The BGS Lexicon of Named Rock Units — Result Details

Allt An T-Strathain Till Member

Computer Code: STTI Preferred Map Code: MST
Status Code: Full
Age range: Devensian Stage (QD) — Devensian Stage (QD)
Lithological Description: Pale brown to mid grey, poorly sorted, clay-rich, matrix-supported diamicton with boulders. The till shows good evidence of subglacial deposition; namely edge-rounded and bullet-shaped striated clasts set within a stiff, highly consolidated muddy matrix. In places the diamict exhibits a strong subhorizontal fissility, is overconsolidated, and was most likely deposited by subglacial lodgement processes. Clasts are predominantly Moine Supergroup rocks, with minor amounts of Eriboll sandstone (quartzite). Sporadic clasts of Torridon Group sandstone, Lewisian gneiss and various local igneous rocks are also present. The till may also contain reworked glaciofluvial and/or morainic deposits of Devensian age or older.
Definition of Lower Boundary: Unconformable on bedrock, predominantly Moine Supergroup psammite.
Definition of Upper Boundary: Overlain locally by Holocene (Flandrian) deposits, typically peat or (glacio)fluvial gravel.
Thickness: Variable; 0.5-8m (maximum observed thickness), probably >10m locally.
Geographical Limits: Relatively widespread across Moine Supergroup rocks in Assynt and northwest Sutherland. Also occurs on Torridon Group sandstone bedrock around Ullapool. May occur locally on other lithologies (eg. Eriboll Sandstone, Durness Limestone, Lewisian Gneiss) in structurally complex terrain.
Parent Unit: Assynt Till Formation (ASTI)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  River cutting, 100m from A835, in south bank of Allt an t-Strathain, 2.5km northwest of Ullapool, Wester Ross. 
Reference(s):
none recorded or not applicable
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable