Recent bores indicate that the basal Permian (Yellow) Sands of N Durham were deposited largely sub-aerially as dunes 50 m or more high on an almost planar Coal Measures surface. Many of these were inundated by a placid sea and covered by the Marl Slate and Passage Beds. Continued subsidence allowed the succeeding lower Magnesian Limestone to accumulate more rapidly on the slopes of thinly covered dunes possibly with relative relief of 30 m and minor slumping occurred over wide areas. Uplift and tilting led to deepening of the basin to the east and partial erosion of the lower Magnesian Limestone and Marl Slate on land followed by reef development. Anhydrite deposits formed contemporaneously or rather later than the reef. Inshore in N Durham these crystallized discretely from sulphate-rich dolomite muds possibly partly in a sabhka environment with ratio of evaporite to dolomite increasing eastwards towards the basin, ultimately to form continuous, bedded deposits some distance from the present shoreline in N Durham but on shore near Hartlepool. Breccias formed contemporaneously with the evaporites partly due to crystallization pressure and later removal of sulphate.

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