The BGS Lexicon of Named Rock Units — Result Details

Ystog Formation

Computer Code: YSTOG Preferred Map Code: notEntered
Status Code: Full
Age range: Devensian Stage (QD) — Holocene Epoch (QH)
Lithological Description: Grey, laminated and massive, silty clay and silt. Gravelly shoreline deposits and sandy deltaic deposits are present locally (Cave and Hains, 2001). Likely to contain dropstones and interbedded thin diamicts at depth.
Definition of Lower Boundary: Unconformable on bedrock and Plynlimon Formation. Laterally it might interdigitate with Head deposits.
Definition of Upper Boundary: Generally the ground surface or unconformable contact with Flandrian alluvial deposits of the Severn and Avon Catchments Subgroup.
Thickness: Likely to be in excess of 10m. A railway cutting at [SO 1696 9105] exposes 7m of sand and fine gravel.
Geographical Limits: Upper Camlad Valley and Caebitra Valley, upper Mule Valley and lower Camlad Valley, Powys/Shropshire.
Parent Unit: Severn and Avon Catchments Subgroup (SACA)
Previous Name(s): Lacustrine Deposits (LDE)
Glaciolacustrine Deposits, Undifferentiated (-2546)
Glaciolacustrine Sand And Gravel (-3153)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  Boggy ground in the Caebitra and upper Camaad valleys, in the vicinity of Church Stoke. Cave and Hains, 2001. The deposits are of Lake Camlad-Caebitra, one of a series of interconnected lakes that developed initially in front of Welsh ice during the Late Devensian and persisted against moraines into the Flandrian. Pollen analyses have failed to provide a date, persistence into the Flandrian is based on geomorphological criteria. 
Reference Section  River bank near Trefeen Bridge exposing up to 7m of soft, grey, laminated clay deposited in the former Kerry Lake. Cave and Hains, 2001. 
Reference(s):
Bowen, D Q, 1999. Wales. Chapter 7 in Bowen, D Q (Editor), A revised correlation of Quaternary deposits in the British Isles. Geological Society of London Special Report, No.23. 
Cave, R and Hains, B A. 2001. Geology of the country around Montgomery. Memoir of the British Geological Survey, Sheet 165 with part of 151 (Welshpool) (England and Wales). 
British Geology Survey. 1994. Montgomery. England and Wales. Sheet 165, Solid and Drift Geology. 1:50 000 (Nottingham: British Geological Survey, Keyworth.) 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E165