MESA AND FOOTHILL VEGETATION 47 



Rosa sayi covers quite extensive areas on the east and southeast slopes; 

 on these slopes it grows as abundantly in the depressions as on the drier 

 ridges separating them. The roses are also quite common in the pine 

 shade on the upper mesa top. 



Birch. Betula fontinalis Sarg. — One tree grows in the upper part of 

 Pole Canyon. 



Hackberry. Celtis reticulata Torr. — One tree is found at the lower 

 end of Cemetery Gulch. 



Summary 



The mesa sides are cut by lateral ravines of various depths. These 

 ravines on the north slope are narrow and some extend to the mesa top ; 

 those on the east and sOuth sides as a rule take the form of amphitheaters 

 or depressions. In all cases the ravines, amphitheaters and depres- 

 sions receive seepage water from the top and sides and are more meso- 

 phytic than the ridges between. In these moister places one may find 

 shrubs common to the main gulches, such as skunk bush, nine bark, 

 choke cherry, roses, currants and thorn apples. On the north slope 

 Indian currant is generally the first shrub to find its way into an embry- 

 onic ravine. It is followed by roses, currants and skunk bush; while 

 nine bark, choke cherry and thorn apple come in after the ravine has 

 deepened and the topography become more favorable for the collection 

 and conservation of moisture. As a final stage thorn apple will dominate. 

 With the deepening of the gulch such deciduous trees as narrowleaf 

 cottonwood, willow, alder, birch and mountain maple will come in. 



In the amphitheaters the shrubs grow just below the rock cap. It 

 is quite evident that their distribution there is due to the fact that they 

 receive seepage water which has followed along on the more impervious 

 shale, which underlies the rock cap. (See p. 18.) Looking at the 

 mesa from a distance a fringe of shrubs just below the edge is quite 

 noticeable. 



The shrubs of the mesa are easily divisible, in most cases, into meso- 

 phytes and xerophytes. Mesophvtic shrubs occur in the gulches, small 

 ravines, depressions and seepage zone of the amphitheaters. They 

 include nine bark, thorn apple, thimble berry, poison ivy, Indian currant, 



