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The origins of the London Basin

Aerial view of the River Thames
An ice sheet re-routed the Thames to come through London half a million years ago.

Geologists describe London as the 'London Basin'.

This is because it is literally a thick layer of clays and sands all piled up into a large depression - a basin.

The basin walls are made of chalk but what does this tell us about London's long and distant past?

The chalk represents the product of a massive global warming.

The result of this event for the Earth was a huge rise in sea-level flooding most land areas.

In this massive sea the area of London was a long distance from land and only the fine white mud we call 'chalk' rained out as billions and trillions of tiny algal skeletons.

Soon, a thick (600m) blanket of white sediment had formed and the sea level gently fell again.

The result was an extremely flat white expanse of chalk exposed as the new land area.

Trafalgar Square
Lions and bears once roamed this famous location.

Weathering resulted in some of the chalk dissolving away, but 50 million years ago an important event happened; Africa started pushing into Europe.

One result of this was the Alps and another was the 'buckle' in the rocks producing the basin shape that was to become London.

Over the following millions of years rivers criss-crossed the area and the sea came and went again, the latter event leaving a thick deposit known as the London Clay.

The final dramatic part of the London rock story was when the Ice Age arrived.

Temperatures plummeted and London became tundra several times over the final 2.6 million years of its history.

THAMES FACTS
The River Thames is 215 miles long
There are 44 locks on the non-tidal Thames, which begins near Cirencester and ends at Teddington Lock.
The first bridge in the capital was located where the current London Bridge stands. It was built by the Romans around 2,000 years ago
French Impressionist Claude Monet painted the Thames three times.
In pre-19th century London, cold winter weather would sometimes freeze the surface of the Thames. 'Frost Fairs' would be held on the ice
fact here
Before engineer Sir Joseph Bazalgette built London's sewerage system, much of the capital's waste was dumped in the river

In between the times when London became an icy wasteland, there were warmer periods when animals, and even ancient man, returned to bask in the warm temperatures.

This has left an interesting set of evidence that is uncovered every time anyone digs into the ground to build office blocks, new roads, or to lay pipes.

For instance, evidence dug up in Trafalgar Square included bones of lions, hippos, elephants, bears, bison and deer - proving that these roamed London before the pigeons.

It was during the Ice Age that an event happened which caused a change in London's future history.

An ice sheet re-routed the Thames to come through London for the first time only half a million years ago.

Before this time the Thames ran from its source area in the Midlands through Oxfordshire then eastwards to Hertfordshire and out into the North Sea at Ipswich.

Once the new path was founded, the river cut down into its valley at least 10 times.

Each new 'cut' left a series of sand and gravel terraces marking each event.

These terraces are named after the area they are best known in, for example: Dartford Heath Gravel, Swanscombe, Orsett Heath, Corbets Tay, Mucking, West Thurrock, Kempton Park, Shepperton, Staines and Tilbury Gravels.

Article courtesy of Dr Jill Eyers Open University Associate Lecturer, freelance geologist and geo-archaeologist.





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