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Essex Field Club
Essex Field Club
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Geology Site Account


Little Chishill Borehole (site of), DUDDENHOE END, Uttlesford District, TL45283637, Historical site only

 
 
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Site name: Little Chishill Borehole (site of)

Grid reference: TL 4528 3637

Brief description of site:

Site of an oil exploration borehole sunk in 1964. It provided an excellent section through the bedrock of this part of Essex and penetrated the ancient, hard rocks of the 'Palaeozoic basement'.

The village of Little Chishill is across the border in Cambridgeshire but this particular borehole (one of several in the area) is in Essex.

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Details

Drilled by an independent American oil company, Superior Oil, in 1964, the Little Chishill No.1 Well is one of only a handful of boreholes to have penetrated the Palaeozoic basement of Essex. These ancient rocks consist of laminated mudstones and siltstones with beds of limestone, laid down in the late Devonian period, about 370 million years ago.

The borehole encountered the Devonian rocks at a depth of 278 metres (912 feet) and continued for a further 10 metres (35 feet) to the bottom of the hole at 288 metres (947 feet). These ancient rocks contained fossil shellfish, such as brachiopods and bivalves, and also crustaceans and stem fragments of crinoids (sea lilies). These creatures were living in a shallow sea, at a time when there were volcanoes in Scotland and Devon, and the first animals were beginning to colonise the land.

The total sequence of rocks encountered in the borehole is as follows: After passing through a 6 metre (20 feet) covering of boulder clay, the borehole encountered the Chalk (Upper, Middle and Lower Chalk) which here was a total of 210 metres (690 feet) thick. Beneath the Chalk was 60 metres (200 feet) of Gault, a silty clay containing scattered fossils, laid down in the mid Cretaceous period about 100 million years ago. The base of the Gault lies directly upon the basement rocks, which demonstrates that here, and beneath most of Essex, a considerable thickness of rocks is missing, the gap representing about 270 million years of Earth history. During most of this time, including the whole of the Jurassic period, the London area is thought to have been land with prolonged erosion of the Devonian landscape and no deposition of rocks.

The borehole is the only hydrocarbon exploration well in the county of Essex. The site of the borehole is on the edge of High Wood, south west of Duddenhoe End, on land owned by the Forestry Commission, and is publicly accessible.

 

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Reference: Lake & Wilson 1990 (p.4 & 5), Butler 1981.

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