Mendips header
 Overview maps
 Locality areas
  Cheddar Gorge
 Burrington Combe
 Shipham & Rowberrow
 Crook Peak & Axbridge
 Banwell to Churchill
 Harptree & Smitham Hill
 Draycott & Westbury-sub
 Wookey Hole & Ebbor
 Great Elm & Vallis Vale
 Mells & the Wadbury Valley
 The Vobster area
 The Whatley area
 Torr Works & Asham Wood
 Beacon Hill
 Stoke St Michael & Oakhill
 Holwell & Nunney
 Shepton Mallet & Maesbury
 Gurney Slade & Emborough
 The Nettlebridge valley
 Rocks of Mendips
 Geological timescale
 Ancient environments
 Geological structure
 Minerals and mines
  Minerals and mines
 Industrial archaeology
  Stone as a resource
 Employment & the economy
 Quarrying & geodiversity
 Quarrying & the environment
 History of quarrying
 Caves and karst
 How caves form
 Dry valleys and gorges
 Dolines and sinkholes
 Mendip caves
 Going caving
  Flora and fauna
 Typical Mendip habitats
 Special Mendip habitats
 Horseshoe bats
 Appendix of names
 Biodiversity of western
 Biodiversity of eastern
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  Coal mining
  Mendip quarry companies
  East Mendip quarries
 Biodiversity of eastern
  West Mendip quarries
 Biodiversity of western
 Site map

The rocks of Mendip

Silurian | Devonian | Lower Carboniferous | Upper Carboniferous | Triassic | Lower to Middle Jurassic

Upper Devonian rocks (385 to 359 million years ago)

Devonian rocks form the highest ground on the Mendips, rising to around 300 m in the cores of the anticlinal folds of Blackdown, North Hill, Pen Hill and Beacon Hill. Where the base of the succession is unproved at Blackdown, in the western Mendips, there are at least 500 m of Devonian strata, reducing to 400 m on Beacon Hill where Devonian rocks unconformably overlie Silurian volcanics. The Devonian of the Mendips is represented by the Portishead Formation, informally known as the Old Red Sandstone.

Portishead Formation (Old Red Sandstone)   Portishead Formation
(Old Red Sandstone)

These rocks were deposited by meandering river systems flowing across an arid continental environment at the southern margin of a large landmass. The rocks comprise dull red sandstone, interbedded with fine-grained mica-rich sandstone and sandy green, red and purplish shales and mudstones. Pebbly sandstones and conglomerates occur in the lower part of the succession, and thick beds of mudstone become more common towards the top. The Portishead Formation belongs to the upper Devonian; older Devonian rocks, represented in the nearby Clevedon area by the Black Nore Sandstone Formation and overlying Woodhill Bay Conglomerate are absent. A plant and fish-bearing conglomerate occurs near the top of the formation at Burrington, on the north side of Blackdown, and may equate with a similar bed in the Bristol district named the Sneyd Park Fish Bed. The fossilised remains of plant spores show that the Devonian / Carboniferous boundary in the Mendips is within the top of the Portishead Formation; the marine conditions that are usually typical of the Carboniferous affected the Mendips slightly later than elsewhere.

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