JOHNSON: GRAND CANON DISTRICT 147 
where they would be farthest from the main stream Hues and covered 
by the thickest and coarsest debris from the monocHne just west. 
The finding of the terraces near the base of the monocline south of 
House Rock Ranch adds one more link to the chain of evidence indi- 
cating an ancient date for the displacements in the Grand Caiion 
district. 
West Kaibab Fault. 
After traversing the summit of the Kaibab Plateau, where we were 
impressed by the maturity of the drainage features, a peculiarity dis- 
cussed at length by other writers, we entered Jacob's Canon, one of 
the canons leading to the Kanab Plateau on the west. The heari of 
this canon was of mature asjject, wide and open, with graded side 
slopes. But as we continued downward to the west we entered a 
narrow gorge cut in the bottom of the broader valley. The change 
from the more mature to the younger portion of the canon is well 
marked. A good road has been built down the gorge, but so active 
is erosion during wet weather that we found many places badly 
washed out, and passing with our wagons was difficult. 
Continuing down the gorge, we suddenly passed from Ijetween 
the canon walls, across the West Kaibab Fault (fig. A, IF. A'.) and out 
upon the Kanab Plateau. Looking back, we obtained the view shown 
in plate 18, figure 2. The more mature portion of the valley is seen 
in cross section, with the steeper-sided gorge cut in its rather fiat 
bottom. The gorge is not located in the center of the older valley, but 
is at this point cut in its southern side. 
It is possible that the existence of harder rock at the level of the 
inner gorge may in part account for its more youthful character. But 
if this were alone responsible, we should expect the retreat of the over- 
lying beds to take place at the same rate on both sides of the valley, 
leaving the gorge symmetrically located with reference to the more 
open upper portion. 
The fact that an inner gorge occupies an unsymmetrical position 
in a more mature valley, suggests that the upper portion of the valley 
may have been developed with reference to a higher and older base- 
level on the soft Permian beds west of the fault. Subsequent regional 
uplift permitted the removal of the soft rocks, and allowed the streams 
to incise a gorge in the hard rocks in whatever portion of the older 
