30 LAKES OF NORTH AMERICA. 



this character occur in the Arid region and especially in Nevada and 

 southeastern California, but probably the most typical example is the 

 one occupied by Abert lake, Oregon. 



Along the east side of Abert lake there is a long line of magnificent 

 palisades, several hundred feet high, formed by the precipitous face of an 

 eastward dipping fault block ; the lake washes the base of this escape- 

 ment and occupies the depression formed by the subsidence of the rocks 

 on the west side of the fracture. Something of the appearance of Abert 

 lake, as seen from the crest of the palisades a few miles to south of its 

 southern end, and also of the general structure of the underlying rocks, 

 may be gathered from the accompanying illustration. The lake is about 

 fifteen miles long with an average width of nearly four miles, and is 

 shallow. It receives the water of a single creek, but does not overflow 

 and is intensely alkaline. 



Many of the lakes of the Arid region are of the Abert type, but 

 usually the great depressions in which they occur have become deeply 

 filled Avith the sediments of older water bodies, and they may be considered 

 as occupying depressions on new land areas, or as belonging to the class 

 of basins here considered, as one prefers. 



In some instances the faulting that gave origin to the characteristic 

 topography of the Great Basin region has been continued to the present 

 time, and jjroduced escarpments across the bottoms of the deeply filled 

 valleys, so that the existing water-bodies are confined in part by recent 

 fault scarps. An instance of this nature is furnished by Mono lake, Cali- 

 fornia, which washes the base of a precipice formed by a recent movement 

 of the great Sierra Nevada fault. A similar association has also been 

 observed in connection with several of the lakes of western Nevada. 



When a fault crosses the course of a river, the edge of the upturned 

 block may rise so slowly that the stream is able to maintain its course 

 and cut a channel through the obstruction as it is elevated, and a lake is 

 not formed. Numerous instances of this nature have been observed by 

 the writer in the central part of the state of Washington, where the 

 Columbia and the Yackima river have eroded deep narrow gorges through 

 the edges of fault blocks that wei^e upheaved across their courses. 



With basins produced by faulting, as in other instances of surface 

 inequalities due to movements of the earth's crust, the question whether a 

 lake will be formed or not, is answered mainly by the climatic conditions. 

 In arid regions the surface effects of orographic movements are counter- 

 acted by erosion but slowly; while in countries with abundant drainage 



