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.