A spectacular submarine cliff over 100 metres high - the Cathedral Escarpment- marked the front of of the algal limestones of the Cathedral Formation. The Cathedral Escarpment was initially thought to be a product of prolific algal growth and sediment accumulation during a period of sea level rise. A recent re-interpretation suggests that the escarpment is the headwall of a regional slide scar, where the over-steepened front of the Cathedral Formation collapsed into the basin shales. In either case, the Cathedral Formation carbonates are older than the adjacent mudstones of the Burgess Shale Formation.
Over approximately two million years, the Burgess Shale Formation mudstones filled the basin in front of the escarpment, and eventually covered the Cathedral limestone when sea level rose. The Stephen Formation mudstones are much thinner on top of the escarpment than the Burgess Shale mudstones in front of it. The Cathedral Escarpment marks this change in thickness and it can be traced across much of Yoho National Park.
All of the soft-bodies fossil localities in Yoho Park, and there are more than a dozen, lie in the thick Burgess Shale Formation immediately beside the Cathedral Escarpment. This submarine cliff was crucial to fossil formation and preservation of the Burgess Shale: no Cathedral Escarpment, no fossils.
Revised for web format from “A Geoscience Guide to the Burgess Shale” by Murray Coppold and Wayne Powell, © The Burgess Shale Geoscience Foundation. To purchase this book, please go to: the Yoho National Park Visitors Centre, Alpine Book Peddlers, Amazon.ca, or Amazon.com