14 UNIVERSITY OF COLORADO STUDIES 



of wider ravines, which, just below the edge of the mesa, widen out and 

 assume an amphitheater-like form. These are of importance in relation 

 to the distribution of moisture, as will be seen later. On Horse and 

 Long Mesas these occur only on the south side. They are indicated 

 on the topographic map by X (Fig. 3). 



Gulches. — Bluebell and Pole Canyons head well up toward the 

 summit of Green Mountain, while Cemetery Gulch is a younger, shorter 

 gulch still working headward into the general surface of the mesa. In 

 all of the gulches are intermittent streams. On account of several good 

 springs in Bluebell Canyon it contains much more moisture than either 

 of the other gulches and water flows in its channel during the greater part 

 of the year. In Pole Canyon water flows during spring and early 



--Mesa Cap 



Sccpa&c Zone 



Gulch 



Fig. 2. — Diagrammatic Representation of Cross-Section of a Mesa. — This 

 shows the underlying rock, usually Pierre shales, and the covering of rock debris. On 

 Horse and Long Mesas the largest fragments are boulders several feet in diameter. This 

 explains the extremely rocky zone at the edge of the mesa. The thin layer of rock waste 

 on the steeply sloping sides is at places entirely wanting, exposing the underlying shales. 

 Reference to the figure explains the seepage zone just below the crest of the mesa. Water 

 percolates readily through the mesa cap to the impervious shales and at this level reaches 

 the edge of the mesa. 



summer, while Cemetery Gulch carries water only during wet weather. 



Bluebell Canyon is a deep V-shaped gulch throughout the greater 

 part of its length. Pole Canyon is deep and narrow at the foothill end 

 of the mesa, but lower down in its course it widens out into a broader 

 U-shaped valley. Cemetery Gulch is V-shaped with fairly steep sides. 



From the above account it will be seen how great is the variety of 

 slope and exposure on these two mesas ; giving a wide diversity of con- 

 ditions for plant growth. There is a difference of elevation of eight hun- 

 dred feet between the highest and lowest parts of the area under study, 



