60 C. R. Longivell — Geology of 



valleys, but near the edges of the block they plunge 

 sharply by a succession of falls which total a height of 400 

 feet or more. The aggradation surface has a general 

 slope conforming to the grades of the larger streams, and 

 also slopes from the mountain walls toward the valley in- 

 teriors. A considerable part of this highest surface was 

 destroyed during the growth of the next one, which is 

 locally 5 or 6 miles in width and connects with smooth 

 benches cut on solid rock. The inner surfaces or ter- 

 races record comparatively short halts of base-level. 

 Within relatively recent time both the Virgin E-iver and 

 Muddy Creek have incised their floodplains, which may 

 properly be considered a third terrace level. 



Outline of Geologic History. 



In the Virgin Mountains the geologic record extends 

 backward into pre-Cambrian time, but in the crystalline 

 rocks the record is complicated and obscure. Appar- 

 ently the clastic Cambrian sediments were deposited on 

 a nearly even surface which cuts across the structure 

 planes of the metamorphic rocks. During much of the 

 Paleozoic era southeastern Nevada was part of the Cor- 

 dilleran geosyncline, although it was east of the area 

 which received the thickest sediments. Further study 

 will be required to determine the amounts and kinds of 

 sediments deposited previous to the Devonian period. 

 The Upper Devonian sea shallowed to the east and deep- 

 ened to the west and north. Earlier Devonian invasions 

 may have covered the region, but f aunal evidence is not 

 available. Marine invasions occurred in both Lower and 

 Upper Mississippian, in Pennsylvanian, and in Permian 

 times. During the Permian and perhaps during the 

 Upper Pennsylvanian, thick continental and littoral de- 

 posits accumulated under conditions of aridity or semi- 

 aridity. At the close of the era the region was uplifted 

 without perceptible tilting, and suffered moderate ero- 

 sion. 



The Calif ornian Lower Triassic sea deposited limestone, 

 gypsum, and clastic sediments, and a great thickness of 

 continental deposits followed the retreat of the sea. An 

 interval of erosion succeeded, and another epoch of con- 

 tinental sedimentation began with the deposition of the 

 coarse Shinarump conglomerate. A thick series of sand. 



