HENDERSON] 



GEOLOGY AND TOPOGRAPHY 37 



canyon meantime would be more protected from such winds, while 

 the rains would strike it at a very low angle and not be so effective. 



Vegetation also is always an important factor in erosion on steep 

 slopes. Very little force is required to move loose soil down a slope 

 of 30° to 40°, but even scant vegetation helps to hold the soil and to 

 permit its accumulation, its effectiveness being proportioned to the 

 completeness with which it covers and sends its roots out into the 

 soil. The quantity and character of the vegetation depends partly 

 on the quantity of light, heat, and evaporation. The intensity of 

 light and heat bear direct ratio to the angle at which the sun's rays 

 strike the surface. Differential evaporation is governed largely by 

 the angle at which both winds and the sun's rays strike the surface 

 and by the velocity of the winds. Applying these laws to the area 

 under consideration is a very complex task, the result of which must 

 be unsatisfactory because of lack of knowledge of the relative values 

 of the various local factors. El Rito de los Frijoles flows approxi- 

 mately southeast, while the canyon north of it runs more nearly east. 

 Hence in the latter the sun shmes on the northern wall practically 

 all day, while at the Frijoles it shines all through the summer on the 

 southern side of the canyon for some time in the morning before 

 reaching the northern wall. However, the northern side receives the 

 hottest rays durmg the middle of the day, and in the winter, when the 

 sun is well south, it receives the rays at more nearly a right angle. 

 The fact that clouds more often obscure the sun durmg the heat of 

 the day than in the early morning is a factor on which it is difficult 

 to place definite value. The direction of the prevailing winds is dis- 

 tmctly favorable to more rapid evaporation on the northern wall. 

 Taking everything mto consideration, the evaporation in all these 

 canyons must be considerably greater on the northern than on the 

 southern side. Innumerable observations in the southern Rockies 

 and thence southwestward demonstrate that m arid and semiarid 

 regions, where moisture is barely sufficient to support vegetation 

 under favorable circumstances, a slight change in the direction of 

 slopes is often vital. While the difference in this respect in the 

 Frijoles canyon is not so radical as in many places, there is an 

 observable difference between the opposite walls m the abundance 

 of the smaller plants, which act as soil bmders. In the course of 

 centuries even a slight difference in the vegetal covering on steep 

 exposures where conditions are otherwise near the critical point may 

 make a marked difference in erosion. It is hardly believable that a 

 factor so nearly universal in operation elsewhere should be wholly 

 inoperative here. 



Another possibihty naturally suggests itself. If, after the canyon 

 had reached two-thirds of its present depth, the whole region had 

 been slightly tilted to the northward, the change woukl have thrown 



