Sherwood Sandstone Group
The Sherwood Sandstone Group is a Triassic lithostratigraphic group which is widespread in Britain, especially in the English Midlands. The name is derived from Sherwood Forest in Nottinghamshire which is underlain by rocks of this age. It has economic importance as the reservoir of the Morecambe Bay gas field, the second largest gas field in the UK, and the Wytch Farm oil field in Dorset, the largest onshore oilfield in Western Europe.
Geographical extent
The Sherwood Sandstone Group is present in a number of sedimentary basins throughout the United Kingdom, where regional subsidence during the Triassic allowed sediments to accumulate and be preserved.Northwest England: the East Irish Sea and Solway Basins
The Sherwood Sandstone Group in onshore Northwest England comprises the Cheshire Basin and the eastern edge of the East Irish Sea and Solway Basins. The Sherwood Sandstone Group is found as bedrock in the Vale of Eden, on the coast of Cumbria, in Lancashire and in Cheshire. The Sherwood Sandstone in Cumbria and Lancashire is mostly obscured by superficial deposits, with a notable exception being the cliffs at St Bees Head which are formed in the Chester Formation, the lowermost formation within the group. Cities which are built on the Sherwood Sandstone include Preston, Lancashire, Carlisle, Manchester, Liverpool, and Chester.Offshore, the Sherwood Sandstone Group is present as bedrock beneath the Irish Sea seabed as far east as the Isle of Anglesey and the Isle of Man. The Sherwood Sandstone also crops out onshore in the northeastern Isle of Man.
Northern Ireland and Scotland
The Sherwood Sandstone is also found in southern Scotland around Annan. The Sherwood Sandstone in Annan is continuous with the Sherwood Sandstone in Northern Ireland under the North Channel. In Northern Ireland, the Sherwood Sandstone is relatively widespread, though much of the unit is buried beneath younger rocks.The Sherwood Sandstone crops out north and east of Limavady, east of Cookstown, between Dungannon and Armagh and along the Lagan Valley beneath Belfast and Newtownards and on the Antrim coast.Northeast England
In northeastern England, the Sherwood Sandstone Group extends in a roughly north-south band running parallel to the Pennines, from Stockton-on-Tees south through York, into Nottinghamshire and then the English Midlands, though is largely concealed by superficial sediments. Offshore in the southern North Sea, the Sherwood Sandstone Group is present under younger rocks, and is the equivalent to the Bunter Sandstone of the Bacton Group.Midlands: Hinkley, Needlewood, Stafford and Knowle Basins
The Sherwood Sandstone occurs widely in the Midlands, particularly to the western side of the region in Shropshire and Staffordshire. The cities of Lichfield, as well as west Birmingham and west Wolverhampton, are built on the Sherwood Sandstone.Southern England: Worcester and Wessex Basins
Southwards, the Sherwood Sandstone is continuously present beneath Worcestershire, Gloucestershire, Wiltshire and Hampshire, and beyond there through Somerset, Dorset and Devon, however outcrops are very limited and it is largely buried under younger sedimentary rocks. The only substantial outcrop in this area is the cliff section from Budleigh Salterton to Ladram Bay and Sidmouth on the coast of East Devon.Cheshire Basin rock succession
The sequence is most thickly developed in the Cheshire Basin, which also extends into north Shropshire. It comprises the following formations:Helsby Sandstone Formation
The Helsby Sandstone Formation comprises around 250m thickness of sandstone with conglomerate and siltstone which occurs across the Cheshire Basin. Older literature includes it as part of the Lower Keuper Sandstone. It is often divided into an upper Frodsham Member and a lower Delamere Member. Faulted blocks of these rocks are largely responsible for the prominent west facing escarpment of the Mid Cheshire Ridge and the Helsby Sandstone is exposed in numerous localities here, southwards from Runcorn through Helsby and Frodsham to Utkinton, spectacularly at the outlier of Beeston Castle hill and lastly within the Peckforton Hills. Further outcrops occur on the Wirral Peninsula, on Thurstaston Common and the Hilbre Islands.Wilmslow Sandstone Formation
The Wilmslow Sandstone Formation comprises up to 900m thickness of early Triassic sandstones with occasional siltstones. It was earlier known as the Upper Mottled Sandstone.In Wirral, the 60m thick Thurstaston Sandstone Member and the 2m thick Thurstaston Hard Sandstone Bed are distinguished at the top of the sequence.