DORSET BUILDING STONE
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 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.
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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.
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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). 
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Sawn slab of shelly Cornbrash limestone.
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Brachiopod in bioclastic matrix. Holwell. Slide PJB 155
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Echinoid spine in bioclastic matrix (XPL). Holwell. Slide PJB 155

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

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Bivalve shell in bioclastic matrix (PPL). Holwell. Slide PJB 155
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Bivalve shell in peloid matrix (PPL), Sparite outer layer after aragonite. Holwell. Slide PJB 155
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
Picture

© DORSET BUILDING STONE
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  • Home
    • About Us
    • Glossary
    • References and Sources
    • Work in Progress
  • Building stones
    • How we study building stones
    • Palaeogene >
      • Heathstone
      • Sarsens
    • Cretaceous >
      • Chalk
      • Flint
      • Upper Greensand
      • Chert
      • Wealden
      • Purbeck >
        • Upper Purbeck (Purbeck Marble)
        • Upper Purbeck (Broken Shell Lst. aka Burr)
        • Middle Purbeck (Upper Building Stones)
        • Lower Purbeck (Cypris Freestones)
    • Jurassic >
      • Lower Purbeck >
        • Cypris Freestones
      • Portland Stone - Dorset
      • Chert
      • Kimmeridge Bay Dolomitic Limestone
      • Corallian
      • Cornbrash
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      • Inferior Oolite
      • Bridport Sands
      • Junction Bed
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    • Non-Dorset Stone >
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        • Bembridge Limestones - Isle Of Wight
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        • Beer Stone - SE Devon
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        • Blue Lias of Somerset
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        • Portland Limestone – Wilts (Chilmark)
        • Normandy >
          • Pierre de Caen (Caen Stone)
        • Pas de Calais
      • Triassic: White Lias of SE Devon
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        • Carboniferous Limestones: Polished Black
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    • Building Stone Trails >
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    • Stone Index >
      • Stone Index A-B >
        • Abbotsbury Ironstone
        • Bath Stone
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        • Blue Lias Stone
        • Bridport Sandstone
        • Broken Shell Limestone
        • Burr
        • Burton Limestone
      • Stone Index C-E >
        • Caen Stone
        • Carstone
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        • Chilmark Lower Building Stone
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        • Chilmark Upper Buiding Stone
        • Clavellata Beds
        • Cliff Stone
        • Clunch
        • Corallian
        • Cornbrash
        • Cypris Freestone
      • Stone Index F-O >
        • Featherbed Limestone
        • Featherstone
        • Forest Marble
        • Flint
        • Fuller's Earth Rock
        • Grey Chalk
        • Ham Hill Stone
        • Heathstone
        • Inferior Oolite
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        • Lower Chalk
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        • Middle Chalk
        • Middle Lias
        • New Vein
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      • Stone Index P-T >
        • Portland Cherty Beds
        • Portland Cherty Series
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        • Rag
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        • Sherborne Stone
        • Tisbury Stone
        • Todber Freestone
        • Trigonia Beds
      • Stone Index U-Z >
        • Upper Chalk
        • Wardour Lower Building Stone
        • Wardour Main Building Stone
        • Weald Clay
        • White Chalk
        • White Lias
        • Yeovil Stone
  • Churches
    • A-Z of Churches
    • East Dorset >
      • Almer
      • Bournemouth >
        • Talbot Village
      • Canford Magna Chapel
      • Corfe Mullen
      • Christchurch Priory
      • Cranborne
      • Edmondsham
      • Gussage All Saints
      • Gussage St. Andrew
      • Hampreston
      • Kinson
      • Knowlton
      • Morden
      • Pamphill
      • Poole St.James
      • Poole, the Spire Church
      • Shapwick
      • Sturminster Marshall
      • Tarrant Crawford
      • Tarrant Keyneston
      • West Parley
      • Wimborne St.Giles
      • Wimborne Minster Exterior Tour
      • Wimborne Minster Interior
      • Wimborne Minster: Purbeck Marble and Decorative Stone
    • Central Dorset >
      • Bloxworth
      • Bradford Peverell
      • Cerne Abbas
      • Hilton
      • Lytchett Matravers
      • Stratton
      • The Piddle Valley >
        • Affpuddle
        • Alton Pancras
        • Bere Regis
        • Piddlehinton
        • Piddletrenthide
        • Puddletown
        • Tolpuddle
      • Winterborne Tomson
    • North Dorset >
      • Batcombe
      • Belchalwell
      • Blandford Parish Church
      • Blandford St. Leonard's Chapel
      • Blandford St. Mary
      • Blandford Cemetery Chapels
      • Bradford Abbas
      • Chetnole
      • Child Okeford
      • Compton Abbas
      • Farnham
      • Folke
      • Gillingham
      • Hammoon
      • Hazelbury Bryan
      • Holnest
      • Ibberton
      • Iwerne Minster
      • Langton Long
      • Leigh
      • Mappowder
      • Margaret Marsh
      • Marnhull
      • Melbury Abbas
      • Melbury Bubb
      • Pimperne
      • Okeford Fitzpaine
      • Purse Caundle
      • Ryme Intrinseca
      • Shillingstone
      • Sixpenny Handley
      • Stockwood
      • Stourpaine
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      • Shipton Gorge
      • Lulworth Castle
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      • Symondsbury
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  • Quarries , Pits and Limekilns
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  • Secular buildings
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    • Blandford Town Hall
    • Cerne Abbas (village and Abbey)
    • Durweston Bridge
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