Geology Site Account

Canvey Island Borehole (site of)

Summary

Historical site only. The Canvey Island borehole is one of very few in Essex that have penetrated the deep 'Palaeozoic basement'. Rocks encountered at the base of the borehole dated from the Devonian period - about 380 million years old.

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Site description

During the Geological Survey's coal exploration programme in the 1950s it was thought that Coal Measures (of Carboniferous age) existed in a concealed depression in the Devonian rocks beneath Canvey Island. This assumption was not unreasonable considering the proximity of a known concealed coalfield in east Kent. Evidence for the existence of this depression came from measuring the variation in the force of gravity over an area and comparing this with an assumed average gravity value, thus producing a contour map with contour lines joining points of equal gravity. The map revealed a 'negative gravity anomaly' at Canvey Island (thought at the time to be due to the low density of Coal Measures sediments present in the depression) and in 1953 a borehole was drilled to test this theory.

The borehole passed through alluvium, London Clay, Lower London Tertiaries, Chalk, Lower Greensand and Gault Clay. Beneath the Gault Clay the 'Palaeozoic basement' was reached at a depth of 1320 feet (400 metres). Instead of coal, these basement rocks consisted of hard Old Red Sandstone of Devonian age and the borehole was abandoned at a depth of 1679 feet (510 metres). Although unsuccessful in its objective, the project nevertheless provided a wealth of information, such as fossil plants from the 380 million year old Devonian rocks, which helped to accurately date the sequence of rocks deep beneath this part of Essex. The borehole was also the first in Britain to measure the direction and dip of sub-surface rocks by the use of sophisticated instruments.

It is an interesting thought that, had the borehole results been different, Canvey Island might have become a coal mining town; an example of how geology affects the character of the landscape.

The entire length of the core (over 500 metres) is available for study at the British Geological Survey's headquarters in Keyworth, near Nottingham.