BGS Research — Sinkholes and karst research

Deneholes are medieval chalk extraction pits. They characteristically comprise a narrow shaft with a number of chambers radiating from the base and their depth reflects the depth to the underlying chalk bedrock. The shaft width was commonly in the order of 2–3 m, widening out into galleries at depth.

More recent equivalents (17th to 19th century) are known as chalk-wells and chalkangles (specific to the Berkshire Downs) and are reported to be cruder in construction; both were commonly located close to the field boundaries at the time of extraction.

Rainham Mark Grammar School: view to show the broadening of the cavity beneath the shaft.
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Denehole at Rainham Mark Grammar School, Rainham, Kent, showing the broadening of the cavity beneath the shaft. BGS © UKRI.

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Why were the deneholes dug?

The chalk was extracted from the deneholes for soil improvement and was usually applied directly to the field. However, sometimes the chalk was processed, which involved burning the chalk in kilns to produce ‘quicklime’ (calcium oxide). The addition of water to quicklime formed ‘slaked lime’ (calcium hydroxide), which was a white powder that was applied to fields to neutralise acidity or used in the preparation of mortar.

Once they had reached their limits, deneholes were commonly capped. A variety of capping techniques were used, such as upturned trees or brick arching. Records pertaining to the distribution of deneholes are incomplete, sometimes being limited to features marked as shafts or occurrence of circular depressions on historic Ordnance Survey maps. In the field, they are most likely to be visible as shallow depressions, if at all.

Denehole at a Kent school

The Rainham Mark Grammar School example near Gillingham, Kent, is interesting because the shaft was situated on relatively high ground, possibly reflecting the location of a former field boundary in an area with a relatively thin cover over the chalk.

Bricks in the base of the hole suggest that this feature may once have been capped with a brick arch.

Access courtesy of Mrs D Capelin (School Business Manager) and Mr S Decker (Head Teacher).

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For more information please contact the Sinkhole team

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