Works from 136 to 1884 provided some descriptions and labels of the locally-known beds, and the divisions of the Purbeck into upper, middle, and lower units.

These were elevated to the status of Lulworth and Durlston Formations by Thompson in 1975, who introduced the Purbeck Group as their parent unit.

[2] The British Geological Survey recognises three formations and five members of the Purbeck Group, exposed differently across the span of southern England.

[1] In East Sussex the Purbeck Group is formally subdivided into the Blues and Greys Limestones members [3] The sequence was traditionally divided into three, though along different lines viz.

The Middle division (50 ft (15 m)), mainly thin limestones with shaly partings, contains the principal building stones of the Swanage district; near the base of this subdivision there is a 5 in (13 cm), bed from which an interesting suite of mammalian remains has been obtained; in this portion of the Purbeck Group there are some marine bands.

In the past, many geologists have ranged the Purbeck Group with the overlying Lower Cretaceous Wealden Group on account of the similarity of its fresh-water faunas; but the marine fossils, including the fishes, ally the Purbeck more closely with the Upper Jurassic rocks of other parts, and it may be regarded as the equivalent of the upper Volgian of Russia.

In north-west Germany three subdivisions are recognized in strata of the same age: in descending order Purbeck Kalk, Serpulit and Münder Mergel.

Deposits of evaporite minerals such as gypsum and anhydrite, within the Purbeck Group are mined and processed in Mountfield, East Sussex.

The rocks predominantly comprise calcareous mudstones though also include clays, shales and marls with marly, tuffaceous and shelly limestones, occasional oolitic and sandy strata, and evaporites.

No zonal fossil has been recognized for the British Purbeckian strata, but the horizon is approximately equivalent to that of Pensphinctes transilorius of the European continent.

The mammals, mostly determined from lower jaws, found in the beds mentioned above include Plagiaulax, Amblotherium, Stylodon, Dorsetodon, Triconodon, Spalacotherium and several others.

Reptile remains diagnostic to the genus level (Megalosaurus sp (theropod indet), Opisthias) are among the fossils that have been recovered from the formation.

[8] The silicified stumps and trunks of cycads and coniferous trees, often surrounded by great masses of calcareous concretions (burrs), are very noticeable in the dirt beds of Portland and near Lulworth.