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~. - +. <br />. .. <br />__ _ ~ -- <br />- i9 <br />ACTION OF THE~VIRGIN RIVER <br />The landforms of the Virgin River basin are primarily erosional <br />rather than depositional. Since the early Cenozoic 1,500 to 1,800 m <br />of ~~lesozoic and Cenozoic strata have been removed via erosion. <br />Gregory (1950, p. 153) described the action of Virgin River thusly: <br />. the streams seem not content with removing <br />the products of t•~eathering; they dig deep trenches into <br />the rock and remove the intervening material by a process <br />of lateral mining. Great plateau b]ocks are first out- <br />lined by a series of master trenches like those occupied <br />by the Virgin [River]; secondary trenches, divide <br />the original earth block into innumerable varied forms. <br />The resulting landscape appears in general view as a <br />succession of terraces miles in width separated by <br />escarpments hundreds of feet high .-canyons seem to <br />be everywhere and vie with the bold cliffs in giving the <br />landscape of the plateau country its astounding expression. <br />Steep walled, narrow canyons are produced as a result of the <br />large abrasive loads of .sediment carried at relatively high velocities <br />down steep gradient valleys. Downcutting and the removal of large <br />amounts of rock debris by the Virgin River continues today.- Gregory <br />(1950) estimated that the Virgin River could cut down an additional <br />300 m and still maintain a sufficient grade to carry sediments to the <br />Colorado River. In some canyons (notably Zion Canyon) downcutting is. <br />so rapid that tributary streams cannot keep pace and the result is <br />"hanging" valleys sometimes more than 300 m above the valley floor. <br />