The BGS Lexicon of Named Rock Units — Result Details

Glen Achall Psammite and Semipelite Member

Computer Code: GACH Preferred Map Code: notEntered
Status Code: Full
Age range: Neoproterozoic Era (AZ) — Neoproterozoic Era (AZ)
Lithological Description: The Glen Achall Psammite and Semipelite Member comprises psammites, with abundant subsidiary semipelite beds that vary in thickness from a few centimetres to tens of metres. The psammites are grey to pink weathering and vary from tabular to cross-bedded, with bedfordms from 10cm to 1m in thickness. Semipelite beds up to a metre thick are common, but the key characteristic of this member is the presence of several garnet-bearing semipeilite units that attain several tens of metres in thickness. The semipelites are typically fine- to medium-grained and strongly foliated, consisting of quartz plus feldspar plus biotite plus muscovite plus garnet. Some of these semipelite units can be traced for several kilometres across moderately exposed ground around Glen Achall, which represents the type area for the Member, however, some of the semipelite units pinch out laterally.
Definition of Lower Boundary: The lower boundary of the Member is defined in Glen Achall as the lowest appearance of a thick (>2m) unit of garnet-bearing semipelite above the Moine Thrust.
Definition of Upper Boundary: The upper boundary of the Member is defined in Glen Achall as the uppermost limit of thick (>2m) garnet-bearing semipelite units. A discontinuous quartzite bed forms the very top of the Member in some outcrops. In this area, the top of the Member also represents the top of the A'Mhoine Psammite Formation.
Thickness: About 200m
Geographical Limits: The Member has so far only been identified in the hills around Glen Achall, east of Ullapool, between the Rappach Water in the north and Glen Douchary in the south. However, continued mapping in the area is likely to result in it being traced for greater distances.
Parent Unit: Altnaharra Psammite Formation (ALPS)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Type Area  Glen Achall, from East Rhidorroch Lodge to Loch an Daimh, including the hills of Meall nan Uan and Cnoc Damh to the north, and Meall na Moch-Eirigh and Meall Dubh to the south. Peach et al., 1907 
Peach, B N, Home, J, Gunn, W, Clough, C T and Hinxman, L W, 1907. The Geological Structure of the northwest Highlands of Scotland. Memoir of the Geological Survey of Great Britain. 
Krabbendam, M, Strachan, R A, Leslie, A G, Goodenough, K M and Bonsor, H C. 2011. The internal structure of the Moine Nappe Complex and the stratigraphy of the Morar Group in the Fannichs-Beinn Dearg area, NW Highlands. Scottish Journal of Geology, 47, 1-20. 
1:50K maps on which the lithostratigraphical unit is found, and map code used: