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 oi'dinary 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 

 larjre 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 conglom- 

 erate form a bamer from 800 to 1,000 feet high and 10 miles in width 

 across the valley. Through this mass the fork has cut a noble cailon, 

 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, occui)icd the eastern jiart of the Great Basin. This lake had an ania about three- 

 fourths as great as that of Lake Superior, and its greatest depth was about 1,000 feet. This lake out- 

 flowed to the north into the Suake River aud thence into the Columbia. The iucre.asing aridity of the 

 climate since the close of the Glacial epoch has dried up most of the sources of the lake and cvai)oratPd 

 the waters of the lake itself, so that now only a few remnants are left. Of thise. Great Salt Lake is by 

 far the most important. Utah Lake is a body of /res7t water, and has an outlet through the Jordan 

 River into Great Salt Lake. Sevier Lake is another remnant of Lake Bonneville. 



t These altitudes are jirobably within 50 feet of the exact truth. 



