164 GRAND CANON DISTRICT. 



of tli.it protection. At first, then, the talus causes the lowest beds to 

 lag behind in the recession. As it mounts up the wall, higher and higher 

 beds gain protection, and they, too, begin to lag behind, until at last 

 the talus mounts up very nearly to the base of the extremely hard, cross- 

 bedded sandstone. Thus the entire lower series of soft strata becomes 

 converted into a slope covered with talus, and at this stage all the beds 

 above stand as a single vertical face. But immediately a second cliff 

 and talus begin to form above the hard sandstone; for since the lower 

 soft beds are protected and their rate of recession reduced by the talus, 

 the upper soft beds, being naked, must recede at a more rapid rate, until 

 they, too, become a slope and receive the protection of a talus from the 

 hard limestone at the summit. 



It appears, then, that the recession of the hard beds is accelerated by 

 undermining, while the recession of the soft beds is retarded by the pro- 

 tection of the talus. The result is the final establishment of a definite 

 profile, which thereafter remains very nearly constant as the cliff con- 

 tinues to recede. Thus the talus is the regulator of the cliff profile. There 

 are many minor features which may be explained as satisfactorily, and 

 one of them is the curvature of the Lower Aubrey profile. 



Throughout the greater part of the chasm the slope of the Lower 

 Aubrey is a very graceful curve, but in the Kaibab division it is usually 

 straight and descends at an inclination of about 30 degrees, the angle 

 of repose, or very nearly so, for the d6bris which occurs here. Taking 

 first the Toroweap section, we remark that at the base of the main 

 palisade is the broad esplanade or plain which forms the floor of the 

 upper chasm. It is from a mile to three miles in width. In a great talus 

 the fragments are slowly and continually creeping down by the action 

 of rain and frost. The plain at the base acts as a check to the descent. 

 Nowhere except, perhaps, at a notable distance away from the base or 

 in the very lowest part of the stratigraphic series are the beds wholly 

 buried in talus. Considerable areas of rock surface project through the 

 covering. The tendency of the descent of talus under the conditions 

 here considered is to give more protection to the lower beds than to the 

 higher. The check given to the descent of the talus by the level plain 

 is felt more strongly at the base of the slope than higher up. Moreover, 

 the finer ddbris is more readily washed down a slope of given declivity 

 than the coarse, and thus the ddbris at the base of the talus is finer 

 than that above; and fine d6bris is a more efficient protection than 

 coarse. In consequence of this greater protection, the recession of the 

 lower beds is less rapid than that of the higher ones, and in gener;il 

 terms the protection of any given bed in the slope is inversely propor- 

 tional to the square or some other complex function of its height above 

 the base. The curved profile at once follows, and it is demonstrably of 

 the hyperbolic class. 



In the Kaibab the case is different. Here the mighty plinth of the 

 Bed Wall limestone cuts off the foot of the Lower Aubrey slope, giving 



