Grand Canyon from Torroweap Overlook.

North rim of the Grand Canyon from Torroweap Overlook. The overlook, at 8, 000ft elevation, is the only viewpoint in the National Park from where the Colorado River can be seen vertically 3, 000ft below. The term Tuweep, a Paiute is a word for "the earth". Toroweap, a Paiute term meaning "dry or barren valley", strictly refers to the valley and the overlook. The three main rock layer sets in the Grand Canyon are grouped based on position and common composition and 1. Metamorphic basement rocks, 2. The Precambrian Grand Canyon Supergroup, and 3. Paleozoic strata. These three main sets of rocks were first described by the explorer and scientist John Wesley Powell during his expeditions of the Grand Canyon in the late 1860s and early 1870s., Photo by DAVID PARKER/SCIENCE PHOTO LIBRARY