 The rocks containing the fossils are on the border between two partially overlapping bands of rock that run along the western face of the Canadian Rockies . On the eastern side of this border is the Cathedral Formation , a platform of limestone formed by algae . The western surface of the Cathedral Formation is steep and consists of the resistant rock dolomite , 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 . A layer of shale lies partly on top of and partly to the west of the Cathedral Formation . 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 . 
