Detailed stratigraphical descriptions are given of the foreshore inter-tidal exposures and undercliff sections of the Jet Rock Series and Alum Shale Series which stretch from Port Mulgrave to Ravenscar on the north Yorkshire coast. Comparisons and correlations are made between the main development of these beds to the north-west of the Peak Fault at Ravesncar and the completely different development exposed in a single outcrop immediately south-east of the fault.

The Jet Rock Series and Alum Shale Series belong to the Falciferum and Bifrons Zones respectively of the Upper Lias. The area discussed is the type area for the subdivisions of the Falciferum Zone, and an area of some importance for the subdivisions of the Bifrons Zone. On the basis of 1450 ammonites belonging to forty-six species collected from known horizons, the stratigraphical sequence and ranges of the species are given, and from these details of the subzonal divisions are deduced. The generic classification of the Dactylioceratidae is discussed in the light of this stratigraphical knowledge of the large number of Yorkshire species.