Deneholes

Rainham Mark Grammar School: view to show the broadening of the cavity beneath the shaft.

Deneholes are medieval chalk extraction pits; characteristically they comprise a narrow shaft with a number of chambers radiating from the base.

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

The depth of the features 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.

Soil improvement

The chalk was extracted for soil improvement and was usually applied directly to the field.

However, sometimes the chalk was processed. The processing involved lime burning in kilns, where the chalk was burnt to produce quicklime (calcium oxide).

The addition of water to quick lime forms slaked lime (calcium hydroxide) a white powder, which can be applied to fields to neutralise acidity or can be used in the preparation of mortar.

Once they had reached their limits deneholes were commonly capped. A variety of capping techniques were used; examples include the use of 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 Gillingham school

Rainham Mark Grammar School: surface expression of denehole collapse that occurred on 12 February 2014.
Rainham Mark Grammar School: view into the base of the denehole collapse.

The Rainham Mark Grammar School example, see recent sinkholes map, is interesting in that 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.

Contact

For more information on deneholes, please contact Dr Vanessa Banks or Dr Andy Farrant