The BGS Lexicon of Named Rock Units — Result Details

Tywi Valley Formation

Computer Code: TYWI 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 Towy (Afon Tywi) and its tributaries. Includes contemporaneous head, colluvium and pedogenic deposits within the catchment area. Clasts principally consist of local, Lower Palaeozoic turbidites, admixed to the west of Llandovery with Old Red Sandstone clasts. Aggradational river terraces and major alluvial fans are dominated by pebble cobble gravels and medium- to coarse-grained sand; the modern floodplain is domiated by silt and clay, and locally includes lenses of peat.
Definition of Lower Boundary: Sharp, unconformable and locally channelised, predominately on bedrock and Late Devensian head and Welsh glacigenic deposits (e.g Brecknockshire and Plynlimon till formations). Locally beyond the ice limit of the Late Glacial Maximum (e.g. north and west of Cwmduad [SN 3748 3125], it might overlie older alluvial, periglacial and glacial deposits. 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 about 10m; it might be locally thicker in the lower reaches of the Tywi.
Geographical Limits: The valley of the River Twyi and its tributaries (Dyfed, Wales).
Parent Unit: West Wales Catchments Subgroup (WWACA)
Previous Name(s): Alluvial Fan Deposits (ALF)
Subsumes River Terrace Deposits, Alluvial Fan Deposits, Head, Alluvium And Lacustrine Deposits (-4947)
River Terrace Deposits (RTD)
Lacustrine Deposits (LDE)
Head (HEAD)
Alluvium (ALV)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  Tywi valley between Pont-ar-gothi and Abergwilli. Strahan et al., 1909. This reach contains numerous well developed alluvial fans, abandoned meander loop lakes, a staircase of terraces and a broad floodplain, mostly developed on glaciofluvial deposits. 
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. 
Strahan, A, Cantrill, T C, Dixon, E E L and Thomas, H H. 1909. The Geology of the South Wales coalfield. Part 10. The country around Carmarthen. Memoir of the Geological Survey of England and Wales. [London: HMSO.] 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable