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.
For the most part, sedimentary rock sequence sites are
classified according either to their stratigraphical age (Stage,
Period) or to a range of stratigraphical ages (e.g. Caradoc–Ashgill
block). Blocks for some stratigraphical ages, however, were
defined not purely by age, but also by geographical area or
environmental setting where there were significant variations in
rocks across Britain formed at the same time. This is why
there are two blocks for the Devonian Period; one for marine rocks
and one for non-marine rocks.
It is not possible in every case to define the stratigraphy
GCR blocks by stratigraphical age. For example, where fossils
are rare or absent it is difficult to locate the boundary between
different geological ages. Such units are named after the
geographical localities where they were defined, for example, the
Wealden Group, which consists of mudstone, shale and sandstone
which only occur in south-east England.
Most invertebrate fossils (e.g. trilobites, echinoderms,
ammonites and other molluscs) are also 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 (fossil)
blocks.