14 University of California PuUications in Botany [Vol. 9 



he considered 1,000 years to be a reasonable estimate of the time which 

 passed since the ice left the basin of the Upper Kern. 0. H. Hershey-^ 

 has stated that in his opinion Sierran glaciation must have been 

 short and comparatively recent, interesting for its alpine features 

 but insignificant in the matter of geological time: "In the Klamath 

 region, I have not seen a trace of any glacial action older than the 

 Wisconsin epoch and I have not heard of anything in the Sierra 

 Nevada region which can be referred to the lowan or any older glacial 

 epoch." More recently, F. E. Matthes^° has described the fresh 

 appearance of the glacial evidences in the Yosemite district : "It seems 

 as if it were only yesterday that the ice had left them. Fresh and 

 unweathered, like new quarries, are the cirque walls, while smooth, 

 glassy 'glacier polish,' the result of long-continued grinding and 

 'sand-papering' by the debris-laden ice, still shines upon their bare 

 rock floors." Matthes considers Sierran glaciation to have been 

 recurrent, the last phase but recently ended: "Indeed, in one sense 

 it has not ended yet, for on the Sierra crest a few small ice bodies 

 still hold their own. The uppermost cirques, there is good reason for 

 believing, have only just been released from the dominion of the ice, 

 but the lower canyons have been free for a considerable lapse of time 

 and subject to normal weathering and stream erosion. ' ' 



Glacial scour and deposition produced such profound changes in 

 the drainage that today the boreal region is preeminently a lake dis- 

 trict. These bodies of water are of all sizes from mere pools but a 

 few yards across to lakes several miles in length. They were long 

 ago-^ divided into two classes: (a) those retained by moraines, and, 

 (h) those occupjnng rock basins. The morainal lakes, for the most 

 part, lie in and below the Canadian life-zone. The smaller rock 

 basins are characteristic of the higher levels. The morainal lakes 

 are being rapidly invaded by vegetation and changed into meadows 

 onto which the forest advances, while the rock basins are nearly barren 

 of plant life and are being very slowly filled. Around the lakelets 

 of the higher region are usually found narrow beaches of white sand 

 and just beyond the characteristic "rock-ramparts" formed of boulders 

 of all sizes and walling in the basin. The relative immunity of the 

 rock basins from plant invasion seems to depend in part upon the 

 forces which form the rampart. The high mountain lakes freeze and 

 thaw repeatedly during the year. After an ice cover has formed over 

 the water, a sharp drop in temperature will cause the ice to contract 

 and split, the cracks become filled with water from below and this 



