Geological Conservation Review (GCR) sites are selected for,
and grouped according to, GCR blocks, which are site-selection
categories for nationally important Earth science sites.
There are around 100 'blocks' and about 3,000 GCR sites in
Britain. Each 'block' represents a particular geological age
or Earth science theme, such as Marine Permian Stratigraphy,
Jurassic–Cretaceous Reptilia, Caledonian Igneous Rocks and
Quaternary of Scotland.
The volcanic (igneous) geology blocks relate to the formation
of rocks originating from magma (molten rock) according to major
periods of mountain building activity, such as during the
Caledonian mountain-building episode.
Major episodes of volcanic activity form the basis of six
igneous Geological Conservation Review blocks, and these are linked
to mountain-building (e.g. south-west England Igneous), and the
opening of oceans (e.g. Tertiary Igneous).