DAVIS: THE GRAND CANYON OF THE COLORADO. t3a 
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 large 
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 Java-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 
ageraded 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 
