178 bulletin: museum of comparative zoology. 



appearing falls and cliffs will remain in evidence for a much longer time 

 if they are up-stream from the master ledge, for there they must recede 

 until they are concealed beneath the backward extension of the gently 

 sloping graded reach or platform below them. Thus a master ledge 

 promotes the development of a high fall or long talus-slope beneath it, 

 and of a long reach or broad platform above it ; and when this relation 

 is established, the number of separate elements of form in valley 

 bottom or on valley side is much less than it was in earlier youth. 

 The great escarpments of the High plateaus exhibit the concentration 

 of all the large and small cliffs of youth in a relatively small number 

 of great cliffs in late maturity ; and thus still again confirm Dutton's 

 opinion that the erosion of the plateaus and the erosion of the canyon 

 took place in different cycles, separated by a strong uplift of the region. 



The general principles here reviewed are of wide application, but in 

 regions of moderate relief and moist climate, the forms of valley sides 

 are seldom analyzed : certainly they are frequently overlooked in 

 geographical descriptions. In the Grand canyon district, where the 

 relief is on a huge scale, and where the arid climate lays bare every 

 topographical detail, the elements of form assumed under moving 

 streams of waste on valley sides are conspicuous ; they are glorified by 

 mere magnitude so that one is tempted to treat them as a new class of 

 topographic forms, until it is recognized that they are only new varia- 

 tions on an old class, examples of which are to be found in all regions 

 of horizontal structure. The canyon walls in the Kaibab and the great 

 mesozoic " terraces " that overlook the plateau from the north exhibit 

 the earlier and later stages of all these forms with great clearness. 



Cirques, Cusps, and Niches. — The many variations in the horizontal 

 pattern of the cliffs in the canyon walls have been briefly described by 

 Dutton (c, pp. 258, 259). The cliff outline, as seen in plan, has two 

 expressions (Figure 1G). In some cases, the re-entrants of a cliff are 

 sweeping concave curves, but little notched, while the intervening 

 salients are sharply attenuated cusps. In other cases, the re-entrants 

 are narrow and acute notches, while the salients are broad and rounded 

 spurs. The difference between these two cases seemed to depend partly 

 on the drainage area of the uplands, whose waters are shed over the 

 cliffs from higher levels, and partly on the amount of erosion that the 

 cliffs have suffered ; these two factors being indirectly connected. 



Where an upland sheds a stream over a cliff, the cliff will be cut back 

 much faster by the stream than it will be weathered back on the inter- 

 stream front; here the re-entrant must be an acute notch; while the 



