DAVIS : THE GRAND CANYON OF THE COLORADO. 



179 



adjoining salients will be spurs with rounded front. But where the 

 upland is of so small an area that it cannot gather streams, then 

 the head of the axial line of a re-entrant has no more importance as a 

 stream way than the lines that join it from either side; as a result the 

 retreat of the walls will be of about uniform value for a considerable 

 length of front, and the head of the re-entrant will here assume a con- 

 cave or cirque-like pattern. At the same time, the widening of the re- 

 entrant will narrow the adjacent spurs to mere skeletons of their original 

 size and sharpen their salients into cusps. 



Figure 16. 



Notches and cusps in cliff patterns. Diagram of two cliff-makers; the lower one showing 

 acute notches between rounded spurs; the upper showing rounded cirques between acute 

 cusps. Slightly modified from Bodiish's map of part of the canyon wall in the Kaibab. 

 (Dutton, c, Plate XLII.) 



If this analysis be correct, rounded spurs and sharp re-entrant notches 

 should prevail in the cliffs near the base of the canyon walls, because 

 streams will generally descend into such re-entrants from the higher 

 slopes ; unless, indeed, time enough has elapsed for the strata overlying 

 a low-level cliff to have been swept away, and the area of its platform 

 reduced by the widening of adjoining side-canyous, so as to imitate 

 conditions that would prevail only at higher levels in an early stage of 

 erosion. On the other hand, cirque-like re-entrants should prevail in 

 the high-level cliffs that rim the sides of the great spurs of the canyon 



