page_title: Geography of Somerset
section_title: Geology
hierarchical_section_title: Geography of Somerset / Geology
caption_attribution_description: English: Cheddar Gorge Rock face on the north rim.
context_page_description: The county of Somerset is in South West England, bordered by the Bristol Channel and the counties of Bristol and Gloucestershire to the north, and Wiltshire to the east, Dorset to the south, and Devon to the west. The climate, influenced by its proximity to the Atlantic Ocean and the prevailing westerly winds, tends to be mild, damp and windy.
  Somerset is predominantly a rural and agricultural county. The main upland areas are the Mendip Hills in the east and the Quantock Hills further west, the Blackdown Hills form the county's southern border, and Exmoor is on the western fringes. Between the Mendips and the Quantocks is the large area of flat, low-lying ground known as the Somerset Levels. The county's main rivers are the River Axe in the northeast, and the Rivers Brue and Parrett which flow northwestward through the levels into the Bristol Channel.
  The landscape is largely determined by the underlying geology. The Carboniferous Limestone that forms the Mendips has been eroded to form gorges and caves. Exmoor is an extensive area of moorland and a National Park and the Somerset Levels contains wetland areas of international importance for birds.
context_section_description: The oldest rocks are of Silurian age (443–419 million years ago). They make up a sequence of lavas, tuffs (volcanic ash), shales and mudstones in a narrow outcrop to the northeast of Shepton Mallet in the eastern Mendip Hills. Rocks from the Devonian (419–359 million years ago) are found across much of Exmoor, the Quantocks, and in the cores of the folded masses that form the Mendip Hills.
  Carboniferous Period (359–299 million years ago) rocks are represented by the Carboniferous Limestone that forms the Mendip Hills, rising abruptly out of the flat landscape of the Somerset Levels. The limestones contain fossils of crinoids (sea-lilies), corals and brachiopods, providing evidence of the abundant marine life that existed in the shallow tropical seas that covered these areas at that time. Some hills, such as in the Mendips, are in excess of 800 feet (240 m) above sea level, and from them flow rivers of substantial erosive power. Examples of erosion may be seen at Cheddar Gorge and the caves within it, where the soft limestone has been scoured into gorges and caverns of great depth and length. At the end of the Permian (299–252 million years ago) and Triassic periods (252–201 million years ago), the Variscan orogeny resulted in the uprising of several mountainous areas including Dartmoor to the south, Exmoor, the Quantocks and the Mendips.
  Much of the landscape falls into types determined by the underlying geology. These landscapes are the limestone karst and lias of the north, the central clay vales and wetlands, the oolites of the east and south, and the Devonian sandstone of the west.
  To the north-east of the Somerset Levels, the Mendips are moderately high limestone hills. The area of the central and western Mendip Hills was designated an Area of Outstanding Natural Beauty in 1972 and covers 198 km² (76 sq mi). The main habitat of these areas is calcareous grassland, with some arable agriculture. The Somerset Coalfield is part of a larger coalfield stretching into Gloucestershire. To the north of the Mendip hills is the Chew Valley and to the south, on the clay substrate, are broad valleys supporting dairy farming and rivers draining into the Somerset Levels.
caption: The Cheddar Gorge was carved by water
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