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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 



Figure 4. 



Sketch map of the landslide district, southwest of Lee's Ferry. Triassic escarpment, 



hacliured, 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 chanuel continuously in a single 

 cycle of erosion, the retreat of the Trias on either side of the notch in 



