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About 56 million years ago the land had once again risen above the sea allowing the Reading Beds to be formed by rivers in a sub-tropical climate and it was here that Hertfordshire Puddingstone formed .
Further earth movements forced part of the county back beneath a shallow sea. Rivers flowing into this sea resulted in the formation of the London Clays which is rich in important fossils.
The last 2 million years features a series of rapid climatic changes. Long cold glacial periods with short milder interludes. Indeed about 300,000 years ago Hertfordshire was covered by ice estimated to be up to a 1000 meters thick.
Before the ice advanced the River Thames (Proto-Thames) flowed through the Vale of St Albans to the North Sea. It was at this time that the large quantities of boulder clay, sand and gravel found throughout the county were deposited by the early Thames and the action of the glaciers
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