THE FOOTHILL VEGETATION 113 



sharp definition of the shore lines or at least interrupting their 

 continuity. 



Some of the foothills are morainal in character, and where 

 unsheltered, support only the herbaceous prairie flora. The 

 higher slopes show numerous outcroppings of the country rock, 

 such as may be found in all stages of disintegration, from jutting 

 bluffs to mere mounds, and usually with more of less exposed 

 talus below. It is significant that such outcroppings, if not too 

 bold, and much of the older talus, is occupied by deciduous 

 shrubs : 



Amelanchier alnifolia Opulaster malvaceus 



PhiladdpJius Lewisii Rosa Woodsii 



Ribes cereum Acer glabrum 

 Prunus demissa 



The species which occupy such situations,, it appears, maj' 

 be for the most part those which are plentiful in the region, and 

 not especially xerophj'tic forms. Some places are occupied by 

 Populiis tremulokles, which may occur here and there as small 

 islands, sometimes on high grassy and otherwise treeless slopes. 

 On the east and west forks of the Bitter Root the southern slopes, 

 grassy and treeless over considerable areasi, support in patches 

 scrubby growth of Cercocarpvs ledifolius. OTer a whole moun- 

 tain side the rock outcrops may be located at a distance by the 

 gray bunches of the mountain mahogany. It clings to the faces 

 of cliffs and seemingly in the most severe and inaccessible places. 

 This obvious feature of the distribution of deciduous shrubs 

 over a slope otherwise unfavorable can have but one meaning 

 and that is the locally greater supply of available moisture. 

 Competition of grasses and other plants over most of the area 

 may operate toward exclusion by withdrawing too much water 

 from the soil, but not by the density of the stand, for there is 

 always sufficient space for many more plants than are actually 

 to be found in the sparse and open stand of the bunch grass. 

 The shrubs above mentioned are often seen aligned up and down 

 -the slope in drainage channels so shallow as hardly to be notice- 

 able, but apparently supplying the roots from a source hidden 

 under the coarse fraji)nents which compose the surface of the 

 talus. 



