New exposures of the Ampthill Clay near Swindon, Wiltshire, and their significance within the succession of Oxfordian/Kimmeridgian boundary beds in southern England

New exposures of the Upper Oxfordian Ampthill Clay Formation in the vicinity of South Marston, Swindon, Wiltshire have provided the first opportunity to study outcrops of this very fossiliferous sequence of strata since the beds were first exposed during the construction of the Great Western Railway through the area in the 1840s. The subsequent assemblage by the author of a large collection of ammonites from South Marston has enabled the ammonite fauna to be placed within both the Boreal and the Sub-Boreal ammonite zonal schemes of Sykes & Callomon (1979). Study of the benthic fauna and the sediments containing it has shown that the sequence accumulated during a substantial late Oxfordian transgressive episode. The Upper Oxfordian succession at South Marston is compared with other Upper Oxfordian successions in England, particularly those on the Dorset coast, and revised, detailed descriptions of the uppermost Oxfordian/lowermost Kimmeridgian successions in Dorset are given.