900 W. LINDGREN—F. H. KNOWLTON 



gravels at the latter vicinity and the abundant occurrence of 

 them northward. The relative post-Miocene uplift has conse- 

 quently been larger northward. During the gravel period a 

 drained lake probably occupied the Tahoe-Sierra Valley moat. 

 During the volcanic period Tahoe became separated from Sierra 

 Valley by masses of andesite. Separate lakes covered Sierra 

 Valley, the Boca-Truckee Valley, Tahoe and Mohawk valleys. 

 Carson Valley east of the Genoa buttress was also occupied by a 

 lake during the volcanic period, as was the vicinity of Verdi 

 and Reno. During the tilting of the Sierra Nevada that crustal 

 block did not act as a rigid mass, but was slightly deformed 

 and arched, as indicated from a study of the Neocene river 

 grades.^ 



Along the western foot of the range a retreat of the shore line 

 occurred during the andesitic eruption as indicated by the uncon- 

 formity separating the andesitic beds from the lone formation 

 and by another dividing the Pleistocene from the lone and the 

 volcanic beds. 



Tlie Pleistocene period. — The last andesitic flows are supposed 

 to mark the close of the Pliocene in the Sierra Nevada. This 

 is a somewhat uncertain line as to its exact age, but it is the only 

 one which can be drawn without creating artificial distinctions. 

 It is true that in referring the lone formation to the Upper Mio- 

 cene and drawing the line between the Neocene and the Pleisto- 

 cene at the close of the tuff and breccia flows, the line limit allotted 

 to the Pliocene becomes somewhat restricted, the volcanic period, 

 as above observed, being of comparatively short duration. It is 

 not improbable that the earlier part of the Pleistocene of the 

 Gold Belt maps includes some of the Pliocene as outlined at 

 other localities in the United States. The Pleistocene of the 

 Gold Belt maps includes one long period of extremely active 

 erosion, chiefly characterized by canyon-cutting; during this 

 time another advance of the shore line took place ; the Great 

 Valley being at this time a lake, the highest elevation of whose 

 shores, well marked by bodies of gravel, extended 400 feet above 



' Two Neocene Rivers, Bull. Geol. Soc. Am., Vol. IV, p. 297. 



