The Search for Coal in Essex
Canvey a Coal Mining Town?
The extract below is from‘Essex Rock. A look beneath the Essex Landscape’ written by Gerald Lucy. The extract tells us how there was a possibility that Canvey could have been a coal mining town if the borehole that was drilled in 1964 had found what was thought to be deep in the Devonian rocks below Canvey. What kind of place would Canvey have been? We have had Gas, we were threatened with Oil and by the sounds of it they thought we had Coal. What is it about Canvey that makes it such an important place?
- The only way to prove which rocks are present deep below the surface is to drill a borehole, but this is expensive and therefore the location is carefully chosen so that it is likely to add to our knowledge of the sub-surface geology. This was particularly important during the Geological Survey’s coal exploration programme in the 1950s when 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 the 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 1964 a borehole was drilled to test this theory (fig. 10). Instead of coal, low density rocks of Devonian age were found, but the borehole nevertheless provided a wealth of information, such as Devonian fossil plants, which helped to accurately date the sequence of rocks. 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.