The Polarstar Formation, a 1-km-thick argillite and sandstone unit, is the uppermost part of a thick Cambrian to Permian sedimentary sequence in the Ellsworth Mountains. The formation gradationally overlies the Whiteout Conglomerate, an Upper Carboniferous-Lower Permian glacial diamictite. The lower part of the Polarstar is mostly argillite, and the middle part consists of coarsening-upward cycles of argillite to sandstone. These cycles begin with lenticular bedding overlain by wavy and flaser bedding and end with ripple-laminated, fine-grained sandstone to cross-bedded, medium-grained sandstone. The upper part of the formation consists of fining-upward cycles of channel-form, cross-bedded, medium-grained sandstone overlain by fine-grained sandstone and of Glossopteris-besaing siltstone, argillite, and coal. The sequence of facies suggests that the depositional environment changed temporally in this area from prodelta to delta and coastal plain. The occurrence of a marginal-marine trace fossil fauna in the middle of the formation and the complete absence of a marine shelly fauna suggest depositional conditions ranging from anaerobic to dysaerobic in a stratified inland sea. Detrital grains in Polarstar sandstone indicate a source terrane dominated by silicic to andesitic volcanic rocks, including tuffs, with minor mafic volcanic and low-grade metamorphic and granitic rocks. The Polarstar Formation was probably deposited in a back-arc basin between the Pacific margin of Gondwanaland and the East Antarctic craton.