DAVIS : THE GRAND CANYON OF THE COLORADO. 133 



former pause in the activity of the erosive forces, and that their fresh 

 exposure to-day results from a revival of these processes, such as is else- 

 where manifested in landslides and in migrating divides. 



The Development of Talus. — It must be confessed, however, that the 

 development of talus-covered slopes in the arid region is a complicated 

 problem. The quantity of talus material will vary (in so far as it 

 comes from the capping cliff) with the relation between the thickness 

 and resistance of the hard cliff-making strata above and the weak slope- 

 making strata below. The texture of the talus will vary chiefly with 

 the manner of retreat of the cliff-maker. The completeness of the talus 

 cloak will vary with the stage of the cycle of erosion. Certain mesas, 

 perhaps of Permian strata, seen from the Santa Fe Western Railroad, 

 east of Winslow, have a very coarse and discontinuous talus of lar^e 

 slabs, derived from the strong but thin cliff-maker, which is there 

 broken into great fragments by the rapid sapping of the weak strata in 

 the slope. A lava-capped mesa of Permian under-slope, seen near the 

 road between Flagstaff and Tuba about ten miles southwest of the 

 Little Colorado crossing, has a very coarse talus. On the other hand, 

 certain cliffs of massive red sandstone, on the line of the railroad east of 

 Gallup, descended to the alluvial floor of their valley without any talus 

 at their base. Dutton (c, p. 228) has suggested that such a relation 

 may be explained by the burial of a normal talus under recently 

 aggraded alluvium, thus assuming the talus to be an essential accom- 

 paniment of the cliffs. In the example east of Gallup, the absence of 

 weak basal strata and the habit of massive sandstones to weather by 

 crumbling, rather than by breaking into blocks, seemed to be at least 

 as important factors as the accumulation of alluvium. That some cliffs 

 are habitually free from talus is recognized by Dutton in his account of 

 the Jurassic escarpment flanking the high plateaus : here " a notable 

 feature is the absence of talus ; or, if it be present, its very small 

 proportions " (c, p. 36). What has been said in the section on the mi- 

 gration of divides regarding the retreat of bare slopes does not particu- 

 larly apply here, because in the examples now under consideration, the 

 scarps of weak strata are capped by harder strata; and the habit of 

 such strata is to conceal themselves under a cloak of waste from their 

 capping cliffs. Even in scarps of so small a measure of retreat as those 

 in the Grand canyon, the weaker strata are often largely cloaked with 

 talus, as is the case with the great slopes of the upper Tonto series 

 in the Kaibab section, and in the slopes of the lower Aubrey that 

 descend to the esplanade in the western part of the Kanab section. It 



