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Geologie velkého Londýna (Geology of London)

22.09.2012  |  21× přečteno      vytisknout článek 

Geologie velkého Londýna (Geology of London)

Greater London lies within the London Basin, a large geological 'dish' that dominates the geology of the Home Counties and which is bounded to the south by the chalk of North Downs, and to the north by the chalk outcrop of the Chiltern Hills.

The London Basin formed around 55 million years ago, and is filled with mostly with sands and clay of Tertiary age. These sediments were deposited in shallow marine, coastal and terrestrial environments and reflect changing sea-levels and periods of land uplift over the 20 million year period that they were laid down.

Over the last two million years the Greater London area has been significantly influenced by the dramatic climatic changes associated with the advances and retreats of the great ice sheets of the Ice Age. Ice certainly impinged on the area some 450,000 years ago, while during the other glacial phases arctic conditions would have prevailed along with the development of tundra-like vegetation.

Evidence for warmer interglacials comes from the fossil shells and bones of mammals that have been unearthed from the suite of gravel and sand deposits on which the City of London is founded. These deposits form a series of gentle terraces on either side of the River Thames and represent phases in the gradual downcutting of the river. This gently terraced landform is now almost completely obscured by the dense urban development.

Cretaceous

During the Cretaceous (142-65 million years ago) much of what is now southern Britain was covered, to varying degrees, by a shallow sea leading to the deposition of marine, coastal and sometimes terrestrial sediments. It was not until the Late Cretaceous that a major rise in sea-level across Europe led to a significant deepening of the Cretaceous sea. Without the influence of sediments washed in from nearby landmasses a very pure marine limestone was deposited in the warm, relatively deep sea. This is the Chalk, which may constitute up to 98% calcium carbonate.

Due to the formation of the London Basin at the beginning of the Tertiary and its infill with sediments of this age, much of the Chalk outcrop has been obscured, and, as a consequence, it is largely limited in its outcrop to the rim of the London Basin, where it forms the Chilterns and the North Downs. The Chalk outcrop of the southern side of the Basin (the North Downs), does, however, encroach into south-east Greater London in the Croydon-Orpington-Dartford area. There is also a small isolated patch of chalk at the surface at Greenwich.

Tertiary (comprising Neogene and Palaeogene)

The beginning of the Tertiary age (65-2 million years old) saw the beginning of the building of the Alps in southern Europe. Associated with this phase of mountain-building was the folding of the rocks in southern England and the initial development of the London Basin. A fall in sea-level led to the emergence of large areas of land and in the London and Thames area a considerable thickness of Chalk was eroded away. However, by Palaeocene times (approximately 60 million years ago) sea-level rose again and a shallow sea invaded the area. The subsequent series of clays and sands deposited in the London Basin reflect changes in sea-level and the comings and goings of this sea.

The oldest Tertiary sediments belong to the Lambeth Group and comprise sands (the Upnor Formation) deposited in a shallow sea and the overlying reddish-brown clays of the Reading Formation. These latter sediments were deposited on marshy mudflats crossed by river channels. Named after the Reading area, these largely fossil-poor clays outcrop in a narrow strip between the Chalk and the overlying London Clay and occur at Harefield and at South Mimms; with small outcrops at Pinner and Ruislip. In the eastern part of the London Basin the Reading Formation is replaced by the Woolwich Formation.

The Woolwich Formation comprises a varied assortment of sediments including clays, sands and lignite and outcrops in south-east London. The most widespread deposit is clay packed with mollusc shells, termed the `Woolwich Shell Beds´. The shells from this clay indicate deposition in a brackish, estuarine or coastal lagoon environment. In the Peckham and Rotherhithe area a thin limestone occurs at the top of the formation. This contains freshwater snails indicating the presence of a freshwater lake in this area some 55 million years ago.

After a rise in sea-level, shallow marine conditions were again established in the area. In what is now the Lewisham, Beckenham and Croydon area of south London, sands with rounded pebbles of black flint were deposited. These are the Blackheath Beds, and in the Lewisham area it has been shown that they infill channels cut into the underlying sediments of the Lambeth Group.

Sea-level continued to rise during the Eocene Period (55-35 million years ago) leading to the deposition of the thick bluish-brown London Clay. This is the most widespread and best known of the Tertiary deposits of the London Basin and underlies much of Greater London. The London Clay contains the fossils of many marine animals such as bivalves, gastropods and sharks teeth and also the remains of many species of plants, such as the seeds of palms, climbers, magnolias and mangroves. These were probably washed out into the shallow sub-tropical sea by large rivers from a nearby landmass.

Although the London Clay underlies much of the area, due to a covering of more recent sediments it is rarely exposed. Where it does rise to the surface it gives rise to the undulating grassy country round Harrow, Chipping Barnet and Elstree.

There was a shallowing of the sea after deposition of the London Clay and the succeeding pale yellow sands and grey clays of the Bagshot Formation were laid down. These sediments largely outcrop in the Ascot-Bracknell-Wokingham to the west of Greater London, but there are small outlying outcrops at Harrow and Hampstead, remnants of a once much wider extent of these sands. These free-draining sediments give rise to nutrient-poor acidic soils which support characteristic vegetation types such as oak-beech woodland and originally in locations such as Hampstead, heathland.

Quaternary

Over the last two million years the climate of Britain has varied tremendously with periods of temperate climate interrupted by repeated advances and retreats of glaciers and ice sheets. Collectively these periods have become known as the Ice Age (we are still in one of the temperate phases) and the actions of the ice sheets have been instrumental in forming the landscape we see today.

Throughout this period, the London area would have been subject to major swings in climate, with tundra-like conditions prevailing during the main glacial phases and temperate to warm temperate climates during the interglacials. There is evidence in the form of till (or boulder clay) containing fragments of chalk found in the Finchley area and at Hornchurch that ice reached this far south during the Anglian glaciation, approximately 450,000 years ago. During the Anglian, the River Thames, which originally flowed well to the north of London, via the Vale of St Albans, was forced southwards by the encroaching ice sheet to take up its present position.

Within the floodplain of the Thames there are wide expanses of terraced river sands and gravels, with the majority of the gravel being flint derived from the Chalk. These terrace deposits represent successive levels of the river and were formed under the variable climatic conditions of the last half a million years.

The fossils present within the sands and gravels, particularly plant pollen and the shells of snails, can be used to document the changing environments and the comings and goings of animals and plants associated with the dramatic climatic fluctuations. These terraces occur along the Thames, with some interruptions, from Kingston to Greenwich, and extend in a wide belt, from Wandsworth south to Croydon, and in a narrower line from Greenwich towards Bromley.

Following the melting of the last great ice sheet at the end of the Devensian Glacial, some 12,000 years ago there has been a significant rise in sea-level, perhaps as much as 65m, although there have been smaller retreats and advances during this period. These fluctuations are recorded in the Thames Valley in east London by deposits of river and estuary alluvium interleaved with freshwater peats that overlie glacial gravels.

More: http://www.naturalengland.org.uk/ourwork/conservation/geodiversity/englands/counties/area_ID14.aspx

ZDROJ:http://www.naturalengland.org.uk


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