The “Portland Beds” of Dorset (Portlandian of English usage) are described in terms of a Group comprising two Formations and seven Members. Facies and thickness variations indicate the presence of a swell separating an East from a West Basin. The swell may be due to the movement of Triassic salt. The environmental history of the Portland Group is described in terms of three cycles consisting of major regressive and minor transgressive phases superimposed on an overall regression. The lower cycle consists of siliciclastics and dolomite deposited in a relatively deep marine environment. The dolomite formed by in situ replacement of lime mud. The middle cycle consists of cherty fine-grained limestones deposited on the outer part of a carbonate shelf. The abundance of replaced sponge spicules adequately accounts for the amount of chert. The upper cycle consists of cherty limestones passing up into shallow-water grainstones. Ooid shoals developed over the swell. These marine limestones are overlain by stromatolites and evaporites which formed on the basin margin.