Chalk
| Series | Stage | Ma | Traditional | Group | Formation | Member | Use | ||
|---|---|---|---|---|---|---|---|---|---|
| Upper Cretaceous | Maastrichtian | 72.1 | |||||||
| Campanian | 83.6 | Dorset | |||||||
| Upper Chalk | Chalk | White Chalk | Studland Chalk | Flint, Lime | |||||
| Portsdown Chalk | |||||||||
| Culver Chalk | Spetisbury Chalk | ||||||||
| Tarrant Chalk | |||||||||
| Newhaven Chalk | |||||||||
| Santonian | 86.3 | Spurious Chalk Rock | |||||||
| South East Devon | |||||||||
| Coniacian | 89.8 | Seaford Chalk | |||||||
| Lewes Nodular Chalk | |||||||||
| Turonian | 93.9 | ||||||||
| Middle Chalk | New Pit Chalk | Clunch, Lime, Flint | |||||||
| Holywell Nodular Chalk | |||||||||
| Cenomanian | 101.0 | Lower Chalk | Plenus Marls | ||||||
| Grey Chalk | Beer Head Limestone | Zig Zag Chalk | Beer Stone, Clunch, Lime | ||||||
| West Melbury Marly Chalk | |||||||||
| Glauconitic Marl | |||||||||
| Lower Cretaceous | Albian | Selborne | Upper Greensand |
||||||
© 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).
Riverside cottages in Litton Cheney. The furthest is built of Chalk, the nearest is Forest Marble. Photo: JT
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).
Lower Chalk.The lowest, the Grey Chalk, has crystals of glauconite and some sand grains. It is used in several buildings, including The Old Bell in Cerne Abbas. Photo: JT
New Inn, Cerne Abbas. A block of Cenomanian Chalk showing a characteristic fossil, in the corner of the New Inn, Cerne Abbas. Photo: JT
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.
Barn interior at Shroton (Iwerne Courtney). Chalk is often used for the interior walls in barns, as it is can be kept clean and healthy for the animals. Photo: JT.
The Shroton estate. The wall round the Shroton estate is built of Chalk, Upper Greensand and Corallian limestone. Photo: JT
These cottages in Sydling St. Nicholas are built of banded flint and Chalk. Photo: JT
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.
|
North Barn in Cerne Abbas was built of large blocks of Middle Chalk from the quarry on Giant's Hill. Photo: JT
Middle Chalk. Detail of the Middle Chalk in North Barn, Cerne Abbas. Photo: JT
Cattistock. A group of cottages with Chalk walls on an Upper Greensand plinth. Photo: JT
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.
Farmyard wall. This farmyard wall in Iwerne Courtney is built of Chalk, Upper Greensand and Flint. Photo: JT
Long Bredy. The barn wall at Baglake Farm, at the eastern end of Litton Cheney, is built of Chalk from the parish quarry, and Corallian limestone from the farm quarry. Photo: JT
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
Feb 2017 
Lamperts cottage in Sydling St. Nicholas is built of Chalk block, including some glauconitic Chalk, which suggests that it may have come from the quarry on Giant Hill in Cerne Abbas. Photo: JT
The Fox & Hounds. The Lower Chalk has been used for the walls of the Fox & Hounds inn in Cattistock. can you spot an interesting fossil? Photo: JT
|
