CHAPTER XIV. 
DETAILS OF EROSION. 
The formation and functions of the talus. — Effect of vertical heterogeneity upon cliff profiles. — ^The 
talus acts as a protecting mantle to the softer strata. — It is the regulator of the profile. — Curva- 
ture of profiles. — Its cause. — Application of analysis to the cliffs of the Grand Cauon. — The upper 
Aubrey. — The cross-bedded sandstone. — The curved profile of the lower Aiibrey. — Stability of 
normal profiles.— The Red Wall cliff. — Differences between the profiles of the Kaibab .and those 
of the other divisions. — Explanation of the differences. — The contours or ground plans of amphi- 
theaters and alcoves. — Rounded inward curves and projecting cusps. — Explanation of these 
features. — Statement of the general conditions to which the peculiar sculpture of the chasm is due. 
In the latter part of the preceding chapter I have formulated the more 
general conditions under which erosion acts. In the present chapter I shall 
consider the methods of operation of the forces which have produced the 
architectural forms displayed in the cliffs of the great chasm and of the 
adjoining country. The key to the problems they present is found when 
we analyze 
THE FOEMATIOlSr AND FUNCTIONS OF THE TALUS. 
Since the attack of erosion under the conditions prevailing in the Pla- 
teau Country is mainly directed against the edges of the horizontal strata, 
and since these strata vary among themselves in respect to hardness or du- 
rability, it follows that the different beds would, if the exposures were 
equal, weather at different rates. The softer beds would disintegrate rapidlj^ 
and undermine the edges of harder beds overlying them. The harder beds 
being robbed of support, cleave off by the joints and the fragments fall. 
The fragmental material thus produced is not immediately carried away, 
but remains in part and forms a talus. This talus, however, is ultimately 
dissipated by solution, disintegration, and transportation, and the rate at 
which it is finally carried off is in the long run sensibly equal to the rate at 
which its material is supplied. It remains to consider the arrangement 
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