Thames the River Thames

Ice age

About 450,000 years ago, in the most extreme Ice Age of the Pleistocene, the Anglian, the furthest southern extent of the ice sheet reached Hornchurch^[29] in east London, the Vale of St Albans, and the Finchley Gap.
It dammed the river in Hertfordshire, resulting in the formation of large ice lakes, which eventually burst their banks and caused the river to divert onto its present course through the area of present-day London.
The ice lobe which stopped at present-day Finchley deposited about 14 metres of boulder clay there.^[30] 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.^[31]
The Anglian ice advance resulted in a new course for the Thames 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, south of what is called Doggerland. The overspill of this lake caused the formation of the Channel River and later the Dover Strait gap between present-day Britain and France. Subsequent development led to the continuation of the course that the river follows at the present day.^[32]
Most of the bedrock of the Vale of Aylesbury comprises clay and chalk that formed at the end of the ice age and at one time was under the Proto-Thames. At this time the vast underground reserves of water formed that make the water table higher than average in the Vale of Aylesbury.^[33]

A geological map of the London Basin; the London Clay is marked in dark brown

The confluence of the Rivers Thames and Brent. The narrowboat is heading up the River Brent. From this point as far as Hanwell the Brent has been canalised and shares its course with the main line of the Grand Union Canal. From Hanwell the Brent can be traced to various sources in the Barnet area.

At the height of the last ice age, around 20,000 BC, 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 waters of the proto-Rhine–Meuse–Scheldt delta^[32] 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.
Upon the valley sides of the Thames and some of its tributaries 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 rivers proceeded to cut into.
The steepness of some valley sides indicates 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 350 to 400 feet (110 to 120 metres) 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 the 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, the isostatic rebound from the weight of previous ice sheets, and its interplay with the eustatic change in sea level, have resulted in the old valley of the 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.

Conversion of marshland[edit]

After the river took its present-day course, many of the banks of the Thames Estuary and the Thames Valley in London were partly covered in marshland, as was the adjoining Lower Lea Valley. Streams and rivers like the River Lea, Tyburn Brook and Bollo Brook drained into the river, while some islands, e.g. Thorney Island, formed over the ages. The northern tip of the ancient parish of Lambeth, for example, was marshland known as Lambeth Marshe, but it was drained in the 18th century; the street name Lower Marsh preserves a memory.^[34]
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 Lea can be considered another boundary.^[35] 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.^[34]
Canvey Island in southern Essex (area 18.45 km², 7.12 sq mi; population 40,000^[36]) was once marshy, but is now a fully reclaimed island in the Thames estuary, 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 Roman times.