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• ~ .~ S.r.l._ . _ r :_ L.-.r ~. Lli'~liY:. ~c :. _ ... .__,. .r_r N ~ -c s.,•y'Lhy,_~.. • ~~ ~ 2~ Erosion is rapid in the Virgin River. basin. The mechanical disintegration of rocks is accomplished by a combination of factors: frost; large diurnal variations in temperature alternately cooling and heating the rocks; sparse vegetation allowing soils to be washed away by heavy runoff. The removal of rock debris is expedited by rain falling on bare rock or sand; localized flooding follows nearly every thunderstorm (Cook 1960). The widening of the canyons is due to lateral mining rather than excavation. Sandstone cliffs are undercut by removal of shales; gravity pulls huge slabs from the walls. But even where the canyons are widest, the rock fails sit near the river's edge indicating that • downcutting has been more rapid than widening (Cook 1960, Gregory 1950). There are four major canyons along the Virgin River: (1) Zion Canyon, containing the North Fork, is over 550 m deep in the "Narrows" and from 7 to 40 m wide. Below the Narrows it is about 600 m deep and 0.8 km wide. (2) Parunuweap Canyon along the lower reaches of the East Fork is over i6 km song and nearly 600 m deep. (,3) Timpoweap Canyan, near Virgin, Utah, is up~to 150 m deep and emerges through the Hurricane Cliffs (Kaibab limestone). (4) The Virgin River Gorge (=Arizona Narrows) south of St. George is cut through the southern Beaver Dam Mountains and emerges into an intermountain basin near Beaver Dam Wash. The Virgin River has not been deterred by any of the major structural features of the basin including Kolob Terrace, the Hurricane fault, the Virgin anticline and the Beaver Dam Mountains a]1 of which