During Triassic time, northwest Europe was subjected to tensional stresses which resulted in the formation of a complex system of rapidly subsiding grabens and wrench-faulted basins. This pattern of regional crustal extension, which is part of the Mesozoic breakup of the Pangean megacontinent, is related to the Triassic opening of the Tethys ocean in southern Europe and rifting in the Arctic North Atlantic, and is a prelude to the Jurassic opening of the southern North Atlantic. Great thicknesses of chiefly continental (non-volcanic) sediments accumulated within the Triassic basins. Within western Britain, a complex series of fault-bounded basins extended from the western approach and channel area, northward to the Irish Sea.

Sediments within the Triassic basins of southwest Britain generally conform to a pattern. In the basin center, haline and marls predominate, and toward the basin margin halite gives way to gypsum-anhydrite (commonly replaced by quartz, dolomite, and calcite). Where highland regions occur at basin margins, alluvial fan sequences are developed and interdigitate down-fan with marls. Beach breccias and shore-flat deposits occur in some marginal areas, as well as wave-cut platforms carbed into Carboniferous limestone bedrock. At times the fault-bounded basins contained substantial water bodies ("lakes") from which the halite was precipitated, and around which beaches developed. Contraction of the lakes produced polygonal structures in the halite and calcretes within basin margin deposits. rare but interesting marginal facies is that of a hyposaline limestone containing fine stromatolites, fenestral fabrics, and tepees.

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