| 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.