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Show 212 GEOLOGY OF THE HIGH PLATEAUS. ward into Lake Bonneville,* and thence through the Snake River into the Columbia. Where the upper tributaries of the South Fork reach the foot of the Markagunt slope the altitude is about 7,000 feet. At the junction of the East Fork it is 6,000 feet, and where the river, enters the Pavant it is 5,000 feet.f In any ordinary region the Sevier would not be dignified by the name of a river. In the early part of July its flow is a little less than 1,000 cubic feet per second, and this volume diminishes to about half that in September. Nevertheless it is the largest stream between Great Salt Lake and the Colorado. The name Sevier Valley might with propriety be given to the entire trough of the stream, but local names have been given to different portions of it which are well separated by transverse barriers through which the river has cut narrow passages. The most important of these is encountered by the Southern Fork, about 17 miles north of (below) the town of Pan-quitch. The great outbursts of trachytic lava which flowed eastward from Dog Valley here stretch athwart the course of the stream and wall against still more ancient coulees, which broke forth from vents situated in the southern half of the Sevier Plateau, and over them have accumulated large masses of conglomerate derived from their ruins. There has also been local uplifting of a few hundred feet transversely to the greater structure-lines, so that now the confused masses of trachyte and conglomerate form a barrier from 800 to 1,000 feet high and 10 miles in width across the valley. Through this mass the fork has cut a noble canon, called Panquitch Canon. Above this barrier (southward) lies a large valley-plain, having on the east long alluvial slopes, which rise gently to the base of the Sevier Plateau, and on the west the still longer and gentler slope of * Although all American geologists are well aware of it, it may not be generally known that the name "Lake Bonneville" has been given to a vast body of fresh water which during the Glacial and Post-Glacial periods, occupied the eastern part of the Great Basin. This lake had an area about three-fourths as great as that of Lake Superior, and its greatest depth was about 1,000 feet. This lake outflowed to the north into the Snake Eiver and thence into the Columbia. The increasing aridity of the climate since the close of the Glacial epoch has dried up most of the sources of the lake and evaporated the waters of the lake itself, so that now only a few remnants are left. Of these, Great Salt Lake is by far the most important. Utah Lake is a body of fresh water, and has an outlet through the Jordan Eiver into Great Salt Lake. Sevier Lake is another remnant of Lake Bonneville. t These altitudes are probably within 50 feet of the exact truth. |