3 8o 



BOTANICAL GAZETTE 



[NOVEMBER 



erosion. Vegetation of cut-off mesas more closely approximates 

 that of open plains. The physiographic parts of a mesa are four: 



i. The mesa-top, flat, slightly inclined (at the angle of the stream gradient), 

 and covered with unassorted rock waste. An exposure of the detritus cap is 

 shown in fig. 3. 



2. The mesa-crest, either abrupt, with loosely strewn surface rocks, and 

 coarse, dry, unstable surface soil; or gently rounded, with soil conditions as 

 on the mesa-top. , 





Fig. 3. — Vertical exposure of the detritus cap of a remnant of mesa-terrace four 

 miles east of Boulder: the layer of mixed soil is about twelve feet deep; the surface 

 layer is darker; the substratum is Fort Pierre shale; 191 2. 



3. The side-slope, either actively eroding or stable. If the slope has been 

 lowered below the bottom of the mesa-cap, the soil of the slope is usually derived 

 from the substratum, with only a few surface rocks. Otherwise, soil conditions 

 are similar to those of the mesa-top. Seepage areas just below the crest are 

 infrequent. 



4. Valleys or draws, originating on the side-slope, usually broad and 

 shallow. Gullies are infrequent, v-shaped ravines, with intermittent 

 streams, are present in the higher mesas. Surface soil is usually fine grained. 



Climatic influences within the region have been studied by 



Ramaley (14) and Robbins (15). 



summary from 



these two papers is given. Climatic variation from 



