52 NORTH AMERICAN FAUNA. [no. 16. 



IIASIN SLOPES. 



Many of tlie jilacial basiiisof the timbeilinerejjion arc broad U-sliai)ert 

 depressions with <4(Mitly sloping bottoms, ending abruptly in terminal 

 moraines, below whicli they may or may not continue on to other 

 moraines. They were excavated by glaciers at a ])eriod when the ice 

 cap of Shasta was much larger and more complete than at ])resent. 

 The upi)er ends of most of these valleys abut against the steej) up]>er 

 slopes of the peak, and are bordered on both sides by lofty ridges, so 

 that they are walled in on three sides and thus converted into basins. 

 Such basins, when they face the southwest, appear to promote the 

 retiection of heat and retard the escai)e of hot air, so that they some- 

 times become hot pockets characterized by species belonging to the zone 

 below. 



LIFE ZONES OF SHASTA. 



Shasta stands on a Transition zone plane, with a dilute tongue of 

 Upper Sonoran approaching its northern base by way of Klamath and 

 Shasta valleys. Its forested slopes rise (juickly through the Boreal 

 zones to timberline, above wiiich its ice-clad summit towers to a height 

 of 5,000 feet. The life zones of Shasta, therefore, beginning with the 

 Upper Sonoran element of Shasta Valley, arc — 



Upper Sonoran Iludsonian 



Transition Arctic-Alpine. 



Canadian 



In a generalized diagrammatic north and south section of the moun- 

 tain the relations of these zones may be shown somewhat as follows: 



N 



'a^a ^a va v^ v^ V/^ v^ ^, 



'f u'IpeV^I^ A V A V A V A ^ TRANSITION^ V a V a ^^ A ^ /077"^^7r^ A V/ 

 ^!S^AVAVAVAVXVAV;VAVAVAVAyAyAVAVAV/ 

 A^X^^ a'^aVa^^aVaVaVaVaVaVaV^VaV/xVaVaVa 



Fl(i. 30. — I)i:ii;raiii oi Sha.sta HhowiDg relations of life zdiiis. 



The altitudes of zone boundaries licrc given are inteiided to repre- 

 sent their average or mean elevation on normal southerly sloi»es. The 

 aridity of the mountain as a whole, with conse(iuent scattered or 

 'spotty' instead of 'continuous' distribution of most of its zone species, 

 comi)licated by the inlluences of hot and cold slopes, springs, and air 

 currents, elsewhere discussed, which frci|uently carry species 1,0(M» feet 

 or more above or below their normal limits, makes it almost impossible 



