It consists of shale, thin-bedded limestone, and siltstone that was deposited during Middle Cambrian time (513 to 497 million years ago).

Detrital sediments were washed in by rivers from the continent, over the limestone reefs which formed the shallow sea floor.

[9] 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.

[1] The fossiliferous deposits of the Stephen Formation are a sequence of slightly calcareous dark mudstones, about 508 million years old.

[6] 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.

[1] It was originally thought that the Burgess Shale was deposited in anoxic conditions, but mounting research shows that oxygen was continually present in the sediment.

[9] The Trilobite Beds, the first Burgess Shale locality to be discovered,[15] mark the southerly extent of fossiliferous exposure on Mount Stephen, although many more sites exist on the inaccessible northeasterly flank of the mountain.