The western surface of the Cathedral Formation is steep and consists of the resistant rock dolomite,[16] which was originally part of the limestone platform, but between the Mid Silurian and Late Devonian was transformed by hydrothermal flows of brine at up to 200 °C (400 °F), which replaced much of the limestone's calcium with magnesium.[21] A layer of shale lies partly on top of and partly to the west of the Cathedral Formation.[16] This shale layer used to be called the "thin" Stephen Formation where it lies above the Cathedral Formation and the "thick" Stephen Formation where it lies to the west; but the "thick" Stephen Formation is now generally known as the Burgess Formation.[22]