Deneholes

Chalk is a porous, sedimentary carbonate rock formed millions of years ago, during the late Cretaceous period, by the compression of microscopic plankton that settled on the sea floor. The chalk section in Royston Cave is thought to be of Early Turonian age, in the bottom layer of the New Pit Chalk Formation, otherwise known as Middle Chalk. This chalk layer is near to the surface and only covered by topsoil or loam, a fertile soil of clay and sand.

Historically, chalk was an important material, particularly for building, so it was heavily mined. Kilns could then be used to burn the calcium carbonate found in the chalk to temperatures around 900 degrees celsius. This produced a form of lime called quicklime (calcium oxide) which was used to make bricks, mortar and fertiliser. The Romans and Saxons were known for mixing lime with water to make a mortar and, in the 12th century, limewash was used to coat walls.

Early, small chalk mines are described as dene holes or bell pits. A narrow, vertical shaft would be dug and, once the chalk layer was reached, a chamber was excavated, often in two sets of three to give a double cloverleaf or trefoil pattern but sometimes just as a single bell shape. The bell shape helped spread the load of the material above and reduce the risk of the cave collapsing.

The chalk would be cut using a short-headed iron pick, working forward in a series of steps or benches. It was then hauled to the surface in a basket using a small winch mounted over the shaft. Most dene holes have deep grooves visible at the base of the shaft where the hauling ropes have cut into the soft chalk. When excavation had finished, tree stumps or bushes were often thrown down the shaft to jam it, before being backfilled to the surface. Sometimes, a brick arch was placed at the top of the shaft to cover the entrance, as seen at the dene holes found at Temple Farm Strood.

As well as a building material, chalk could be spread onto agricultural land to either improve the soil’s texture, if it was clay based, or to raise its pH if it was of high acidity. By excavating a narrow shaft to reach the chalk below the farmland, rather than collecting chalk from the surface, the miners used as little of the productive agricultural land as possible. Chalk taken from further below the ground was also thought to have a greater mineral content.

Chalk within Royston Cave. © Royston Cave

Royston Cave is a manmade, subterranean structure cut approximately 9m down into the chalk that lies beneath Royston. The lowest part of the cave is cylindrical, approximately 3m tall and 5m in diameter. The apex of the cave, at the time of its discovery in 1742, was a dome of chalk and tiles, said to be only 30cm from the surface level.

Some people believe that Royston Cave was originally a dene hole or mine, particularly because of its bell shape and location. It was common practise to locate dene holes as close as possible to the destination of the chalk, to save transporting heavy loads by horse and cart. As the cave is situated so close to an ancient crossroad, it is thought, perhaps, that the chalk from the cave might have been used for road building and maintenance. Then, once there was no longer need for a mine, the cave may have been repurposed, the tiled dome put in place, and a new entrance shaft built to complete the alterations.

Similar pits were also dug to mine flint embedded in the chalk. At Grimes Graves in Norfolk, for example, Neolithic miners dug shafts up to 13m deep to where the best flint lay, prising the flint from the rock with picks made from antlers. The chalk around Royston, however, contains only occasional, small flint nodes which is unsuitable for flint axes or tools, so it is unlikely that Royston Cave was ever a flint mine.

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References

Beamon, S.P. (1992). Royston Cave: Used By Saints or Sinners? Baldock: Cortney Publications.

A special thank you to Rod Legear from Kent Underground Research Group for all of his knowledge and support