JOHNSON: GRAND CANON DISTRICT. 
145 
upper courses these same streams have broadly open, mature valleys. 
Inasmuch as the geological series cut by the streams is the same in 
both parts of their courses, there is a suggestion that the uj^per courses 
of these streams may have been developed before the lower courses. 
This would be the case if the upj^ter part of the Kaibab arch were 
exposed above a baselevel of erosion which bevelled across softer 
rocks concealing the lower part of the arch. During the baselevelling 
period mature valleys would develop on the exposed part of the arch. 
Subsequent uplift would permit the removal of the soft rocks, exposing 
the lower part of the arch, and allowing the extended portions of the 
Fig. B. — Diagram illustrating relation of House Rock Valley to curving monocline. 
mature streams to cut new gorges in the newly exposed lower portions 
of the monoclinal slope. 
Where we crossed the East Kaibab Monocline on our way to the 
summit of the Kaibab Plateau, at a jioint a few miles south of House 
Rock Ranch, there was a distinct difference in the amount of stream 
dissection observed in the two parts of the fold. As already described, 
the lower member of the fold is but little dissected, the surface over 
large areas arching gracefully with the curve of the rocks, and occa- 
sional gorges alone destroying the symmetry of the monocline. When 
we reached the upper member of the fold, however, we noted a mature 
dissection of the slope of the monocline; closely spaced ravines, 
separated by rather sharp-crested divides, have consumed most of the 
original surface. We made the ascent of the first part of the fold by 
passing up over the surface of the arch itself, because there is plenty 
of surface left, and verv little room in the bottoms of the gorges which 
