DUTTON.) 



DEVELOPMENT OF CLIFF PROFILES. 



163 



but far less so than the overlying beds. They break down always into 

 a steep slope, covered with a talus partly of their own debris and 

 partly of the cherty nodules weathered out from above. Beneath the 

 limestones lies the cross-bedded sandstone, one of the most conspicu- 

 ous members of the cailon. Of all the strata it is the hardest ; in truth, 

 is about as adamantine as any rock to be found in the world. It forms 

 everywhere the vertical frieze of the upper wall and is very seldom 

 broken down into a slope. Underneath it comes the great series of 

 Lower Aubrey sandstones, a thousand feet thick, made up of very many 

 individual beds. They are similar in character, and all of them weather 

 rapidly. We have, then, in the Upper and Lower Aubrey Groups, which 

 form the outer chasm wall at the Toroweap (and which are almost ex- 

 actly the same elsewhere throughout the Grand Canon), four groups of 

 strata which are alternately hard and soft (Fig. 1G), (1) a hard cherty 

 limestone, (2) a softer limestone, (3) an extremely hard sandstone, (4) a 

 great thickness of much softer sandstones. 



Fig. 16. — Development of cliff profiles by recession in the nppor wall of the chasm. 



It has already been explained that the attack of erosion is made chiefly 

 upon the scarp walls and steep slopes of a country and only feebly upon 

 level surfaces. Imagine, now, a cut made by a corrading stream into 

 such a series of strata as that which has just been described. It will 

 soon appear that it is quite immaterial whether the cut be made very 

 gradually or instantly,— by a miracle, as we may suppose. Weathering 

 at once attacks the face of the wall. The softer beds yielding much more 

 rapidly, gradually undermine the harder ones above, and the latter cleave 

 off by their joints and great fragments fall down. If we suppose the 

 corraded cut to have been made instantly and the river to be flowing in 

 it, the fragments would at first fall into the stream and be devoured by 

 corrasion as fast as they fall. But after a time the widening of the cut 

 so produced would leave a platform on the margin of the stream where 

 the fragments would begin to form a talus. As the recession by waste 

 goes on, the talus grows larger and larger, and gradually mounts up on 

 the breast of the lower wall, i?ow the effect of a talus is to protect the 

 beds upon whose edges it lies, and to retard their rate of decay by virtue 



