The BGS Lexicon of Named Rock Units — Result Details

Lleyn Till Member

Computer Code: LLEYN Preferred Map Code: notEntered
Status Code: Full
Age range: Devensian Stage (QD) — Devensian Stage (QD)
Lithological Description: Stiff, generally ill-sorted, massive to crudely stratified, variably pebbly, sandy clay, characteristically containing exotic clasts and shell fragments. Subordinate clayey gravels and stratified sand and gravel are locally present. Colour and dominant clast composition variable and generally reflect the local bedrock geology; in Ceredigion and Pembrokeshire it is generally green-grey with abundant Lower Palaeozoic rock clasts.
Definition of Lower Boundary: Base not seen at type locality, but could potentially overlie pre-Late Devensian deposits (Young et al.,2002); locally the formation overlies bedrock and Devensian head deposits (Gibbons and McCarroll, 1993). In Ceredigion and Pembrokeshire the base is unconformable on bedrock, head and regolith (Hambrey et al., 2001); near Llanrhystud it is on the Plynlimon Till Formation (Campbell and Bowen, 1989; Davies et al., 1997) or Plynlimon Glacigenic Formation (McMillan et al., in prep. 2009). Along Liverpool and Conwy bays the base overlies bedrock and, locally, tills of Welsh origin (Warren et al., 1984).
Definition of Upper Boundary: Generally the ground surface or unconformable contact with West Wales or Cheshire-North Wales catchment subgroups. However, at the eastern end of the type section the upper boundary is defined by contact with till of the overlying Eryri Glacigenic Formation.
Thickness: Highly variable: locally more than 17m in the Cardigan district (Maddison et al., 1994); more than 60m in the Dee Valley (Davies et al., 2004); and more than 30m on the Lleyn Peninsula (Gibbons and McCarroll, 1993).
Geographical Limits: Coastal fringe of Ceredigion and Pembrokeshire approximately between Llanrhystud and St Brides Bay; Anglesey; north coast and western tip of Lleyn Peninsula as far east as Porth Neigwl; coastal fringe of Conwy and Liverpool bays.
Parent Unit: St Asaph Glacigenic Formation (SAGL)
Previous Name(s): St Asaph Formation (*201)
Irish Sea Till (-3528)
Alternative Name(s): none recorded or not applicable
Reference Section  Coastal cliff section at Traeth y Mwnt. Rijsdijk, 2001. Exposes highly deformed, well stratified and massive diamictons alternating with sands, gravels and laminites. Interpreted as having been deposited in a small ice-dammed rock basin, probably subaqueously, with deformation occurring penecontemporaneously. 
Type Section  Coastal cliff section in Porth Neigwl (Lleyn Peninsula). Young et al., 2002. Most of the section is dominated by a clay-rich diamicton, locally with some crude stratification. Layers and lenses of sand and gravel are generally uncommon but, where present, tend to occur towards the top of the section. The southeastern end of the section is more complex and shows the Lleyn Till Member to be overlain by till of the Eryri Glacigenic Formation and to be separated from it by glaciofluvial sand and gravel and glaciolacustrine deposits. The section records the early advance of Irish Sea Ice into the area, followed by Welsh Ice, with ponding on the surface of the Irish Sea Ice during stagnation being recorded by intervening laminated deposits. 
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. 
Thomas, G S P. 1985. The Late Devensian glaciation along the border of northeast Wales. Geological Journal, 20, 319-340. 
McMillan, A A, Hamblin, R J O, and Merritt, J W. 2011. A lithostratigraphical framework for onshore Quaternary and Neogene (Tertiary) superficial deposits of Great Britain and the Isle of Man. British Geological Survey Research Report, RR/10/03. 343pp. 
Hambrey, M J, Davies, J R, Glasser, N F, Waters, R A , Dowdeswell, J A, Wilby, P R, Wilson, D and Etienne, J L. 2001. Devensian glacigenic sedimentation and landscape evolution in the Cardigan area of southwest Wales. Journal of Quaternary Science, 16, 455-482. 
Campbell, S and Bowen, D Q. 1989. Quaternary of Wales. Geological Conservation Review. (Peterborough: Nature Conservancy Council.) 
Maddison, J D, Siddle, H J and Stephenson, B. 1994. St Dogmaels landslide: Report on investigation. (Cardiff: Sir William Halcrow and Partners Ltd.) 
Davies, J R, Wilson, D and Williamson, I T. 2004. Geology of the country around Flint. Memoir for 1:50 000 Geological Sheet 108. 
Young, T P, Gibbons, W and McCarroll, D. 2002. Geology of the country around Pwllheli. Memoir of the British Geological Survey, Sheet 134 (England and Wales). 
Gibbons W and McCarroll D. 1993. Geology of the country around Aberdaron, including Bardsey Island. Memoir of the British Geological Survey, Sheet 133 (England and Wales). (London: H.M.S.O.). 88pp. 
Davies, J R, Fletcher, C J N, Waters, R A, Wilson, D, Woodhall, D G and Zalasiewicz, J A, 1997. Geology of the country around Llanilar and Rhayader. Memoir for 1:50 000 Geological Sheets 178 and 179. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E193 E194 E211