The Stephen Formation is a geologic formation exposed in the Canadian Rockies of British Columbia and Alberta, on the western edge of the Western Canada Sedimentary Basin. It consists of shale, thin-bedded limestone, and siltstone that was deposited during Middle Cambrian time (513 to 497 million years ago). It is famous for the exceptional preservation of soft-bodied fossils: the Burgess Shale biota. The formation overlies the Cathedral escarpment, a submarine cliff; consequently it is divided into two quite separate parts, the 'thin' sequence deposited in the shallower waters atop the escarpment, and the 'thick' sequence deposited in the deeper waters beyond the cliff. Because the 'thick' Stephen Formation represents a distinct lithofacies, some authors suggest it warrants its own name, and dub it the Burgess Shale Formation. The stratigraphy of the Thin Stephen Formation has not been subject to extensive study, so except where explicitly mentioned this article applies mainly to the Thick Stephen Formation. Sedimentary setting The Stephen Formation formed at a low-latitude miogeoclinic continental margin, at the western limit of a continental craton. Detrital sediments were washed in by rivers from the continent, over the limestone reefs which formed the shallow sea floor. At the top of sequence-stratigraphic cycles, oncoids were sometimes washed in to the Thin Stephen formation from the shallower waters closer to the shore. The fossiliferous deposits of the Stephen Formation are a sequence of slightly calcareous dark mudstones, about old. The beds were deposited on top of and at the base of a cliff about tall, below the depth agitated by waves during storms, and thus at a water depth of around 200 m. This vertical cliff was composed of the calcareous reefs of the Cathedral Formation, which probably formed shortly before the deposition of the Burgess shale. The precise formation mechanism is not known for certain, but the most widely accepted hypothesis suggests that the edge of the Cathedral formation reef became detached from the rest of the reef, slumping and being transported some distance — perhaps kilometres — away from the reef edge. Later reactivation of faults at the base of the formation led to its disintegration from about . This would have left a steep cliff, the bottom of which would be protected, because the limestone of the Cathedral formation is difficult to compress, from tectonic decompression. This protection explains why fossils preserved further from the Cathedral formation are impossible to