Cornbrash – Middle Jurassic (Lead author: PJB)
The Cornbrash Formation (Great Oolite Group) ranges in age from Late Bathonian to Early Callovian (c.167-165my). This shallow marine formation varies in thickness from 19m off the Dorset coast to about 10m in boreholes inland. Like the similar micritic Fuller’s Earth rock (Early Bathonian) this is an uncommon building stone, outcropping in the core of the Weymouth Anticline and east of Bridport and in the South and East of Sherborne. At the main outcrop in North Dorset (Bishops Caundle – Stalbridge area) it is divided into Lower Cornbrash consisting of 3-5m of shelly bioclastic lime mudstones, overlain by Upper Cornbrash consisting of 2-4m of calcareous sandstones, sandy micritic limestones and peloidal limestones. Bivalves, brachiopods, ammonites and serpulids are common.
Historically burned for lime, Cornbrash Limestone is also a good insulator. Well-cemented micritic beds of the Lower Cornbrash may have been used in vernacular buildings behind impervious FM, as well as in mixed stone walls.
Cornbrash is most commonly seen as coursed rubble and mixed with Forest Marble but where not too shelly, uniformly micritic and still hard enough, it can occasionally be found as coursed hammer-dressed rubble walling. Weathering and porosity go against this stone so even in villages where it was once most easily quarried and used in stone-built cottages, these buildings are now commonly rendered (cement or pebble-dash) and this vernacular building stone has been almost forgotten.
As a building stone in North Dorset it is most commonly a white, pale grey to cream peloidal lime mudstone with occasional unbroken shell, microfossils and bioturbation. The texture of this stone is what gave it its name. Victorians were all familiar with the brash of cornhusk and chaff remaining on any threshing floor and it well fits the close-up texture of this stone to the eye. At low magnification and in thin section, burrows can also be seen set in this stone now described as a ‘peloidal textured’ limestone. At Puncknowle and in the core of the Weymouth Anticline Cornbrash limestones were used as rubblestone (cream-coloured and occasionally iron-rich blue-hearted).
This close-up of a sawn slab of Cornbrash limestone and these thin section photomicrographs reveal the shelly bioclastic peloidal nature of these limestones.
All photos & thin sections by PJB
Away from Dorset, the harder shelly Cornbrash is more akin to the Forest Marble, too hard to dress as an architect’s choice of stone, so historically both were mainly used in the walling of farms, cottages and rural buildings. The regularly interrupted Cornbrash to Forest Marble sequence extends NNE through Stalbridge into Somerset and almost to Frome.
All text and images by PJB, May 2017