London Basin

The London Basin is an elongated, roughly triangular syncline approximately convert|250|km|mi long which underlies London and a large area of south east England and south eastern East Anglia.

Boundary

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 escarpments of the Chilterns and Marlborough Downs to the north and the North Downs and Berkshire Downs to the south. To the south lie the Weald and Salisbury Plain and to the north is the Vale of Aylesbury. The approximate western limit is in the Marlborough area of Wiltshire. The eastern end merges with the basin of the North Sea, extending on land along the north Kent coast to Reculver and up the east coast of Essex and into Suffolk, where it is overlain by Pleistocene 'Crag' deposits which cover much of eastern Suffolk and Norfolk and 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]

ettlements

Much of the basin is urbanised. Besides the whole Greater London Urban Area of over 8.5 million people, the basin contains most of the M4 Corridor including Newbury, Reading, Bracknell, Maidenhead and Slough; the urbanised area to the south-west of London (Camberley, Farnborough, Aldershot and 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 Bay and Canterbury; Essex towns including Brentwood, Chelmsford, Braintree, Colchester, Clacton-on-Sea and 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, Luton and High Wycombe.

Geology

Underlying the basin is the chalk of 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, Eocene and 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, Lewisham and 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 Beds etc, forming sandy heaths.

The main fold axis of the basin runs west-east from Marlborough and Newbury Berkshire to Chertsey (Surrey) before swinging slightly north of east through Westminster, passing midway between Chelmsford and 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 ]

London Platform

Most of the basin is underlain at depth by a block of Palaeozoic rocks known as the London Platform. This is separated by faulting from the Wessex Basin (underlying the Weald and 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 Mesozoic rocks underlying the neighbouring Weald are not present, or are very much thinner. Rocks of Triassic age are absent under London, occurring at depth only at the western edge of the basin. Jurassic rocks 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 Greensand series, 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.

During the Palaeogene the 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 Anticline and the London Platform subsiding to form the London Basin. Up to convert|320|m|ft of Palaeocene and Eocene sediments were deposited in the basin. The Pleistocene saw the sea retreat from the basin as global sea-level fell due to accumulation of ice sheets.

Drainage

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 Horse and 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]

During the Pleistocene ice 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 Sea by rivers including the Crouch, Blackwater, Stour and Orwell.

References