1921] Smiley: Flora of the Sierra Nevada of Calif oriiia 13 



at their periphery. In many places along the crests two cirques at 

 the heads of glaciers moving normal to the divide have intersected 

 and produced a col. The debris thus removed was deposited in the 

 great moraines lying at from 5,000 to 7,000 feet, on the lower edge of 

 the boreal region. These moraines are generally sharp-crested, of very 

 regular contour, and often of huge size. The moraines of the Fallen 

 Leaf Glacier southwest of Lake Tahoe are 1,500 feet high and three 

 miles long. Morainal deposits also occur in the higher regions, fre- 

 quently as ground moraine filling the bottoms of the high lying valleys, 

 as in the valley of the Tuolumne at Tuolumne Meadows. The morainal 

 matter is composed of coarse sand, cobblestones, and angular rock 

 fragments loosely compacted to porous soil. Much of the surface from 

 which this debris was taken now lies a.bsolutely bare of soil, forming 

 true rock deserts. "Above this (morainal zone) extend vast stretches 

 of bare rock surfaces, dazzling white smooth outcrops of granodiorite 

 and reddish-brown slate areas. "^^ These denuded rock surfaces un- 

 doubtedly explain in part the relative poverty of the Sierra in the 

 true ' ' alps, ' ' such as distinguish the Swiss mountains or may be found 

 in the mountains of Washington and British Columbia. Plate 4 shows 

 a typical glaciated valley in the central Sierra west of Lake Tahoe. 



Glacial phenomena in the higher Sierra Nevada are characterized 

 by their fresh, scarcely altered appearance ; the rock surfaces preserve 

 their striae sharp and distinct and even so superficial a character as 

 glacial polish is only beginning to disappear. Many years ago Rus- 

 selP® observed that the balance between the conditions favoring the 

 formation of glaciers and those causing them to disappear is, in the 

 region about Lake Mono, in nice adjustment. A slight alteration in 

 the present climate would again cause the valleys to become filled with 

 glacial ice, for to him it appeared probable that at no time in the 

 glacial epoch could the climate of the Sierra have been of really arctic 

 type. The glaciers were always controlled by the topography and 

 the difference between the temperatures of the sun and shade sides 

 of the ridges was, in his opinion, too small to cause such a control 

 had the climate been truly arctic. A few years later G. F. Becker^^ 

 called attention to the probable shortness of the time which has elapsed 

 in California since the end of the Sierran glacial age : ' ' The period 

 which has elapsed in California since the glaciers disappeared is a 

 very brief one and the canon erosion has no doubt been correspond- 

 ingly small. ' ' Professor A. C. Lawson^ saj^s that very late in Quater- 

 nary time an epoch of alpine glaciation occurred in the Sierras and 



