The BGS Lexicon of Named Rock Units — Result Details

Teifi Valley Formation

Computer Code: TEIFI Preferred Map Code: notEntered
Status Code: Full
Age range: Holocene Epoch (QH) — Holocene Epoch (QH)
Lithological Description: Unconsolidated, stratified gravel, sands, silts and clays forming the alluvial deposits of the River Teifi (Afon Teifi) and its tributaries. Includes contemporaneous head, colluvium and pedogenic deposits within the catchment area. Clasts principally consist of local Lower Palaeozoic turbidites but, west of a line approximately running between Talgarreg [SN 4267 5096] and Pentrecagel [SN 3385 4030] a substantial mixture of "Irish Sea" material derived from the Lleyn Formation can be present. 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.
Definition of Lower Boundary: Sharp, unconformable and locally channelised on bedrock, Devensian head deposits and Late Devensian glacigenic deposits (particularly the Plynlimon Till Formation and, west of Llanysul [SN 4190 4043], the Teifi Clay Formation and Lleyn Till Formation). In Tregaron Bog it overlies the Tregaron Formation. Interdigitates in a complex manner with Tidal Flood 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 4 to 5m, but ranging up to 10m.
Geographical Limits: The valley of the River Teifi and its tributaries (Dyfed, Wales).
Parent Unit: West Wales Catchments Subgroup (WWACA)
Previous Name(s): Alluvial Fan Deposits (ALF)
River Terrace Deposits (RTD)
Head (HEAD)
Alluvium (ALV)
Lacustrine Deposits (LDE)
Alternative Name(s): none recorded or not applicable
Type Area  Teifi valley between Henllan and Newcastle Emlyn, defined by bedrock gorges at either end (Glasser et al., 2004). Waters et al., 1997. This reach contains numerous and well developed alluvial fans, a broad floodplain and a staircase of terraces incised into glaciofluvial deposits (of Welsh Ice derivation). The floodplain prediminantly overlies sediments of Teifi Clay Formation. 
Type Area  Teifi valley between Lampeter and Pencarreg. Jones et al., 2006. This reach consists of a broad floodplain, largely overlying Late Devensian glaciolacustrine deposits, and encompasses a number of well defined terraces incised into glaciofluvial deposits. Parts of the alluvial sequence have been radiocarbon dated and indicate deposition up to 11170-10740 BP (cal). 
Waters, R A, Davies, J R, Wilson, D and Prigmore, J K. 1997. A geological background for planning and development in the Afon Teifi catchment. British Geological Survey Technical Report, WA/97/35. 
Glass, N F, Etienne, J L, Hambrey, M J, Davies, J R, Waters, R A and Wilby, P R. 2004. Glacial meltwater erosion and sedimentation as evidence for multiple glaciations in west Wales. Boreas, Vol.33, 224-237. 
Jones, A F, Johnstone, E, Brewer, P A and Macklin, M G. 2006. Dating and correlating Late Pleistocene and Holocene alluvial sequences in Welsh river catchments. Final Report, British Geological Survey University Collaboration Contract GA/02E/01. (Aberystwyth: University of Wales). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable