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Abstract

Surficial sediment analyses, bottom photographs and current-meter data, coupled with studies of sidescan and 3.5 kHz echodata clearly show the imprint of a strong, seasonally-affected, slope current on the Hebrides shelf and slope. The present day shelf sediments are in fact relict, coarse-grained material (gravel with boulders) of Pleistocene glacial derivation, which have been modified during the Holocene by winnowing, sea-floor polishing and transport of the finer fraction across the shelf to the northwest. On the outer shelf and upper slope the sharp change from gravel to sand is marked by a physiographic-textural boundary, herein termed the sandline, which occurs at a depth of between 170 m and 300 m. Further down the continental margin on the lower slope, the lower limit of the sand-rich facies is marked by the mudline. This typically occurs at a depth of around 1200 m, and marks the depth of substantially increased clay-sized material. Above the mudline the long-term (Holocene to present), time-integrated signature of bottom current flow has resulted in the relatively slow accumulation of a mid-slope sandy contourite deposit, the Barra contourite sand sheet. This covers an area of 1000–1500 km2 with an estimated sand volume of 30 000 m3. Below the mudline the clay-rich deposits represent a hemipelagic drape with only minor bottom current influence and intense reworking by benthic organisms.

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