DAVIS : THE GRAND CANYON OF THE COLORADO. 



123 



the Triassic escarpment should by this time have given the bordering 

 cliffs — Vermilion on the west, and Echo on the east — a relatively 

 stable profile, such as they usually have elsewhere ; but as a matter of 

 fact, it is precisely in the notch of the escarpment that the cliffs are 

 most unstable and that landslides are most numerous. The cliffs have 

 not retreated here far enough to allow the weak underlying strata — 

 especially the blue clays of the lower Trias — to be concealed beneath 

 a graded slope ; it is because the cliffs are sapped by the rapid removal 





Figure 5. 



Landslides of Vermilion cliffs. The Triassic cliffs rise to the rim of the Paria plateau. 

 Monoclinai slides lie on the Shinarump bench, one of whose promontories is seen in the 

 centre and another on the right, half smothered in tumultuous slides that descend to 

 the plain of Marble platform in the foreground. Drawn from rough sketch. 



of the clays that the landslides result. Yet twenty miles northwest of 

 the river, in House-rock valley between the Kaibab and Paria plateaus, 

 a graded basal slope is well established beneath the cliffs and no slides 

 occur. The same is true along the foot of the Echo cliffs, twenty miles 

 south of Lee's Ferry ; the weak blue clays are there concealed under 

 a well-graded monoclinai valley floor, and slides are wanting. It is only 

 as the river is appi-oached that the clays are laid bare, and that, with 



