The BGS Lexicon of Named Rock Units — Result Details

Conwy Valley Formation

Computer Code: CONWY 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 Afon Conwy (River Conway) and its tributaries. Includes contemporaneous head, colluvium and pedogenic deposits within the catchment area. Clasts principally consist of local Lower Palaeozoic turbidites admixed with Ordovician volcanic rock clasts derived from Snowdonia and the Harlech Dome (either primarily, or secondarily from the reworking of glacigenic deposits). Aggradational riverr terraces and major alluvial fans are dominated by pebble cobble gravels and medium- to coarse-grained sand, and locally by silty clay with pebble and cobble beds (Warren et al, 1984), the modern floodplain is dominated by silt and clay, and locally includes lenses of peat.
Definition of Lower Boundary: Sharp, unconformable and locally channelised, predominantly on bedrock and Late Devensian head and Welsh glacigenic deposits (e.g. Eryri Till Formation). Interdigitates in a complex manner with Tidal Flat Deposits (Gwynllwg Formation of Bowen, 1999) in the estuary.
Definition of Upper Boundary: Generally the present ground surface or, locally, overlain by peat.
Thickness: Generally to around 10m, with up to 2m exposed locally (Warren et al. 1984).
Geographical Limits: The valley of the River Conwy and its tributaries (Gwynedd and Clwyd), northern Wales.
Parent Unit: Cheshire - North Wales Catchments Subgroup (CNWCA)
Previous Name(s): Alluvial fan deposits (ALF)
Head (HEAD)
River terrace deposits (RTD)
Lacustrine deposits (LDE)
Alluvium (ALV)
Alternative Name(s): none recorded or not applicable
Type Area  Conwy valley between Tal-y-Bont and Llanrwst. This reach contains numerous well developed alluvial fans, large terraces, a broad floodplain and sections of abandoned channel filled with peat. The formation largely overlies the Eryri Till Formation and undifferentiated glaciofluvial deposits. 
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. 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable