A redefinition of the Great Oolite Formation of the South Cotswolds to the Chatfield Oolite Formation is proposed; and a refined classification of this formation into four informal units, based largely upon the occurrence of regionally persistent hardgrounds, is defined. These units are viewed as stacked depositional prisms, each of which successively prograded southward into the mudstone sequence of the Wessex Basin. The succeeding Upper Rags Member of the Forest Marble Formation spread extensively over both South and Mid Cotswolds. The Reef Bed at its base, which embodies scattered, small, coralline patch reefs, is shown to successively transgress northward all four units of the Chatfield Oolite. This interpretation leads to the conclusion that the Fairford Coral Bed in the Cirencester-Fairford area, like the Reef Bed, forms the basal unit of the Forest Marble Formation there.
