((drawing in field book.))
The storms under such circumstances would cause the more rapid recession of the NE wall than the SW, but would not change its shape, as they would beat equally upon all portions of the NE wall. However, there are several, broad ill defined, rather irregular hard zones in the tufa, one of which is at the top of the nearly vertical cliff. These hard zones of course exist on both sided of the canyon. They are much reddened by iron oxides. The material below it is very yielding. If the storms beat more fiercely against the NE wall, they would weather the material on that side very rapidly without much affection the hard zone, and would have much less influence upon the SW wall. Consequently the tendency would be to produce a bluff on the NE side and none on the SW side. If the hard zone protecting the cliff were a massive, comparatively unjointed rock it would be undermined and great caverns formed under overhanging ledges as in the Mesa Verde districts thus: ((Drawing in field book)). The reason this
