Definition: a channel cut by glacial meltwater under, along or in front of an ice margin.
Meltwater may flow under hydrostatic pressure within the glacier and so locally move up-gradient - these channels will show up-down long profiles. More widely, meltwater flows down-gradient under gravity. Meltwater channels are often recognised from their anomalous topographic positions - crossing watersheds and running parallel with former ice margins along valley side. Meltwater channels are often very large relative to the streams that now occupy the valleys - the streams are regarded as too small to have cut the valley and are termed misfit streams.
Rock-cut subglacial drainage channel excavated along a fracture line, Lumbister, Yell
Systems of meltwater channels are developed locally on Shetland but in many areas channels are not an obvious feature in the landscape. It is possible that small rock-cut channels are masked by covers of peat and that systematic mapping would reveal the patterns of meltwater flow across the islands. Alternatively, the development of meltwater channels may have been restricted on the current land area due to its position close to the axis of the former Shetland ice cap.
Channel systems have been identified in association with moraines at the margins of former ice fronts on Papa Stour, Sandness and Burn of Mail. Flinn (1994) has identified a zone on north Yell and northwest Unst where systems of meltwater channels relate to an ice margin, with possible glacial lakes. Flinn suggest that the former margin may date from the Last Glacial Maximum but it remains undated. Ross (1996) argues that the channel systems were formed during a stillstand in the general retreat of the last ice sheet.