True reef limestones discovered in the mid-Viséan (Upper Mississippian) reef complex of northwest Derbyshire are composed for the most part of massive calcareous algae of the Collenia-Cryptozoon types. The reefs developed at two horizons and are not continuous either in time or space. They developed at the margin of a shallow shelf and formed an incipient barrier reef which separated the open seas of the English Midlands from a shallow shelf sea. Each reef was elongate parallel to the margin of the shelf and was up to 1200 feet long. The reefs averaged 30 feet wide and accumulated in thicknesses up to 100 feet. At the shelf margin in front of the reefs was a steep slope leading to the adjacent basin floor which was as much as 400 feet below the reef surface.
The shelf limestones are calcirudites, calcarenites, and calcilutites. Spergenites, which formed in shallow, warm sea water supersaturated with calcium carbonate, compare closely with sediments on the Bahama Banks. Aphanitic limestones within the reef complex possibly represent recrystallized algal deposits.
Few species lived in the shelf province, but the reef-complex fauna was rich both in species and individuals. The reef—built for the most part of algal deposits—had a distinctive sponge and bryozoan fauna.
Identical reefs occur in the Craven reef complex of Yorkshire, but no such fades occurs in C zone reef knolls of Bowland, Lancashire.