52 NORTH AMERICAN FAUNA. [No. 16. 
BASIN SLOPES, 
Many of the glacial basins of the timberline region are broad U-shaped 
depressions with gently sloping bottoms, ending abruptly in terminal 
moraines, below which they may or may not continue on to other 
moraines. They were excavated by glaciers at a period when the ice 
cap of Shasta was much larger and more complete than at present. 
The upper ends of most of these valleys abut against the steep upper 
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 
reflection of heat and retard the escape 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 quickly through the Boreal 
zones to timberline, above which 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, are— 
Upper Sonoran Hudsonian 
Transition Arctic-Alpine. 
Canadian 
In a generalized diagrammatic north and south section of the moun- 
tain the relations of these zoues may be shown somewhat as fellows: 
KSTA vy, 
Shopper Atlen 
Us Z SONORAN??? 
Fic. 30.—Diagram of Shasta showing relations of life zones. 
The altitudes of zone boundaries here given are intended to repre- 
sent their average or mean elevation on normal southerly slopes. The 
aridity of the mountain as a whole, with consequent scattered or 
‘spotty’ instead of ‘continuous’ distribution of most of its zone species, 
complicated by the influences of hot and cold slopes, springs, and air 
currents, elsewhere discussed, which frequently carry species 1,000 feet 
or more above or below their normal limits, makes it almost impossible 
