Geology Site Account
Stock borehole, STOCK , Chelmsford District, TL70540045, Historical site only
Site name: Stock borehole
Grid reference: TL 7054 0045
Brief description of site: Historic borehole sunk in 1970 by the British Geological Survey to obtain a complete record of the entire thickness of the London Clay.
Before the 1970s what was known about the London Clay had been obtained from isolated boreholes, pits and coastal cliffs and so in 1970 the Institute of Geological Sciences (now the British Geological Survey) decided to sink a deep borehole specifically to obtain a complete section through the entire thickness of this formation. It was important to choose a site where the London Clay was capped by Claygate Beds and Bagshot Sand to ensure that no part of the London Clay was missing and so the borehole was sunk on the high ground north of Stock, the exact site being behind the Ship Inn on the Stock Road.
The total thickness of London Clay encountered was 150 metres (500 feet) down to the base of the formation where the borehole entered shelly sands which were classified as Oldhaven Beds. From the information obtained it was then possible to subdivide the London Clay into units which were each recognisable by the nature of the clay and the fossils present. This borehole, together with another at Sandpit Hill, Hadleigh, enabled the complete succession to be studied for the first time. It has also shed more light on the changing nature of the muddy floor of the London Clay Sea together with the evolution of the creatures that inhabited it 50 million years ago.
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Reference: King 1981 (p. 45-46)
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