The BGS Lexicon of Named Rock Units — Result Details

Gwent Levels Formation

Computer Code: GLEV Preferred Map Code: notEntered
Status Code: Full
Age range: Holocene Epoch (QH) — Holocene Epoch (QH)
Lithological Description: Formation encompasses marine, estuarine and terrestrial deposits that were formed in the Gwent Levels area during the Holocene transgression. The deposits are dark blue-grey silty clays and silts with subordinate sands and beds of peat, submerged forests and gravel. The deposits rest on a rockhead platform at -5 to -7.5m OD, intricately dissected by river valleys, and their upper surface is approximately level at c 4.5 to 7m OD. The river valleys are infilled with gravels and sands that become shelly upwards. Typically two beds of peat are included, up to 0.6m thick. The lower of these forms the lowest unit in the formation, except where it rests upon the valley-fill sands and gravels, and was formed c 8500-8000 years BP when a birch forest was inundated by the rising sea. The other formed c.5000-4500 years BP during a slowing down of the rising sea level, and is now found resting horizontally at OD.
Definition of Lower Boundary: Rests unconformably on Triassic bedrock.
Definition of Upper Boundary: Surface.
Thickness: 10 to 15m, increasing to 35m in buried channels.
Geographical Limits: The Gwent Levels. From Caldicot, Monmouthshire, to Cardiff.
Parent Unit: British Coastal Deposits Group (COAS)
Previous Name(s): Estuarine Alluvium [Obsolete Name And Code: Use TFD] (ESAL)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  The north shore of the Severn Estuary, comprising the Gwent Levels from Caldicot to Cardiff. Welsh and Trotter, 1960. 
Reference(s):
Welch, F B A and Trotter, F M. 1961. Geology of the country around Monmouth and Chepstow. Explanation of one-inch geological sheets 233 and 250. Memoir of the Geological Survey of Great Britain. 
Allen, J R L. 1987. Late Flandrian shoreline oscillations in the Severn Estuary: the Rumney Formation at its typesite (Cardiff area). Philosophical Transactions of the Royal Society of London, Vol.B315, 157-174. 
Squirrell, H C and Downing, R A. 1969. Geology of the South Wales Coalfield, Part 1, the country around Newport (Monmouthshire). Memoir of the Geological Survey of Great Britain, Sheet 249 (England and Wales). 
Allen, J R L. 2001. Late Quaternary stratigraphy in the Gwent Levels (southeast Wales): the subsurface evidence. Proceedings of the Geologists' Association, Vol.112, 289-315. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E249 E250 E263