114 forest distelbutiox 



The '"Slide Rock" Stxcessiox. 



One feature which is noticeable in the topography alike 

 toward all points of the compass are the rock fields which com- 

 pose extensive slopes of taliis and are known as "rock slides" 

 or "slide-rock" Such areas consist of rock fragments mostly 

 under a foot in diameter. The succession of vegetation on these 

 areas is a matter of much interest and importance. As might 

 be expected the rate of development varies much with the ex- 

 posure. 



One of these areas which faces the south in Hellgate Can- 

 yon a mile east of ^lissoula, offers considerable difficulty from 

 the standpoint of vegetation. This slope which rises at an angle 

 of about 40 degrees is one of the older slide rock areas since the 

 cliffs which formed it no longer are evident and the talus merges 

 smoothly into the grassy slope above. Viewed from a distance 

 the shallow channels of occasional drainage are evident and these 

 have been preempted by Eihes cereiim, PhUadelphus Lewisii and 

 Amelanchier alnifolia, which extend up and down the slope in 

 bushy, hedge-like rows. In about half a mile of this slope one 

 may count a score of Douglas spruce trees, old and misshapen. 

 A slightly greater number of yellow pines, and an equal num- 

 ber of smaller trees of each species, ten feet in height or less. 

 None of these occur above the line of the slide-roek. Upon 

 closer inspection one finds old stumps of former large trees of 

 200 to 250 years of age. The larger living trees proliably vary 

 from 75 to 200 years of age. There is immediate evidence that 

 on this area of approximately Sd acres there have been seed- 

 bearing trees for full\- 200 years, yet the total stand of pine and 

 Douglas spruce combined numbers less than 100 trees, while the 

 number of young seedlings number a scant two score, the progeny 

 of about fifty seed-liearing trees. 



The fragments of the talus, at first gray, soon become spotted 

 with minute lichen growth which finally covers the surface with 

 a coating almost lilack. ilost of these lichens are close encrust- 

 ing forms without even so much as a foliaeeous margin of the 

 thallus. This change pr( needs slowly, and much more slowly 

 on the southern than on the northern exposures. In the case 

 now under discussion considerable parts of the rock slope are 



