 The Burgess Shale is a series of sediment deposits spread over a vertical distance of hundreds of metres , extending laterally for at least 50 kilometres ( 30 mi ) . The deposits were originally laid down on the floor of a shallow sea ; during the Late Cretaceous Laramide orogeny , mountain @-@ building processes squeezed the sediments upwards to their current position at around 2 @,@ 500 metres ( 8 @,@ 000 ft ) elevation in the Rocky Mountains . 
