AURIFEROUS GRAVELS—INDEPENDENCE HILL FLORA 901 



the present sea level. Mr. L. F. Ransome' has expressed a 

 belief that the Great Valley during the early Pleistocene was 

 chiefly above water, though transient lakes might have existed. 

 An extensive acquaintance with the formations along the Sierra 

 foothills can hardly fail to convince anyone of the reality of the 

 Pleistocene body of water extending up to the level indicated, 

 though for the later part of the Pleistocene after the drainage of 

 the lake by the subsidence of the region about San Francisco,^ 

 Mr. Ransome's view is probably correct. A subsidence of only 

 50 feet would at the present time create a very large lake in the 

 valley. 



During the earlier and middle Pleistocene many minor erup- 

 tions of basalt took place, chiefly near the crest of the range. 

 Lake Tahoe had first been dammed up by andesite during the 

 latter part of the Neocene and during the earliest Pleistocene, 

 its outlet, the Truckee River, cut a canyon through the andesite 

 and through the northern continuation of the great buttress east 

 of the lake. In middle Pleistocene time this canyon was again 

 dammed up by a basaltic eruption, and again the lake cut an 

 outlet through this second dam. 



The second division of the Pleistocene period comprises the 

 glaciation of the High Sierra, the drainage of the lake of the 

 Great Valley and corresponding formations of fluviatile deposits. 



During the glacial period the whole crest-region of the Sierra 

 was covered with a continuous sheet of ice, neve and snow from 

 which glaciers extended down along the principal river courses. 

 The glacial basins were swept bare, the morainal debris accu- 

 mulating at lower elevations, around the projecting ice tongues. 

 The block east of Lake Tahoe and in fact the whole eastern slope 

 was only glaciated to a small extent. The outlet of Lake Tahoe, 

 the canyon in andesite between Tahoe City and Truckee City, 

 has apparently never been filled with glaciers though ice streams 

 from lateral canyons reached it in one or two places. Lake Tahoe 

 does not appear to have been filled with glacial ice at any time. 



'Bull. Dept. Geol. Vol. I, No. 14, p. 386. 



^^Cf. A. Lawson, Univ. Calif. Bull. Dept. Geol. Vol. I, No. 8, p. 266. 



