Everything about The London Basin totally explained
The
London Basin is an elongated, roughly triangular
syncline approximately 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's overlain by
Pleistocene 'Crag' deposits which cover much of eastern Suffolk and
Norfolk and are better considered as part of the North Sea basin.
Settlements
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, with
fresh 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. 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.
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 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.
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 glaciation. The north eastern part of the basin is now drained to the
North Sea by rivers including the
Crouch,
Blackwater,
Stour and
Orwell.
Further Information
Get more info on 'London Basin'.
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