This is a sortable list of the groups into which the rock succession of Great Britain and surrounding seas is formally divided. Rock sequences are described by geologists by dividing them hierarchically thus: individual 'beds' of rock (or in the case of certain volcanic rocks, 'flows') are grouped into 'members', members are grouped into 'formations', formations into 'groups' and groups occasionally into 'supergroups'. Some groups are also subdivided into 'subgroups'. Not all of these hierarchical layers are necessarily present or defined within a particular rock succession. Many of these groups will be encountered as 'series' in older geological literature or indeed simply as the proper name e.g. 'Dalradian' or 'Millstone Grit' though care needs to be exercised as many of the names have o