DAVIS: THE GRAND CANYON OF THE COLORADO. 155 



of adjustment to weak structures thau is now seen would have been 

 almost inevitable. This opinion is fortified when it is noted that the 

 two streams in question are of moderate size, and still more when it is 

 seen that they head to the northwest against the retreating Tertiary 

 escarpment, thus suggesting that they are now longer and larger than 

 they were formerly. Their antecedent origin seems improbable, to say 

 the least. 



The Summit Valleys of the Kaibab. — The irregular longitudinal de- 

 pressions, including Summit valley and De Motte park, in the highlands 

 of the Kaibab plateau, have been regarded as of especially obscure origin. 

 Dutton thought them all as the work of a single south-flowing antece- 

 dent stream whose waters had been withdrawn in the change from the 

 moist Miocene to the dry Pliocene climate, and whose bed had been 

 deformed by the local uplift of the Kaibab (c, pp. 193-195). This view 

 seems open to question because of certain improbabilities that it in- 

 volves and of certain possibilities that it omits. The association of the 

 Summit valley depression with the axis of the Kaibab uplift, a broad 

 flat arch, with stronger flexure on the east than on the west, seems too 

 close to be the work of chance, — as would necessarily be the case if 

 the depression were the work of a stream whose origin antedated the 

 uplift. The axial line of the longitudinal depression does not now de- 

 scend continuously towards the canyon ; a long northern stretch (Sum- 

 mit valley proper) slopes northward and discharges to the east into 

 House-rock valley and thence to Paria river ; a more southern por- 

 tion (De Motte park) also slopes northward, and, except for shallow 

 sinks on its floor, discharges eastward by a deep ravine down the east 

 Kaibab monoclines. Two other shorter portions also slope northward. 

 Certain intermediate parts slope southward, but they are much shorter 

 than the parts just mentioned. Dutton concludes that all these parts 

 once had a continuous southward slope, and that the present discon- 

 tinuity of slope is due to a reversal of grade by the uplift of the 

 Kaibab. 



The continuous southward slope assumed for the depression seems 

 open to question, especially when it is remembered that the recession of 

 the Triassic cliffs demands a more remote date for the Kaibab arch than 

 early Pliocene. The retreat of the heavy Triassic strata around the 

 north end of the Kaibab suggests that at least some of the mesozoic 

 strata once stretched partly over the uplifted area : they may indeed 

 have stretched all over the Kaibab when the uplift was formed, as sug- 

 gested by TValcott on account of the absence of fractures in the east 



