The BGS Lexicon of Named Rock Units — Result Details

Brecknockshire Glacigenic Formation

Computer Code: BNOCK Preferred Map Code: notEntered
Status Code: Full
Age range: Devensian Stage (QD) — Devensian Stage (QD)
Lithological Description: Generally stiff, structureless, stony, sandy and silty diamict and clayey gravel, characteristically containing abundant Old Red Sandstone clasts (derived from Black Mountain, Fforest Fawr and the Brecon Beacons) and sparse, if any Lower Palaeozoic clasts. Subordinate stratified sand and gravel are locally present. Colour and dominant clast composition are variable and generally reflect the local bedrock geology: in the Usk Valley it is red and exclusively composed of Old Red Sandstone material.
Definition of Lower Boundary: At the type locality it unconformably overlies Early Devensian head deposits and Ipswichian raised beach deposits of the Pennard Formation (of Bowen, 1999); elsewhere it is generally unconformable on bedrock or regolith (Barclay et al, 2005).
Definition of Upper Boundary: Generally the ground surface or overlain by Late Glacial organic deposits infilling kettleholes (see Bowen, 2005, p.155)(Britannia Catchments Group), or Kenfig Formation or Gwent Levels Formation (British Coastal Deposits Group).
Thickness: Highly variable, locally to 25m (Waters and Lawrence, 1987).
Geographical Limits: Usk Valley west of Talybont; Swansea Bay, Cardiff, Newport, Fforest Fawr and Brecon Beacons.
Parent Unit: Wales Glacigenic Subgroup (WALES)
Previous Name(s): Brecknockshire Drift [Obsolete Name and Code: Use BNOCK, LDTI] (-4481)
Brecknockshire Formation [Obsolete Name and Code: Use BNOCK, WALES] (-4482)
Welsh till [Obsolete Name and Code: Use MNTI, PLYNT, BNOCK] (-3232)
Alternative Name(s): Langland Member
Stratotypes:
Type Section  Coastal section at Rotherslade in Langland Bay exposing 14m of lodgement till, crudely stratified ablation till and well imbricated glaciofluvial sand and gravel of the Brecknockshire Glacigenic Formation overlying 3.5m of early Devensian Head and 0.6 m of Ipswichian raised beach deposits of the Pennard Formation (of Bowen, 1999) on bedrock. Locally, colluvium and loess of possible Loch Lomond age overlie the Brecknockshire Glacigenic Formation. Campbell and Bowen, 1989. 25-26. 
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. 
Bowen, D Q. 2005. South Wales. In Lewis, C A and Richards, A E (editors), The glaciations of Wales and adjacent areas. (Logaston Press.) 
David, J W E, 1883. On the evidence of glacial action in south Brecknockshire and east Glamorgan. Quarterley Journal of the Geological Society, London, Vol.39. 39-54. 
Waters, R A and Lawrence, D J D. 1987. Geology of the South Wales Coalfield - Part III: the country around Cardiff. Memoir of the British Geological Survey, Sheet 263. (England and Wales). 
Campbell, S and Bowen, D Q. 1989. Quaternary of Wales. Geological Conservation Review. (Peterborough: Nature Conservancy Council.) 
Barclay, W J, Davies, J R, Humpage, A J, Waters, R A, Wilby, P R, Williams, M and Wilson, D. 2005. Geology of the Brecon district - a brief explanation of the geological map Sheet 213 Brecon. Sheet explanation of the British Geological Survey. 1:50000 Sheet 213 Brecon. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable