborators suggested that drumlin creation occurs by a catastrophic flooding release of highly pressurized water flowing underneath the glacial ice. However this is perhaps the most widely disputed of the proposed mechanisms of formation. The more widely accepted proposed methods can be split into two camps:〔(【引用サイトリンク】title=Automated drumlin shape and volume estimation using high resolution LiDAR imagery (Curvature Based Relief Separation): A test from the Wadena Drumlin Field, Minnesota )〕 # Constructional; the landforms are the result of sediment being manipulated into shape, for example via subglacial deformation. # Remnant/erosional; the landforms are part of a landscape that has been formed by erosive processes removing material from an unconsolidated bed. The recent retreat of a marginal outlet glacier of Hofsjökull in Iceland〔A satellite image of the region of Hofsjökull where drumlin growth has been observed (see ). The drumlins can be observed between pools of water.〕 provided the opportunity for direct study of a drumlin field with formation of more than 50 drumlins ranging from in length, in width, and 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「drumlin」の詳細全文を読む スポンサード リンク
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