Cathedral Formation

The Cathedral escarpment was a limestone submarine cliff, the wall of an algal reef, which formed part of the Laurentian continental shelf during the Cambrian period. It is associated with the exquisite preservation of the Burgess Shale. Approximately 100 to 300 m high, it runs for around 100 km through and around Yoho national park, British Columbia. Only small portions of it are exposed. During the Cambrian period mudflows ran down and along the escarpment, trapping and quickly burying organisms at the base of the cliff, and preventing their decay, permitting the preservation of soft tissue in the rocks that now comprise the Stephen formation.

...
Wikipedia