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.
Fossil (palaeontological) blocks address the evolution and
diversity of significant animal and plant groups which are not
included in the stratigraphy (rock sequence) blocks and
therefore have independent block status. Most invertebrate
fossils (e.g. trilobites, echinoderms, ammonites and other
molluscs) are addressed within the stratigraphical blocks, because
these fossils are widely used in correlating rock strata.
However, because of the relative rarity of fossils such as
reptiles, fish, mammals, birds, terrestrial plants, insects and
other arthropods (excluding trilobites), these are covered in
separate palaeontological blocks. Geological time is used as
the basis to refine the definition of some blocks, for example,
Jurassic–Cretaceous Reptilia.