ALAN SIRULNIKOFF / SCIENCE PHOTO LIBRARY ALAN SIRULNIKOFF / SCIENCE PHOTO LIBRARY
Burgess Shale fossil quarry. These rocks contain large numbers of fossils from 500 million years ago. They were formed when an avalanche of fine mud buried and preserved a large number of marine animals. The site is remarkable for the excellent preservation of invertebrates. It recorded a key moment in Earth's history, the Cambrian Explosion, when an unprecedented period of evolution produced a wide diversity of body types, including those of today's animals. The site, in Canada's Rocky Mountains, was discovered in 1909 by Charles D. Walcott. It is now a UNESCO World Heritage Site. The sedimentary rock layers are clearly seen. Photographed at Walcott Quarry.
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