216 ILLINOIS ACADEMY OF SCIENCE 



ample. In its upper course it has cut through the entire Mesa 

 Verde series and is developing a wide valley in the softer 

 Mancos shale. Similarly, Morfield and Prater canyons have 

 cut through the sandstone in the upper courses and have de- 

 veloped comparatively broad and open valleys. 



The situation of the mesa on the border of the Pla- 

 teau Province, and at the foot of the La Plata moun- 

 tains is also important. The mesa on the north is bounded by 

 a sharp escarpment averaging about 1,000 feet in height, over- 

 looking lower country from which the Mesa Verde sandstone 

 and the Mancos shale have been removed. This has been ac- 

 complished by stripping, a gradual recession of the escarpment 

 due to sapping. This stripping is comparatively rapid because 

 of the much greater rainfall in the mountain area than in the 

 plateau country. The La Platas are regularly mapped with 

 the areas having the heaviest precipitation in the state. Their 

 drainage is predominantly to the southwest and erosion, there- 

 fore, is unusually active on the soft shales at the base because 

 of the abundance and clearness of the waters. 



THE CHARACTER OF THE EROSION 



The erosion is predominantly by canyon cutting and by cliff 

 recession through stripping. Erosion on the flat plateaus, ex- 

 cept by wind, is almost negligible in comparison. Where water 

 runs over an impervious surface the result is sheet wash. 

 But where the soil is very porous, as on the greater part of the 

 plateau, the absorption is rapid enough to prevent even much 

 sheet wash. Only where the water drops over the rim rock 

 and in the bottoms of the canyons can there be much ero- 

 sion. The canyons, therefore, are deep and narrow, the upper 

 end usually box-like. The erosion is headward, up slope, 

 toward the source of the supply. Since the slope 

 is all in one general direction, the growth of the canyons 

 is in one direction, also, producing the parallel arrange- 

 ment of canyons so conspicuous on the mesa. 



The box-like endings of canyons is especially noticeable in 

 the small canyons where the headward cutting now is extremely 

 slow. In these small canyons the massive layer is well sup- 

 ported on the sides, and the undercutting into the softer layers 

 is sometimes several hundred feet. Many of these cavern-like 



