Orwell Clunch Pit LGS

This site is an ancient pit created by extensive quarrying of the Upper Cretaceous West Melbury Marly Chalk. It now forms a south facing hollow beneath Toot Hill. Much of the ground is chalk grassland with some exposures of Chalk debris and bedrock. This is a good site to see the effects of periglacial activity in the interaction of the chalk with the diamicton at the very top of the cliff. Sites showing clear evidence for periglacial activity over the millennia are generally quite rare, but here it is very evident in the involutions of chalk putty in brecciated Chalk and diamicton in the top 50-100 cm of the cliff.  The diamicton dates from the Anglian glaciation and possibly includes material derived from Oadby Till.

There is no other place in the county where this can be easily seen since the only other recognised occurrence was at the Barrington Chalk Pit where it has now been removed by quarrying. The site is proposed as suitable for further educational field-studies examining periglacial geological processes. This Chalk pit shows the use made by early settlers here of the Chalk for building purposes – both of clunch blocks and lime-wash for rendering – as well as for liming the fields for agriculture.

The site is significant in its setting in the wider Cam Valley as it has spectacular views across the Rhee Valley towards the Chalk hills near Great Chishill near the county border. It is a well-used recreational area and known to local people and botanists for its chalk grassland flora. It is designated as an SSSI for its biodiversity value.

These small, local parish pits are a feature of south east Cambridgeshire. Most have now been lost to development and infill. Although this pit does not contain any of the more extensive harder bands that were used in more spectacular buildings such as the Cambridge colleges and the cathedrals of the area, it did produce some the distinctive clunch building stone that can be seen in local churches and vernacular buildings. The clunch used in the church from the twelfth century almost certainly came from this site. Clunch was still being excavated here as recently as the 1930s for use on farmland to correct soil acidity and to repair roads.

 

The Parish Council have erected information boards which mention the geology of the pit. This is an easily accessible, visible example of the exploitation of Chalk for the purposes of building and agriculture that has produced a beautiful area of Chalk grassland below the viewing area above the cliff.

Bronze and Iron age settlements have been found on Toot Hill, which the quarry cuts into. The presence of springs near the pit (at the base of the Chalk contact with the underlying Gault clay) would have attracted early settlers to the area.

In terms of lithology, the West Melbury Chalky Marl is a buff/grey, soft, clayey chalk with some traces of Glauconite: the high clay content makes it almost impermeable.

Whereas the BGS 1:50K digital geological mapping indicates that periglacial deposits are absent from the site, work by the CGS confirms earlier paper mapping (BGS, 1976) in that these deposits are present at the site, with the diamicton thinning markedly towards the cliff face.