24 



University of California Publications. 



[Geology 



by the writer in 1906 T. H. McG-hee very kindly indicated to us 

 the situation of some of the most important fossil localities in 

 the region. 



GENERAL PHYSICAL FEATURES OF THE REGION. 



The region in which the mammal-bearing beds of Virgin 

 Valley and Thousand Creek are situated lies between two fairly 

 defined areas of quite different topographic nature. To the east 

 and south there is a succession of sharply defined mountain 

 ranges with a general north and south trend, between which are 

 broad and remarkably flat valleys. The Pine Forest and Pueblo 

 ranges, which form the eastern border of the region in which 

 the principal field work was carried on, rise to a height of over 

 9000 feet. The broad valleys have an elevation of about 4000 

 feet, 



As is so frequently shown in the Basin region, the develop- 

 ment of the mountain chains to the east of Virgin Valley is 

 evidently due in a large part to faulting. The remarkably even 

 filling of the valleys was accomplished in part by alluvial wash, 

 and partly by accumulation in lakes. A series of terraces dis- 

 tinctly shown bordering the flats near Sodhouse Point and 

 near Mason's Crossing, in the valleys immediately to the east of 

 Pine Forest Range, represents the shore-line of a body of water 

 which must have covered a large part of the valley floor in this 

 region during some portion of Pleistocene time. Associated with 

 the terraces are remnants of marginal deposits containing a 

 fresh-water molluscan fauna. Along certain levels, the marginal 

 deposits of the ancient lake show an extraordinarily heavy cal- 

 careous deposit. According to Russell 3 this region was occupied 

 by the northern extension of Lake Lahontan, and the terrace 

 deposits to which reference has just been made were evidently 

 formed during that period of deposition. 



In the region to the west of Virgin Valley the country is 

 largely lava-covered, and is evidently a southward extension of 

 the great lava plateau in the Oregon region to the north. The 

 valleys here are either broad and comparatively shallow depres- 



s Russell, I. C, Lake Lahontan, Monog. U. S. Geol. Surv. no. 11. 1885. 



