Clifton Down Limestone of the carboniferous Period at Westbury-sub-Mendip Quarry.
At the beginning of the Carboniferous the arid terrestrial environment of the Devonian gave way to shallow marine conditions. The Mendip area became part of a broad, southward shelving, shallow tropical sea that stretched from Belgium westwards into Pembrokeshire. The initial flooding of the region produced the mud-rich Avon Group (Lower Limestone Shale), followed by a succession of coral-rich limestones comprising (in ascending order): the Black Rock Limestone, Burrington Oolite and Vallis Limestone, Clifton Down Limestone and Oxwich Head Limestone. It is these limestones that form the dominant features of the Mendip landscape.
At the beginning of the late Carboniferous the growth of the carbonate ramp that developed during the lower Carboniferous in the Mendip region was terminated by the southward spread of deltaic sandstones. Lush vegetation, comprising forests of giant tree ferns, club mosses and horsetails, colonised the delta, the rotted remains of which formed thick peat layers that were eventually transformed into coal. These events are reflected in the late Carboniferous rocks of the Mendips, which comprise the Quartzitic Sandstone Formation overlain by the Coal Measures.
The Avon Group (or Lower Limestone Shale), is up to 150 m thick in the western Mendips. The mud-rich nature of the succession reflects the environmental transition from arid desert to shallow sea. Ripples, scours and cross-bedding in the limestones show that deposition occurred in a shallow, high energy environment, and some of the limestones are distinctly reddened due to high concentrations of the iron mineral haematite. The higher part of the formation contains greenish-grey shales and black crinoidal limestones, which were probably deposited in a slightly more open-water marine setting.
A dark grey, fine grained and fossiliferous limestone, containing broken fragments of braschiopods and crinoids (sea lillies). The Black Rock Limestone is well exposed in Burrington Combe in the West Mendips and Vallis Vale in the East Mendips.
A crinoidal limestone which locally replaces the Burrington Oolite in the East Mendips. It is more than 150 m thick in the Binegar-Ashwick area, and completely replaces the Burrington Oolite further east near Mells and Whatley.