At the end of the Devonian the sea spread across the Mendip area, marking the beginning of a long phase of limestone deposition. Initially, muddy sediments accumulated (Avon Group) in shallow, turbid waters, but gradually the reef limestones that are characteristic of the Carboniferous Limestone (Pembroke Limestone Group) became predominant. The environment in which these rocks formed was analogous to the Bahamas today, with tropical, warm, clear shallow water conditions covering the Mendip region in the early Carboniferous.
Deposition of the Carboniferous Limestone was abruptly terminated by the southward spread of sandy sediment that today forms the Quartzitic Sandstone Formation. This sandstone formed part of a delta complex, built out by rivers that drained a landmass (St George's Land) to the north of the Mendip region.
Throughout the Upper Carboniferous the river deltas that now covered the Mendip region were densely colonised by lush forests of giant tree ferns, horsetails and club mosses. River channels meandered through the forest and periodically the delta was flooded by the sea. Thick peat deposits formed in this tropical swamp, eventually becoming buried and transformed into coal.