OCT., 1899.1 TIMBERLINE. 29 
Tesult is that this slope, unless too steep or otherwise unsuited to tree 
Stowth, or within the track of avalanches, is commonly covered with 
white-bark pines. As arule the timbered area on these westerly slopes 
takes the form of a broadening tongue, beginning at the highest alti- 
tude attained by trees on the crest of the ridge and increasing in width 
at lower altitudes until the bottom of the valley is reached and skirted, 
and the limit of avalanche movement passed, when the trees again 
strike out boldly. The pines in the basins are much larger and more 
erect than those on the summits of the ridges; they decrease in size 
with increase in elevation. The long oblique line which on the west 
side of each ridge marks the lower limit of tree growth commonly 
Sere a 
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& } Le; ae a Ee a ea 
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Fie. 14.—High timberline ridge, showing effects of slope exposure. The dark patches on the left 
(west) side of the ridge are dwarf white-bark pines. (Photographed by John H. Sage.) 
marks also the upper limit of the area in shadow during the late 
afternoon. In local spots other factors may account for the absence of 
trees. ‘hus, they are always absent from the avalanche-swept bot- 
toms of the valleys, and from ground kept wet by springs or melting 
snow. Studied with these facts in view, comparatively few treeless 
areas will be found which can not be explained, and the position of 
true timberline may be fixed with some degree of certainty. This is 
really a very important matter inasmuch as it affects the zone position 
of a great many species. 
It is necessary to remember that the reason trees are absent from 
the cold east and northeast slopes of the ridges whose summits are 
