The sedimentary succession in the Pensacola Mountains, Antarctica, which is more than 9000 m thick, ranges in age from late Precambrian to Permian. The Patuxent Formation of late Precambrian age is a thick sequence of weakly metamorphosed eugeosynclinal slates and subgraywackes. The Wiens Formation of Cambrian(?) age contains subgraywackes, which are rich in volvanic detritus; this detritus was derived from nearby Cambrian volcanic rocks. The overlying Neptune Group and Dover Sandstone of early and middle(?) Paleozoic age consist mostly of quartzite deposited on a slowly subsiding epicontinental shelf; upward decrease in amount of volcanic rock fragments indicates a shift in time from local provenance to a sediment source in the East Antartic Shield. The Gale Mudstone of Permian age is a tillite, which has the composition of a subgraywacke. The youngest rocks in the succession comprise the Pecora Formation (new name) of Permian age and consist of graywackes and carbonaceous siltstones.