The BGS Lexicon of Named Rock Units — Result Details

Eryry Glacigenic Formation

Computer Code: ERYG Preferred Map Code: notEntered
Status Code: Full
Age range: Devensian Stage (QD) — Devensian Stage (QD)
Lithological Description: Moderately stiff, dark grey to blue-grey, generally massive, pebbly, silty clay, characteristically containing Snowdonian Ordovician volcanic clasts but dominated by local Lower Palaeozoic clasts. Colour and dominant clast composition variable and generally reflect the local bedrock geology. Also includes other glacigenic deposits of similar composition, notably glaciofluvial sand and gravel and morainic deposits.
Definition of Lower Boundary: Generally unconformable on bedrock, but locally may overlie Devensian Head deposits (McCarroll, 2005, p.29); Lleyn Till Member of St Asaph Glacigenic Formation (Lleyn Formation of Young et al., 2002), or post-Ipswichian pre-glacial peats (Addison and Edge, 1992).
Definition of Upper Boundary: Generally the ground surface or unconformable contact with overlying alluvial deposits, peat and Head (Britannia Catchemnts Group). May locally be unconformably overlain by red diamicton of the Lleyn Formation (of Harris et al., 1997), Lleyn Till Member, St Asaph Glacigenic Formation (McMillan et al., in prep, 2009).
Thickness: Highly variable, locally likely to greatly exceed 10m (Warren et al., 1984).
Geographical Limits: Mountains of Snowdonia, Rhinogs and Cadair Idris; eastern Lleyn Peninsula.
Parent Unit: Wales Glacigenic Subgroup (WALES)
Previous Name(s): North Welsh Boulder Clay (-1529)
Welsh Till (-3390)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Pen-y-bryn brickworks, Caernarfon Succession of Irish Sea-derived till on Welsh-provenanced till and glaciofluvial gravels on biogenic muds sands and gravels (Addison and Edge, 1992; Bowen, 1999) 
Reference(s):
McCarroll, D. 2005. Northwest Wales. In Lewis, C A and Richards, A E (editors). The glaciations of Wales and adjacent areas. (Logaston Press.) 
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). 
Harris, C, Williams, G, Brabham, P, Eaton, G and McCarroll, D. 1997. Glaciotectonized Quaternary sediments at Dinas Dinlee, Gwynedd, North Wales and their bearing on the style of deglaciation in the Eastern Irish Sea Basin. Quaternary Science Reviews, 16, 109-127. 
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. 
Whittow, J B and Ball, D F. 1970. North-west Wales. In Lewis, CA. (ed) The glaciations of Wales and adjoining regions. Longman Press. 
Addison, K and Edge, M J. 1992. Early Devensian interstadial and glacigenic sediments in Gwynedd, North Wales. Geological Journal, 27, 181-190. 
Chambers,F M, Addison, K, Blackford, J J, and Edge, M J. 1995. Palynology of organic beds below Devensian glacigenic sediments at Oen-y-Bryn, Gwynedd, North Wales. Journal of Quaternary Science, Vol. 10, 157-173. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable