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.

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.