© GR 2020
Chalk – Late Cretaceous limestone
The geological unit The Chalk Group is up to 300 metres (1000’) thick in Dorset but less than 100m in SE Devon due to erosion of most of the sequence. The traditional divisions were Lower Chalk (Cenomanian), Middle Chalk (Turonian) and Upper Chalk (Coniacian to Maastrichtian). The Chalk Group slowly accumulated over 34 million years (100 to 66 my).
Chalk is a generally soft white marine lime mudstone. The dominant component is microscopic planktonic calcareous algae debris (coccoliths), invisible to a hand lens or standard microscope. Sometimes, sand-sized bioclastic debris (bivalve & echinoderm) reaches a high proportion and a hard “gritty” rock develops. At times planktonic organisms and sponges contributed silica to the sea bed which lead to the formation of flint (qv).
The quarry on Giant Hill, Cerne Abbas, reaches up to the flint-rich Seaford Chalk Formation (middle of the White Chalk Subgroup, formerly Upper Chalk), but only the Cerne Abbas North Barn, with large flint nodules, is clearly from this Formation.
Note: The stone used for building in Cerne Abbas and Sydling St. Nicholas could have been re-used from the Abbey, or could have come straight from the quarry on Giant Hill. My instinct is that the early buildings in the Abbey used Cypris Freestones from Poxwell, and that later building used the various Chalks from Giant Hill. This is based on the order in which the stone was used in buildings such as the Old Bell in Cerne Abbas, and the dates of individual buildings given in the architectural history.
Villages in the Chalk downlands often have a Parish Quarry in the Chalk and, where a stream has cut down to the Zig-Zag Formation (Grey Chalk Subgroup, formerly upper Lower Chalk), this has been successfully used for building in cut blocks known locally as Clunch. The Zig Zag Chalk has a greyish look in a building as it contains scattered grains of quartz sand, glauconite and clay minerals (comprising a marl). In some blocks there are phosphatic nodules and characteristic Cenomanian fossils.
Stone from the White Chalk Group tends to be soft and is often rendered. However, it was often used for interior walls, both in domestic and farm buildings. All the Chalk was used to make cob, but this was also often rendered. Chalk pits were often for production of lime mortar and whitewash (rather than for spreading on the chalky land). Text: WGT & JT. Photos: JT