The BGS Lexicon of Named Rock Units — Result Details

Clwyd Valley Formation

Computer Code: CWYDV Preferred Map Code: notEntered
Status Code: Full
Age range: Holocene Epoch (QH) — Holocene Epoch (QH)
Lithological Description: Unconsolidated, stratified gravels, sands, silts and clays forming the alluvial deposits of the River (Afon) Clwyd and its tributaries. Includes contemporaneous head, colluvium and pedogenic deposits within the catchment area. Clasts comprise variable proportions of Lower Palaeozoic turbidites, Ordovician volcanic rocks, Permian/Trias sandstone and Carboniferous Limestone and Coal Measures reflecting the underlying bedrock geology and complex glacigenic sequence. North of approximately Denbigh, the deposits in the Clwyd and the Wheeler valleys, as well as those in the lower reaches of the Elwy valley, may contain exotic (Irish Sea Ice-derived) materials reworked from the Lleyn Formation. Aggradational river terraces and major alluvial fans are dominated by pebble cobble gravels and medium- to coarse-grained sand; the modern floodplain is dominated by silt and clay, and locally includes lenses of peat (Warren et al., 1984).
Definition of Lower Boundary: Sharp, unconformable and locally channelised, predominately on bedrock and Late Devensian head and glacigenic deposits (e.g Ruabon Member and Wheeler Member of Bowen, 1999, p.89, and Lleyn Formation). Interdigitates in a complex manner with Tidal Flat Deposits (Gwynllwg Formation of Bowen, 1999) in the estuary (Warren et al, 1984).
Definition of Upper Boundary: Generally the present ground surface or, locally, overlain by peat.
Thickness: Generally to around 10m; 8m were recorded by Warren et al (1984) in the Vale of Clwyd and 6m are exposed near Plasnewydd [SH 9460 6640].
Geographical Limits: The valley of the River Clwyd and its tributaries (Clwyd), northern Wales.
Parent Unit: Cheshire - North Wales Catchments Subgroup (CNWCA)
Previous Name(s): River terrace deposits (RTD)
Lacustrine deposits (LDE)
Head (HEAD)
Alluvium (ALV)
River terrace deposits (RTD)
Lacustrine deposits (LDE)
Head (HEAD)
Alluvium (ALV)
Alluvial fan deposits (ALF)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  Clwyd valley between Bodfari and Rhewl. Warren et al. 1984. This reach contains numerous well developed alluvial fans, abandoned peat filled channels, staircases of terraces and a broad floodplain, mostly developed on till and glaciofluvial deposits. 
Reference(s):
Rowlands, B M. 1955. The glacial and post-glacial geomorphological evolution of the landforms of the Vale of Clwyd. Unpublished M A Thesis. Liverpool University. 
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. 
Warren, P T, Price, D, Nutt, M J C and Smith, E G. 1984. Geology of the country around Rhyl and Denbigh. Memoir for 1:50 000 geological sheets 95 and 107 and parts of sheets 94 and 106. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable