The River Thames can first be identified as a discrete drainage line as early as 58 million years ago, in the Thanetian stage of the late Palaeocene epoch.[18] Until around 500,000 years ago, the Thames flowed on its existing course through what is now Oxfordshire, before turning to the north east through Hertfordshire and East Anglia and reaching the North Sea near Ipswich. At this time the river system headwaters lay in the English West Midlands and may, at times, have received drainage from the North Wales Berwyn Mountains. Streams and rivers like the River Brent, Colne Brook and Bollo Brook either flowed into the then river Thames or went out to sea on the course of the present-day river Thames.
The arrival of an ice sheet in the Quaternary Ice Age, about 450,000 years ago, dammed the river in Hertfordshire, causing large ice lakes which eventually burst their banks and caused the river to be diverted onto its present course through London. Progressively, the channel was pushed south to form the St Albans depression by the repeated advances of the ice sheet. This created a new river course through Berkshire and on into London, after which the river rejoined its original course in southern Essex, near the present River Blackwater estuary. Here it entered a substantial freshwater lake in the southern North Sea basin. The overspill of this lake caused the formation of the Dover Straits or Pas-de-Calais gap between Britain and France. Subsequent development led to the continuation of the course which the river follows at the present day.[19]
Most of the bedrock of the Vale of Aylesbury is largely made up of clay and chalk that was formed at the end of the ice age and at one time was under the Proto-Thames. Also at this time the vast underground reserves of water that make the water table higher than average in the Vale of Aylesbury were created.
The last advance from that Scandinavian ice flow to have reached this far south covered much of NW Middlesex and finally forced the Proto-Thames to take roughly its present course. At the height of the last ice age around 10,000 BCE, Britain was connected to mainland Europe by a large expanse of land known as Doggerland in the southern North Sea basin. At this time, the Thames' course did not continue to Doggerland, but flowed southwards from the eastern Essex coast where it met the Rhine, the Meuse and the Scheldt[19] flowing from what are now the Netherlands and Belgium. These rivers formed a single river—the Channel River (Fleuve Manche)—that passed through the Dover Strait and drained into the Atlantic Ocean in the western English Channel.
The ice sheet which stopped around present day Finchley, deposited Boulder clay to form Dollis Hill and Hanger Hill. Its torrent of meltwater gushed through the Finchley Gap and south towards the new course of the Thames, and proceeded to carve out the Brent Valley in the process.[20] Upon the valley sides there can be seen other terraces of brickearth; laid over and sometimes interlayered with the clays. These deposits were brought in by the winds during the periglacial periods, suggesting that wide flat marshes were then part of the landscape, which the new river Brent proceeded to cut down. The steepness of the valley sides is witness to the very much lower mean sea levels caused by the glaciation locking up so much water upon the land masses, thus causing the river water to flow rapidly seaward and so erode its bed quickly downwards.
The original land surface was around 110 to 130 metres (350 to 400 ft) above the current sea level. The surface had sandy deposits from an ancient sea, laid over sedimentary clay (this is the Blue London Clay). All the erosion down from this higher land surface and sorting action by these changes of water flow and direction formed what is known as the Thames River Gravel Terraces. Since Roman times and perhaps earlier, however, the isostatic rebound from the weight of previous ice sheets, and its interplay with the eustatic change in sea level, has resulted in the old valley of river Brent, together with that of the Thames, silting up again. Thus along much of the Brent's present-day course one can make out the water meadows of rich alluvium, which is augmented by frequent floods.
After taking its present-day course, much of the banks of the Thames Estuary and the Thames Valley in London was partly covered in marshland, as was the adjoining Lower Lea Valley. The streams and rivers like the River Lea, Tyburn Brook and Bollo Brook drained into the river, while some islands like Thorney Island formed over the ages. The northern tip of the ancient parish of Lambeth, for example, was a marshland known as Lambeth Marshe, but it was drained in the 18th century and is remembered in the Lower Marsh street name. Sometime after the opening of Waterloo railway station in 1848 the locality around the station and Lower Marsh became known as Waterloo. Lower Marsh is a street in the Waterloo neighbourhood of London. It is the location of Lower Marsh Market.[21]
The East End of London, also known simply as the East End, was the area of London east of the medieval walled City of London and north of the River Thames, although it is not defined by universally accepted formal boundaries; the river River Lea can be considered another boundary.[22] Most of the local riverside was also marshland. The land was drained and became farmland; it was built on after the Industrial Revolution. Use of the term "East End" in a pejorative sense began in the late 19th century,[23]
Canvey Island in southern Essex (area 18.45 km2; pop. 37,479[24]) is a civil parish and once marshy, but now fully reclaimed island in the Thames estuary in England. It is separated from the mainland of south Essex by a network of creeks. Lying below sea level it is prone to flooding at exceptional tides, but has nevertheless been inhabited since the Roman invasion of Britain.
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