Heydon Chalk Pit Local Geological Site

The disused pit, owned by Heydon Parish Council, was designated a Local Geological Site in February 2023.  The site is on the north west facing scarp slope of the Chalk ridge that becomes the Chilterns further to the south west. As with the rest of the county, the bedrock strata dip slightly to the south east, so older Chalk occurs at the surface to the north west and younger to the south east. As mapped by the British Geological Survey, the site lies on bedrock of the Lewes Nodular Chalk, which is the youngest of the Chalk strata found in Cambridgeshire and only found in the far south east of the county near the borders with Hertfordshire, Essex and Suffolk. There are few exposures of the Lewes Nodular Chalk in Cambridgeshire.

Complicating the geological picture in the area of Heydon and Great Chishill is the presence of glacio-tectonic structural features that are indicative of disturbance of the Chalk at an ice-contact margin. There is Chalk breccia here, either glacial or periglacial in origin, and this is overlain at the southern end of the Pit by Glacial Till (diamicton), which is regarded as part of the Lowestoft Formation, dating from the Anglian glaciation, which ended about 425,000 years ago.  Also, it seems apparent that at least part of the exposures in the Pit represent Chalk rafting, where large blocks (rafts) of Chalk from older (and topographically lower) strata have been removed by ice and carried up the scarp slope to be left overlying younger Chalk. The evidence is mainly from the presence of silty layers of till within the Chalk, marking the boundary of the raft(s), and these can be seen at this site. The (Quaternary) Till is much younger than the (millions of years old) bedrock and any Chalk lying over it must have been moved after the Till’s deposition. The strata appear to be faulted, with laminae of red-brown silt along bedding planes that are tilted in a direction contrary to the Chalk’s regional dip to the south east. The quarry is mentioned as a site for Chalk rafts in relation to the Anglian glaciation, in at least two publications. Barkway Chalk Pit in Hertfordshire is a good example of another Chalk raft site.   .

The quarry may have been dug here to reach the Chalk Rock, a hard band of Chalk that lies at the base of the Lewes Nodular Chalk. Its linear outcrop forms a band around the Chalk slope just to the north. This would have provided a harder rock than the rest of the Lewes Nodular Chalk or the underlying New Pit Chalk. Flint nodules are found in the Lewes Nodular Chalk and, less commonly, tabular flint.

It is a safe and accessible site to visit (maintained by the Parish Council as a community green space) but care must be taken on approaching the Chalk exposures and no attempt should be made to disturb any of the Chalk. It is an excellent site to tell the story of the formation of the Chalk, and the effects of glacial and periglacial conditions.