The BGS Lexicon of Named Rock Units — Result Details

Assynt Glacigenic Formation

Computer Code: ASGL Preferred Map Code: notEntered
Status Code: Full
Age range: Devensian Stage (QD) — Devensian Stage (QD)
Lithological Description: A mixed assemblage of morainic and proglacial deposits. The morainic deposits occur both onshore and offshore and principally comprise heterogeneous clast-rich diamictons and interbedded gravels. The diamictons are commonly weak- to well-consolidated, grey to red-brown, up to boulder grade, clast-to-matrix supported, with a silty-sandy matrix composition, and mainly massive. They contain a diverse suite of clast lithologies, including Lewisian gneiss, Torridon Group sandstone, Eriboll Formation sandstone (quartzite), Moine Supergroup psammite, Durness Group limestone, Canisp porphy, Borralan syenite and other minor igneous rocks. The till is typically autochthonous, being largely derived from the underlying bedrock, however it may also contain reworked glaciofluvial and/or morainic deposits of Devensian age or older. Gently dipping stratified gravel units and debris-flow related deformation structures are commonly observed. Offshore, contemporary water-lain deposits include massive, grey and grey-brown colour-laminated clay and silty clay, containing sporadic gravel clast and in situ shelly material.
Definition of Lower Boundary: Sharp, unconformable contact on bedrock or Loch Broom Till Formation.
Definition of Upper Boundary: Onshore it is overlain locally by Holocene peat or fluvial deposits; offshore, it commonly occurs at or near to sea bed (below a Holocene lag), though it is overlain by the Annat Bay and Summer Isles formations in the deeper-water basins.
Thickness: Locally up to 100 m thick offshore; mostly <10 m onshore.
Geographical Limits: Discontinuous extent across Wester Ross and western Sutherland, west of the main Highland watershed. Extends seaward into the coastal waters of the eastern Minch.
Parent Unit: Northwest Highlands Glacigenic Subgroup (NWHG)
Alternative Name(s): none recorded or not applicable
Type Section  Morainic facies: roadside cutting 500 m NW of Achduart. 
Reference Section  57.9681 deg N, 05.3579 deg W. Glacimarine silty clay facies, BGS vibrocore 57-06/262, 3.80 m-thick section (0.26-4.10 m below sea bed), NW extension of Martin Bank. 
Stoker, M S and Bradwell, T. In press. Quaternary geology of the Summer Isles region - a brief explanation of the marine environment map. Sheet Explanation of the British Geological Survey. 1:50 000 Special Sheet Summer Isles (Scotland). 
Stoker, M.S., Bradwell, T., Howe, J.A., Wilkinson, I.P. & McIntyre, K. 2009. Lateglacial ice-cap dynamics in NW Scotland: evidence from the fjords of the Summer Isles region. Quaternary Science Reviews, 28, 3161-3184. 
Bradwell, T. & Stoker, M S. 2010. Quaternary Lithostratigraphy of the NW Highlands. In: Lukas, S. & Bradwell, T. (eds), The Quaternary of Western Sutherland and adjacent areas. (London: Quaternary Research Association), 29-37. 
Bradwell, T. 2003. The Quaternary deposits and glacial history of the area around Inchnadamph, Sutherland. BGS Internal Report IR/03/120. 
1:50K maps on which the lithostratigraphical unit is found, and map code used: