Shale layers, Mile 220, Grand Canyon, Arizona, USA. As the Colorado River descends from an elevation of about 3100 feet at Lee's Ferry, some 280 miles to Lake Mead (1200 feet); it is also a journey back in time, cutting through some 1.7 billion years of geologic and paleontologic history. Every few miles reveals another formation, each a fossil record of a particular episode of marine sedimentation, punctuated by tectonic deformation, displacement and volcanism. From prehistory through the pre