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
 External links
 Detailed site information
  Coal mining
  Mendip quarry companies
  East Mendip quarries
 Biodiversity of eastern
  West Mendip quarries
 Biodiversity of western
 Site map
The Geology of the Mendip Hills
With the exception of a small area of Silurian volcanic rocks, the Mendips are made up of sedimentary rocks ranging in age from Late Devonian (approximately 385–359 million years old) to Mid Jurassic (about 161 million years ago), dominated by the Carboniferous Limestone. Sedimentary rocks are formed by the accumulation of debris, such as sand and mud or the remains of organisms such as shells, in a range of different environments. Over time sediments are buried and harden into rock, a process known as diagenesis, thus forming sandstone, mudstone or limestone, depending on the original sediment. These rocks have often been uplifted, folded and faulted into complex geological structures, or eroded to produce unconformities.

Life in the Silurian sea. Artwork  Paul Stevenson
  The classic De la Beche unconformity at Vallis Vale

Sediments are usually deposited in a series of layers or beds, which may vary in thickness and character and contain features that are diagnostic of certain environments, such as ripple marks. A sequence of beds that forms a mappable unit of rock is known as a 'formation' and is given a name, for example the Charmouth Mudstone Formation. A series of successive formations that show broadly similar characteristic are known collectively as a 'group', or 'subgroup' and also given a name, for example the Avon Group.

The Mendips are characterised by a wide variety of sedimentary rock types formed in a range of different ancient environments from tropical seas, to coal swamps and arid deserts, and more recently cold glacial climates. This section outlines the geological history and describes the major rock units and fossils that can be seen as you explore the area.

Pyroclastic flow on Montserrat, a similar volcanic eruption occurred in the Beacon Hill area over 400 million years ago.
goto the British Geological Survey home page