- London Basin
The London Basin is an elongated, roughly triangular
synclineapproximately convert|250|km|mi long which underlies Londonand a large area of south east England and south eastern East Anglia.
The edges of the basin are rather arbitrary, since the syncline merges seemlessly into neighbouring folds such as the
Weald anticline. The generally accepted limits are the chalk escarpmentsof the Chilternsand Marlborough Downsto the north and the North Downsand Berkshire Downs to the south. To the south lie the Weald and Salisbury Plainand to the north is the Vale of Aylesbury. The approximate western limit is in the Marlborougharea of Wiltshire. The eastern end merges with the basin of the North Sea, extending on land along the north Kentcoast to Reculverand up the east coast of Essexand into Suffolk, where it is overlain by Pleistocene'Crag' deposits which cover much of eastern Suffolk and Norfolkand are better considered as part of the North Sea basin. [Ellison R.A. et al 2004, "Geology of London: Special Memoir for 1:50,000 Geological sheets 256 (North London), 257 (Romford), 270 (South London) and 271 (Dartford) (England and Wales)", British Geological Survey, Keyworth, ISBN 0-85272478-0] ["Chilterns: Sheet 51N 02W Solid Geology", 1:250,000 Geological map series, British Geological Survey, Keyworth, 1991]
Much of the basin is urbanised. Besides the whole
Greater London Urban Areaof over 8.5 million people, the basin contains most of the M4 Corridorincluding Newbury, Reading, Bracknell, Maidenheadand Slough; the urbanised area to the south-west of London ( Camberley, Farnborough, Aldershotand Guildford); north and south Thames-side ( Thurrock, Tilbury, Basildon, Southend-on-Sea, Northfleet, Gravesend etc); north Kent towns including Chatham, Rochester, Gillingham, Sittingbourne, Faversham, Whitstable, Herne Bayand Canterbury; Essex towns including Brentwood, Chelmsford, Braintree, Colchester, Clacton-on-Seaand Harwich; and towns to the north and northwest of London including Bishops Stortford, Harlow, Hertford, Stevenage, Harpenden, Hatfield, Welwyn Garden City, Potters Bar, St Albans, Lutonand High Wycombe.
Underlying the basin is the
chalkof the Cretaceous, which is exposed on the dip slopes of the Chilterns and North Downs. Within the centre of the basin the chalk is mainly covered by Palaeocene, Eoceneand younger rocks, glacial and riverine deposits, though the chalk is also brought to the surface by localised folds and faults, for example at Windsor Castle, Lewishamand Purfleet. The chalk forms an artesian basin, withfresh water springs emerging on the bed of the Thames. In the greater part of the basin the surface 'rock' is Eocene London Clay, flanked at the margins by older deposits such as the Reading Beds. In large areas towards the western end the London Clay is overlain by rather younger deposits of the Bagshot Bedsetc, forming sandy heaths.
The main fold axis of the basin runs west-east from Marlborough and
NewburyBerkshire to Chertsey(Surrey) before swinging slightly north of east through Westminster, passing midway between Chelmsfordand Southend-on-Sea(Essex) to the east coast between the estuaries of the Crouch and the Blackwater.Sumbler M.G. (4th Ed 1996), "London and the Thames Valley", British Regional Geology series, British Geological Survey, ISBN 0-11-884522-5] Though north of the current mouth of the Thames, this line is well to the south of the centre-line of the basin which is asymmetric, its southern limb dipping more steeply than the northern. ["Thames Estuary: Sheet 51N 00 Solid Geology", 1:250,000 Geological map series, British Geological Survey, Keyworth, 1989 ]
Most of the basin is underlain at depth by a block of
Palaeozoicrocks known as the London Platform. This is separated by faulting from the Wessex Basin (underlying the Wealdand Hampshire Basin) to the south and the Worcester Basin to the west. Vertical movements of this block have affected both deposition and structure, with the result that many of the Mesozoicrocks underlying the neighbouring Weald are not present, or are very much thinner. Rocks of Triassicage are absent under London, occurring at depth only at the western edge of the basin. Jurassicrocks occur over a wider area to the west and south but are also absent under London itself. During early Cretaceous times the platform was uplifted and eroded, with material deposited to the south as the Wealden Group. Sea level rose during deposition of the Lower Greensandseries, but only completely covered the platform during deposition of the Gault Formation. The whole area remained submerged during much of the Upper Cretaceous, though the chalk contains levels indicating periods of erosion.
Palaeogenethe south-east of England was affected by movements related to the formation of the Alps(the Alpine Orogeny). One consequence of this was relative vertical movements, with the eastern part of the Wessex Basin being uplifted as the Weald-Artois Anticlineand the London Platform subsiding to form the London Basin. Up to convert|320|m|ft of Palaeoceneand Eocenesediments were deposited in the basin. The Pleistocenesaw the sea retreat from the basin as global sea-level fell due to accumulation of ice sheets.
The basin is mainly drained by the
River Thames, but does not coincide with the Thames drainage basin. The upper Thames cuts through the Chilterns via the Goring Gap, and consequently the Thames drains parts of the Cotswolds, Vale of White Horseand Vale of Aylesbury. The main headstream within the London Basin proper is the Kennet, which flows along the axis from the Marlborough area, joining the Thames at Reading.
To the south rivers such as the Mole and Medway, draining from the Weald, cut through the North Downs into the basin; these are presumed to date from before the erosion of the Weald dome. [Clayton K. in Jones K.C.D. (Ed), "The Shaping of Southern England", Institute of British Geographers Special Publication 11, Academic Press, 1980, ISBN 0-12-388950-2]
Pleistoceneice age the lower Thames is believed to have been been diverted well to the south of its earlier course closer to the main axis of the basin, probably by the Anglian Stage. The north eastern part of the basin is now drained to the North Seaby rivers including the Crouch, Blackwater, Stour and Orwell.
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