DAVIS: THE GRAND CANYON OF THE COLORADO. 131 



ing, and washing on the slope above the head of some deep-cutting 

 stream. Water acts on such a slope only at time of rains and thaws. 

 All this is strikingly illustrated in the plateau region, for here even the 

 stream beds are habitually dry, and the whole process of rain and 

 stream washing is intermittent and exceptional. But when the streams 

 work, they work furiously, as is proved by the coarse stony fans of 

 gentle slope at the ravine mouths on the flanks of the Kaibab and else- 

 where. A very striking example of this kind was seen at the mouth 

 of a ravine near the road from Flagstaff to Tuba, about ten miles south- 

 west of the Little Colorado crossing. The ravine was sharply eroded 

 in the sloping monoclinal surface by which the uppermost Aubrey 

 layers descended eastward under the red shales and sandstones of the 

 Permian ; the lower ground opposite the mouth of the ravine was 

 strewn over with huge limestone boulders that had been swept forward 

 at time of flood. 1 



On the bare clay slope of Cedar ridge and on the red shale wall west 

 of Pipe spring, erosion is accomplished by weathering and rain-washing 

 rather than by stream corrasion. In spite of the aridity of the plateau 

 climate, rain-washing is important ; for the general absence of vegeta- 

 tion allows a single shower to effect a significant amount of work. The 

 bare slopes retreat speedily, and the gentle grade of the valley floor 

 or platform at the base of the slope is actively prolonged headward. 

 The processes here at work resemble very closely those by which the 

 recession of cliffs is accomplished : little destructive work is done on the 

 graded platforms above or below the bare cliffs; nearly all the erosion is 

 effected by the erosion of the cliff face and under-slope, as Dutton has 

 pointed out (c, pp. 64, 221). But while cliff faces are determined by 

 the outcrops of resistant strata, the bare slopes at the divides above 

 described are independent of structure and are determined only by the 

 difference of altitude of two graded valley floors or platforms. The 

 bare slopes retreat rapidly while the graded platforms are hardly 

 changed. 



There is no reason for thinking that the average declivity of the bare 

 slopes at these divides is notably different to-day from what it long has 

 been or what it long will be. Hence there should be some systematic 

 relation between the descent of waste from a bare slope and the re- 



1 It may be noted that if the transition here noted from limestone to yellowish 

 and red shales be taken as marking the division between upper Aubrey and Per- 

 mian, then the boundary of these formations is placed about five miles too far east 

 in this locality on sheet XXIII. of Dutton's Atlas of the Grand canyon district 



