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r- , r , of up to 400 m/km (800 feet/mile). Because the Colorado River cuts across the southern end of the Ka i bab Upwarp, the tJorth Rim is about 305 m (1,000 feet) higher than the South Rim. Due to the regional dip of rock strata to the south and the fact that the tJorth Rim receives more precipitation than does the South Rim, tributary streams from the t-lorth Rim are considerably longer than those from the South Rim. The Harth Rim of the canyon consi sts of a stair-step sequence of pl ateaus separa ted by major north- south-trendi ng high angl e nOnTIa 1 faults. From IoJr.st to east and in ascending order, these are the Shiv\'Jits, Uinkaret, Kanilb, and Kaibab plateaus. The Kaihab Plateau reaches .elevations of 2803 r.l (9,200 feet) and is bounded on the east by the East Kaibab l!1onocline, a )road Laramide flexUl~e or fold that plunges some 610 m (2,000 feet) to the i1arble Platforl1 ina distance of less than 10 km. The South Rim, at an elevation of about 2,132 m (7,000 feet) is the northernmost extension of the Coconino Plateau, and like the North Rim and the t4arble Platform, is capped by resistant Permian Kaibab 1 inestone. Three major rock groups are exposed in the wall s of the Grand Canyon. The youngest rocks are the Paleozoic sedimentary rocks (about 600 to 240 million years old), \.,.hich form the prominent cliffs and slopes of the upper 1219 m (4,000 feet) of the canyon. The Younger Precambri an Grand Canyon Supergroup sedimentary and igneous rocks, oating between about 1240 and 820 million years ago, are locally preserved in the eastern portion of the canyon and near Phantom Ranch and Bass Canyon where they occur ;n down-faul ted and til ted wedges, which were spared from erosion and removal during the "Great Unconformity" erosion interval between about 820 and 570 million years a90. Finally, the Older Precambrian rocks consist of metamorphic sctlists and gneisses and intrusive granites that date back to about 2 to 1. 7 bi 11 ion years. These rocks generally 1 ine the Inner Gorge of the Grand Canyon and are usually overlain unconformably by the Cambri an TaDeats sandstone that forms the TClnto platfolT.l (McKee 1969). Dif ferenti al eros; on of vari ous rock strata causes alternating cliffs and slopes throu9hout the canyon. Relatively nonresistant shales such as the Precamhrian Dox and the Paleozoic Bright Angel and Hermit shal esform gentle slopes while more res; stant sandstones, limestones 14