C. R. Lougivell — Muddy Mountains, Nev. 39 



Art. IY. — Geology of the Muddy Mountains, Nevada, 

 with a Section to the Grand Wash Cliffs in Western 

 Arizona^ ; by Chester R. Loxgwell. 



Introduction. 



The area with which this paper is concerned is in south- 

 eastern Nevada and northwestern Arizona, as indicated 

 on the accompanying index map (fig. 1). The general 

 region of which this area is a part has more than one 

 strong appeal to the geologist. It is practically un- 

 mapped, and hence has the lure of the unknown. The 

 pioneer scientists who traversed the Plateau and Great 

 Basin regions gave only passing notice to the Virgin and 

 Muddy mountains, and the inaccessibility of the region 

 has discouraged later efforts. As a result, a large area 

 still presents opportunities for scientific work of an ex- 

 ploratory nature. Moreover, the location of the region 

 near the edge of the Basin Range coun);ry and imme- 

 diately adjacent to the Plateau Province gives it a criti- 

 cal interest, because the stratigraphic, structural, and 

 physiographic relations of Basin to Plateau must be 

 determined largely by a study of this border zone. Are 

 the rocks west of the Grand Wash Cliffs fundamentally 

 different from those to the east! What is the nature and 

 Avhat the age of structure lines dividing the two prov- 

 inces? To what extent does the physiography of the 

 region help in unravelling the more recent geological his- 

 tory? These questions concern matters which are not 

 only of interest in themselves, but are fundamental in the 

 larger problems touching the origin and development of 

 the Plateau, the eastern part of the Great Basin, and the 

 Colorado River system. 



Previous reports. — The Whipple, Ives, and Wheeler 

 parties did not map the area considered in this report, but 

 made reconnaissance surveys of closely adjacent areas. 

 Dutton included in his atlas of the Grand Canyon district 

 a generalized geologic map of the Grand Wash and the 

 eastern part of the Virgin Mountains. The first repre- 

 sentation of the geology of the Muddy Mountains and the 

 lower Virgin Valley ajjpeared in 1903, on Spurr's map of 

 southern Nevada, included in Bulletin 208 of the U. S. 



^ Published by permission of the Director of the United States Geological 

 Survey. 



