48 UNIVERSITY OF COLORADO STUDIES 



currants and roses. The xerophytic shrubs are common on the crests 

 and top of the mesa; they include Oregon grape, New Jersey tea and 

 Rosa sayi. Skunk bush and sumac are found growing in both meso- 

 phytic and xerophytic places. There is a noticeable absence of shrubs on 

 the mesa top. With the exception of sumac and Indian currant, those 

 shrubs which do occur there are almost always found in the shade of the 

 pine trees. Such shade-seeking shrubs are Ribes pumilum, thimble 

 berry and roses. In the open on the mesa top, grass is a successful com- 

 petitor and excludes the shrubs, while in tthe shade of the pines, not 

 only is there less grass on account of the shade, but a somewhat moister 

 soil on account of the same condition, and hence shrubs find it easier to 

 get a start. 



As a rule the relative ages of topographic forms can be determined 

 by a comparison of their vegetative covering. A good example of this 

 is to be found in comparing the deciduous tree growth of Pole Canyon 

 with that of Cemetery Gulch. It is seen that the number of willows, 

 cottonwoods, box elders, maples, choke cherries and wild plums is far 

 greater in Pole Canyon than in Cemetery Gulch. Furthermore, 

 birches and alders are entirely absent from the latter gulch. This 

 condition is due to a greater water supply in Pole Canyon, which in turn 

 indicates that it must be deeper and older than Cemetery Gulch. Such 

 comparisons, however, are frequently of no consequence. In some cases 

 topographic changes occur so rapidly, with their accompanying rapid 

 changes in the habitats, that the ecological successions are not able to 

 keep pace. 



Explanation of Plates 



Fig. i. — General view of Long and Horse Mesas looking south from roof of a Uni- 

 versity building. Long Mesa extends farther out to the east; only the eastern part of 

 Horse Mesa is shown. The even and gentle slope of these mesas is very characteristic. 

 It is seen that trees are present only on the portions nearer the foothills. The dark areas 

 on Long Mesa are formed by shrubs, chiefly Indian currants, in the ravines and depressions. 



Fig. 2. — A view looking up Cemetary Gulch toward the foothills. Horse Mesa is 

 on the right, Long Mesa on the left. The pointed mountain in the center of background 

 is South Boulder Peak (alt. 8,460 ft.). Pine trees extend out along the north crest of 

 Long Mesa. In the gulch there is a thick growth of thorn apple, choke cherry, skunk 

 bush, nine bark and thimble berry. The thorn apples form, however, the most abundant 

 growth in the gulch. The dark patches of vegetation on the north slope of Long Mesa 



