The name of this clay is taken from Ampthill in Bedfordshire, where it is well exposed. In the Cambridge district it lies conformably on what is considered to be the Lower Calcareous Grit (the Elsworth Rock). The upper limit of the Ampthill Clay is drawn at the base of a phosphatic nodule bed, which is taken as the basement bed of the Kimeridge Clay; this nodule bed occurs at Haddenham and in the well-boring at Chettering, a description of it will be given in the account of the Kimeridge Clay. If these conclusions be correct, the Ampthill Clay must have been deposited during the period when the calcareous, argillaceous and arenaceous material of the Coralline Oolite, Coral Rag and Upper Calcareous Grit was being laid down in Yorkshire and the southern counties of England. In other words the Ampthill Clay is the equivalent of all the Corallian Rocks, with the exception of a portion of the Lower Calcareous Grit.

Much of the Ampthill Clay has been mapped by the Geological Survey as Oxford Clay; it will be seen, however, that it is distinct from that formation and also from the overlying Kimeridge Clay. There is no great physical break separating the three clays, although a slight unconformity probably separates the Kimeridge and Ampthill Clays. Their lithological characters are somewhat similar. In this district therefore, “approximately uniform physical conditions prevailed throughout the Middle Oolite and the earlier part of the Upper Oolite period.” As a necessary result of this, it will be seen that the fauna is somewhat peculiar, since it contains a mixture of Oxford Clay, Corallian and Kimeridge Clay forms, and some which are new.