MESA AND FOOTHILL VEGETATION 



13 



to an elevation of 8,100 feet, 2,000 feet above the level of the mesas. 

 South Boulder Peak, farther to the south, is somewhat higher (Plate I, 

 Fig. 2). These two mesas extend from the foothills in a northeasterly 

 direction. Their length is about three-fourths of a mile. Eastward 

 the slope of the plains is gentle. 



Topography of the Mesas. — The two mesas are separated from each 

 other and from adjacent mesas by sharp ravines. The tops are very even 

 and unbroken. The eastward slope on the part farthest from the foot- 

 hills is about seven degrees (Plate I). This even top is from 150 to 250 

 feet above the level of the adjacent ravines. The slope from the crest 



/FlATlRON* 



Fig. 1. — Diagrammatic Representation of a West-east Section at the Edge 

 of the Foothill Area. — At the west the igneous rocks of Archaean age form the mass 

 of the foothills. To the east of the foothills in the mesa and plains region the edges of 

 the sedimentary rocks are exposed. There are two prominent and continuous north and 

 south ridges, one formed by the resistant strata of the Fountain conglomerate and the 

 other by the Dakota sandstone. In places the Niobrara limestone also forms a low 

 ridge. The diagram shows the relation and structure of the mesa. This, it will be seen, 

 extends eastward from the Dakota "Hogback" and is underlain chiefly by Pierre shales. 

 The rock waste of the mesa cap is derived chiefly from fragments of the Fountain and 

 Dakota sandstones. This layer of rock waste is quite thick on the top of the mesa and 

 much thinner on the steeply sloping sides. (After Fenneman.) 



of the mesa to the bottom of the ravines is abrupt. Erosion is going 

 on actively on the sides where the easily eroded shales are not protected 

 by the mesa cap. Thus the top remains flat and .even and the sides steep. 

 The mesa cap also serves to keep the upper part of the slope, just below 

 the crest of the mesa, steeper than the rest of the slope (Fig. 2). 



The sides of the mesas are cut by numerous gullies, mostly small and 

 narrow and not very deep. The slope of the sides is broken by a number 



