LAKES OF GLACIAL ORIGIN IN THE SIEEEA NEVADA. 95 



originally existed in it, as shown by the form and position of the Tertiary 

 gravel deposits,* have long since been filled up. Lake Tahoe, however, as 

 already mentioned,! is a fine, and for California an exceptional, example of 

 an orographic lake of pure water and large dimensions. Situated in a de- 

 pression between two elevated ranges, on the very summit of the Sierra, it 

 is chiefly supplied by melting snow, and is kept fresh by the abundance of 

 this supply while its overflow is carried to a lower level, within the Great 

 Basin, there to disappear by evaporation in Pyramid and Winnemucca lakes. 

 The great depth which the depression occupied in part by Lake Tahoe must 

 originally have had, becomes evident when it is considered that, in spite of 

 the detrital material to it from the adjacent lofty ranges, it has still a depth 

 of considerably over a thousand feet. 



No one of the lakes mentioned above could for a moment be supposed 

 to be of glacial origin; but there are others, mostly of minute size t and 

 very numerous, in the formation of some of which ice has undoubtedly had 

 a share. The greater part of the more important of these are so-called 

 " moraine-lakes," the origin of which is exceedingly simple. A stream of 

 water is dammed back by an accumulation of glacial debris, and a lake, of 

 greater or smaller size, is the x'esult. Such lakes ai'e usually shallow and not 

 permanent. The barrier may be broken away at any time ; and with the 

 universal shrinking of the glaciers which has taken place in later geological 

 times, the majority of the lakes thus formed have also disappeared. 



There are other lakes which are usually even smaller than those of moraine 

 origin, and which occur high up in the mountains at the very heads of the 

 streamlets, and whose basins have unquestionably been formerly occupied by 

 glaciers. The basins of such lakes are always shallow, usually very much 

 so, and of solid I'ock, with little or no debris about them. As these small 

 bodies of water are quite at the heads of the streams, and at very high alti- 

 tudes, they must occupy positions where, during the glacial epoch, snow or 

 nev^ existed. T\\cy are, in most instances, higher than the ice could have 

 reached, and therefore liigher than the region where the glacier did most 

 of its work. Their extreme shallowness, however, rendei's the question how 

 they are kept filled with water one of more importance than tliat of their 



* See Auriferous Gravels, passim. 



t See ante, p. 57. 



X These lakes are by far too small to be shown on onlinavy maps : more than fifty of them are indieatcd on the 

 Geological Survey "Map of a Portion of the Sierra Nevada, adjacent to the Yosemite Valley," which is on a scale 

 of two miles to an inch. 



