- Hampshire Basin
The Hampshire Basin is a
geological synclinein southern Englandunderlying parts of Hampshire, the Isle of Wight, Dorset, and Sussex. Like the London Basinto the northeast, it consists of an area of sands and clays of Paleoceneand younger age surrounded by a broken rim of chalk hills of Cretaceousage.
The Hampshire Basin is the traditional name for the landward section of a basin underlying the northern
English Channeland much of central southern England, known more fully as the Hampshire-Dieppe Basin. It stretches a little over 100 miles (160 km) from the Dorchester area in the west to Beachy Headin the east. Its southern boundary is marked by a monocline resulting in a near-vertical chalk ridge which forms the Purbeck Hillsof Dorset, running under the sea from Old Harry Rocksto The Needlesand the central spine of the Isle of Wight and continuing under the English Channel as the Wight-Bray monocline. The northern limit is the chalk of the South Downs, Salisbury Plainand Cranborne Chase. The basin at its widest is around convert|30|mi|km from north to south between Salisburyand Newport.
The basin includes areas of forest and heath including
Wareham Forest, Arne and the New Forestand the large south coast settlements of Bournemouth, Southamptonand Portsmouth. The coast has many drowned valleys ( rias) including The Solent, Poole Harbour, Southampton Water, Portsmouth Harbour, Chichester Harbour, Langstone Harbour, Pagham Harbour, Yarmouth, Cowesand Bembridge. In addition to the northern half of the Isle of Wight which lies within the basin, the harbours contain inhabited islands including Brownsea Island, Portsea Island, Hayling Islandand Thorney Island.
The Hampshire Basin has no single dominant
river. In former times the Frome and Solent rivers would have drained much of the basin from westto east, fed by tributariesflowing from the northand south. [Jones K.C.D. (Ed), "The Shaping of Southern England", Institute of British GeographersSpecial Publication 11, Academic Press, 1980, ISBN 0-12-388950-2] [Bird, E (1997), "The Shaping of the Isle of White", Bradford on Avon:Ex Libris Press, ISBN 0-948578-83-1] At the end of the last ice agethis system was disrupted by rising sea levels, which separated the Isle of Wight from the mainland. Today the western part of the basin drains via the rivers Frome and Piddle into Poole Harbour, and via the Stour and Avon directly to the English Channel. The central part drains into the Solent (directly or via via Southampton Water), through the Lymington River, Test, Itchen, Meon, Hamble, Western Yar, Medina and Eastern Yar. The eastern part of the basin is a narrow coastal plaindraining into the many harbours via small streams, and is crossed by larger rivers draining the Wealdincluding the Arun and Adur.
The main basin consists of an asymmetric
synclinein the Cretaceous chalk, with a gentle dip southwards from Salisbury Plainterminating abruptly at a near vertical monocline in the south. The chalk surface dips from around 170 metres (560 ft) above sea level west of Winchesterto convert|600|m|ft below sea level at Newport,Melville, R.V. & Freshney E.C (4th Ed 1982), "The Hampshire Basin and adjoining areas", British Regional Geology series, Institute of Geological Sciences, London: HMSO, ISBN 0-11-884203-X ] before rising abruptly to around convert|200|m|ft above sea level in parts of the central hills of the Isle of Wight and the Purbeck Hills. The chalk can be visualised as a thin layer draped over rigid blocks of older rocksat depth, which have moved vertically due to the Alpine Orogeny.
Beneath the chalk the geology is complex. It is believed that a series of major blocks separated by faults trending NW-SE is fragmented into smaller blocks by numerous east-west trending faults; one such block acts as a trap for the
Wytch Farm oil field. These blocks have moved vertically relative to each other during late Palaeozoicand Mesozoictimes, resulting in considerable variations in the thickness of the various formations deposited over them before the chalk. Rocks of Lower Greensand( Aptian) age were deposited over an erodedsurface, which ranges from Great Oolite(Early Jurassic Bathonian) to Weald Clay(mid-Cretaceous Hauterivian) in age.Andrews I.J & Balson P.S. (1995), "Wight: Sheet 50N 02W Solid Geology", 1:250,000 Geological map series, Keyworth: British Geological Survey, ISBN 0-75183088-5.] The chalk itself varies considerably in thickness, with the results of a marine trangression progressing across into Dorset from the east. There is evidence that the top of the Upper Greensandto the west is the same age as the oldest chalk in the Isle of Wight.
Above the chalk the basin contains
Palaeogenesediments ranging from Thanetian( Palaeocene) age to Rupelian( Oligocene) age. The oldest beds, the Lambeth Group('Reading Beds') and ThamesGroup (' London Clay') outcrop in narrow bands towards the perimeter of the basin, from the coast at Studland, around the perimeter of the Dorset heathlands, Edwards R.A., Crosby A. & Briden J.C (1983), "Portland: Sheet 50N 04W Solid Geology", 1:250,000 Geological map series, Keyworth: British Geological Survey, ISBN 0-75181256-0] north and east past Romsey, swinging southeast past Eastleighand eastwards towards Chichester, Worthingand Shoreham-by-Sea. Within this rim, over the greater part of the basin is an outcrop of younger Eocenedeposits, the Bagshot, Bracklesham and Barton beds. The youngest, Oligocene beds (Bouldnor Formation) only occur on the northern Isle of Wight; the Headon Hill beds of the New Forest, formerly classified as Oligocene, are now regarded as late Eocene.
The simple basin structure is complicated by localised folding. A smaller outcrop of Palaeocene and Eocene deposits occurs in a small syncline to the south-east of
Salisbury. [Jackson, A.A. (1991), "Chilterns: Sheet 51N 02W Solid Geology", 1:250,000 Geological map series, Keyworth: British Geological Survey, ISBN 0-78181900-X.] An anticlineto the north of Portsmouthresults in the significant chalk ridge of Portsdown Hillwithin the younger sediments; the London Clay to the north contains the Forest of Bere[Trueman A.E. revised by Whittow J.B. and Hardy J.R (1971), "Geology and Scenery in England and Wales", Harmondsworth: Penguin books, ISBN 0-14-02-0185-8] . A similar structure further east causes chalk to outcrop between Bognor Regisand Worthing, separated from the chalk of the South Downs by a belt of Reading Beds and London Clay continuing from Havantthrough Chichesterand south of Arundelto the coast at Lancing.
It is likely that the London and Hampshire basins were initially part of a single larger area of deposition covering the whole of
southeast Englandduring the Palaeocene. The two basins were progressively separated by the emergence of the Weald-Artois Anticlineduring the Eocene. By the Oligocene the London Basin was wholly dry land, and only a small part of the Hampshire Basin, centred on the modern Solent, was an area of marine deposition. [cite web|url=http://www.qpg.geog.cam.ac.uk/research/projects/tertiaryrivers/ |author=Gibbard P.|coauthors=Lewin J.|title=History of the major rivers of southern Britain during the Tertiary|publisher=Quaternary Palaeoenvironments Group|date=2007-10-29 |accessdate=2008-03-04]
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