Robinson — Tertiary Peneplain of the Plateau District. 123 



southeastern part of the plateau about Mount Taylor the area 

 is estimated at not less than 5,000 square miles. The region 

 over 100 miles wide intervening between these two areas may 

 have been a peneplain or lowland. The encanyoned course of 

 the Znni River,* and also of the Puerco of the West, across 

 the Defiance monocline and against the dip of the strata, sug- 

 gesting a superposed origin for these streams, points to this 

 conclusion. In fig. 5 the location of the various localities that 



Fig. 5. 



Southern portion of the Colorado Plateaus in Arizona and 

 New Mexico. 



have been described is shown and the approximate position of 

 the boundaries of the peneplain indicated by the dotted line. 



While it is possible to show the existence of the peneplain 

 at many localities, the cpiestion whether the several parts 

 represent a single peneplain or not is more difficult to answer. 

 Incomplete knowledge concerning the extent of the later fault- 

 ing and warping by which the peneplain was raised from a low 

 altitude to its present height and different portions of it rela- 

 tively displaced, introduces an important element of uncer- 

 tainty. The Mohave peneplain, as represented by the surface 

 underlying the older basalts of the Sheavwits and Uinkaret 

 plateaus, is certainly a unit, as is also the area in the Little 

 Colorado Valley. The portion in the Black and Mogollon 

 mesas and Black Hills is a unit, and the same appears to 

 be true of the Mount Taylor region. It is believed that they 



■ Dutton, op. cit. (h), pp. 145, 146. 



