Thursday, September 23, 2010

The Burgess Shale

The Burgess Shale is found in an area of the Canadian Rocky Mountains known as the Burgess Pass, and is located in British Columbia's Yoho National Park. Part of the ancient landmass called Laurentia, centered in Hudson Bay, the Burgess Shale represents one of the most diverse and well-preserved fossil localities in the world.

These fossils have been gathered from shales of the Stephen Formation in two quarries opened between Mount Wapta and Mount Field. The upper quarry is known as Walcott's quarry and contains the most famous fossil-collecting site, the Phyllopod Bed.

The lower quarry is known as Raymond's quarry, named after Professor Piercy Raymond of Harvard, a visitor of the site who opened the quarry in 1924. It is now appreciated that the Burgess Shale is a site of exceptional fossil preservation, and records a diversity of animals found nowhere else. In 1981, to protect the site from overgathering, UNESCO designated the Burgess Shale as a world heritage site.

The Burgess Shale contains the best record we have of Cambrian animal fossils. The locality reveals the presence of creatures originating from the Cambrian explosion, an evolutionary burst of animal origins dating 545 to 525 million years ago. During this period, life was restricted to the world's oceans. The land was barren, uninhabited, and subject to erosion; these geologic conditions led to mudslides, where sediment periodically rolled into the seas and buried marine organisms. At the Burgess locality, sediment was deposited in a deep-water basin adjacent to an enormous algal reef with a vertical escarpment several hundred meters high.

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