708 ARTHUR C. TROWBRIDGE 



wall is the steep eastern slope of the Sierras. Owens Valley, 

 between these two ranges, is occupied partially by Owens Lake, 

 and drained by Owens River which flows into the north end of 

 the lake. The surface of the valley is broken in several places by 

 the Alabama Hills, Poverty Hills, and a series of recent volcanic 

 cones and lava flows (see Plate I). 



The eastern face of the Sierra Nevada Mountains is a precipi- 

 tous fault scarp, probably of late Miocene age, attaining a height 

 of 10,000 ft. above the bottom of Owens Valley. In this slope, 

 streams and valley glaciers have carved numerous deep canyons, 

 whose lower portions are choked with drift and whose upper por- 

 tions are the cirques and bare surfaces of glacially eroded regions. 

 The rock of the mountains in this region is massive, coarse-grained 

 igneous rock, chiefly granite. This rock is weathered chiefly by 

 mechanical processes. Temperature changes and the wedge work 

 of ice cause pieces of rock varying in size from a fraction of an inch 

 to a score or more of feet in diameter to break off and roll down the 

 steep slopes, each piece being broken or worn smaller as it goes. 

 Plants, animals, and ground water are relatively unimportant as 

 weathering agents here, because by reason of the steep slopes, they 

 are not present in abundance. On the other hand, because of these 

 steep slopes, gravity is more than usually important. Oxidation, 

 hydration, carbonation, solution, etc., as usually performed by 

 atmosphere and ground water, do not take place sufficiently rapidly 

 to produce great results on the rocks before these last are disrupted 

 and taken away. That is, the mechanical processes of weathering 

 and transportation take place more rapidly than the chemical 

 processes, and the result is arkose material carried down the moun- 

 tain canyons and deposited in the valley below. These are the 

 materials to be described as the terrestrial deposits of the valley. 



Unlike the Sierras, the Inyo Mountains contain both igneous 

 and sedimentary rock, in about equal abundance. Ordovician, 

 Carboniferous, and Triassic sedimentary formations have been 

 interbedded with Triassic lavas, and intruded by Cretaceous 

 granite and diorite. Though these mountains are not so high by 

 4,000 ft. as the Sierras, and the slopes are not so steep, still here 

 also mechanical processes of weathering keep ahead of chemical 



