A major linear reef ultimately more than 100 meters high protected the seaward edge of the carbonate shelf of the Upper Permian Middle Magnesian Limestone in northeastern England, and is overlain by an extensive stromatolite biostrome. Rocks of both structures are almost completely dolomitized. The reef is founded on a patchy lenticular coquina, and much of its lower parts is formed of typically unbedded bryozoan biolithite which appears to have formed subaqueously and grew mainly upwards. Some contemporaneous lithification and rigidity is indi? cated by the presence of biolithite debris in associated talus. Middle stages of reef growth are characterized by a progressive increase in the proportion of algal rocks and laminar organic or inorganic encrustations at the expense of the bryozoa that dominated in early stages, and a tendency towards bedding may indicate shallowing towards the end of this phase. Stromatolitic and other laminar rocks became dominant in latest phases of reef growth, where the evidence of active contemporaneous erosion, roughly horizontal bedding and lateral rather than upwards growth is thought to indicate proximity of the top of the reef to sea level. The growth and increasing asymmetry of the reef led to progressively more complete separation of environments to landward and seaward of the reef, culminating when the reef approached sea level in the formation of a lagoon and a starved or semi-starved basin. Scattered small patch-reefs occur locally in lagoonal beds in the north of the area and considerably larger masses of reef rock in the same area are probably also patch-reefs but could be outliers of a much widened main reef.
The stromatolite biostrome is a relatively uniform tabular body up to 30 meters thick formed of finely-laminated subtidal algal stromatolites on the flat top of the Middle Magnesian Limestone reef. Algal growth forms are diverse only at the lagoonal and basinal margins of the biostrome, the basinal margin also being varied by the presence, in its lower part, of conglomerates composed of rolled cobbles and boulders of biolithite possibly derived from the underlying reef.