The London Thames is lies within a chalk trough, with the North Downs to the south and Chilterns Hills to the north. Over much of the course of the London Thames geological deposits comprise London Clay, which is a stiff bluish-grey marine deposit, famous for its fossil inclusions. This deposit can be up to 150 metres thick below the city. This London Clay was formed during the 60-50 million years ago during a time when Britain was under warm tropical seas.

Around 40 million years ago these tropical seas gradually receded and dry land emerged. It was during this period that the origins of the Thames river system developed. This gradual development of the Thames continued on a steady trajectory until approximately 2-3 million years ago, when a much enlarged proto-Thames formed a tributary to the ancestor of the modern Rhine. During this period, the Thames would have followed a course to the north of its current course.

However. the development of this proto-Thames was severely disrupted with the coming of the last Ice Age about 110,000 years ago. The glacial advance had the effect of pushing the course of the river gradually southward, approximating something like its present course.

As the Thames was gradually diverted southwards, the river eroded the London Clay valley and created a series of sand and gravel terraces, whilst depositing heavier sediments, such as sands and gravels on its margins as sea levels continued to fall. In places, there are deposits of brick-earth, which is a mixture of clay and sand that has been the source of London’s long-standing brick-making industry.

Towards the end of the last glaciation, the release of water contained within the melting glacial ice caused a global rise in sea level which had the effect of halting the down-cutting Thames and terrace formation. Improvements seen at the end of the last Ice Age, roughly 12,000 years ago, continued up until the Bronze Age, about 4000 years ago, when climatic changes caused the development of peat deposits all along the London Thames foreshore.