The guidelines for the selection of geological and
physiographical SSSIs are published by JNCC in
volume 1 of the Geological
Conservation Review (GCR). 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 3000 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.