Conglomerate, 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 displacement and volcanism. From prehistory through the present the can