122 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
gradually northeastward and descend below the level of the river, and 
the removal of the weak Permian and lower Triassic strata causes the 
valley to widen thereabouts, so that it loses the canyon form that is so 
marked further up-stream in Glen canyon, where the river cuts the 
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ML 
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Figure 4. 
Sketch map of the landslide district, southwest of Lee’s Ferry. Triassic escarpment, , 
hachured, above Permian belt, dotted. Reduced from Dutton’s Atlas. 
heavy Triassic sandstones, and farther down-stream in Marble canyon, ; 
where the river has reached the resistant Carboniferous sandstones and 
limestones. Had the river cut down its channel continuously in a single | 
cycle of erosion, the retreat of the Trias on either side of the notch in 
