The BGS Lexicon of Named Rock Units — Result Details

Oldbury and Avonmouth Levels Formation

Computer Code: OALEV Preferred Map Code: notEntered
Status Code: Full
Age range: Ipswichian Stage (QI) — Holocene Epoch (QH)
Lithological Description: The formation excompasses the marine, estuarine and terrestrial deposits that were formed in the Oldbury and Avonmouth 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 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: c.10 to 13m, increasing to c.21m in buried channels.
Geographical Limits: The Oldbury and Avonmouth Levels, from Berkeley, Gloucestershire, to Portishead, Somerset.
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
Type Area  The Oldbury and Avonmouth Levels, from Berkeley to Portihead, on the south shore of the Severn Estuary. Welch and Trotter, 1960. 
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. 2000. Sea level, salt marsh and fen: shaping the Severn Estuary Levels in the later Quaternary (Ipswichian-Holocene). Archaeology in the Severn Estuary, Vol.11, 13-34. 
Allen, J R L and Rae, J E. 1987. Late Flandrian shoreline oscillations in the Severn Estuary: a geomorphological and stratigraphical reconnaissance. Philosophical Transactions of the Royal Society of London, B315, 185-230. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E250 E251 E264