. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 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
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Harvard University. Museum of Comparative Zoology. Cambridge, Mass. : The Museum
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