 The fossil beds are in a series of shale layers , averaging 30 millimetres ( 1 @.@ 2 in ) and totalling about 160 metres ( 520 ft ) in thickness . These layers were deposited against the face of a high undersea limestone cliff . All these features were later raised up 2 @,@ 500 metres ( 8 @,@ 000 ft ) above current sea level during the creation of the Rocky Mountains . 
