118 Robinson — Tertiary Peneplain of the Plateau District. 



Marvine's description shows that there is a beveling of the 

 strata in the Mogollon Mesa similar in character to that found 

 in the Black Mesa and suggests that the surface underlying the 

 older lava of the mesa may very likely be peneplained. It is 

 to be notedj however, that several outcrops of Cretaceous 

 strata have been found among the lavas in the vicinity of Min- 

 eral (St. Johns, Ariz., topographic sheet) and north of Fort 

 Apache, from which it is to be inferred that the surface was 

 not so flat as farther west. 



It seems probable that the peneplain originally stretched 

 northward beyond the Little Colorado, as was the case to the 

 west, and that it may also have extended somewhat farther 

 south. Eastward the distance to the next locality where traces 

 of the peneplain are found is 125 miles, and as the intervening 

 region has never been geologically studied or topographically 

 mapped, it is not possible to say whether the peneplain for- 

 merly existed there. It is safe to say, however, that if it was 

 not developed, the region must have possessed an extremely 

 mature topography. 



The Mount Taylor region of New Mexico. — The peneplain 

 thus far described occurs in the southwestern and southern 

 portions of the plateau. There is evidence, also, that it exists 

 in the eastern part — in the Mount Taylor plateau and sur- 

 rounding country. Dutton in his report on the Zuni Moun- 

 tains* gives the following description: 



" Just north and northeast of Grants the map shows two 

 detached lava-capped masses, separated from the great volcanic 

 mesa on which Mount Taylor stands. These two masses are 

 separated from each other by a high, narrow saddle. The two 

 are really one long tongue of sedimentary rock. Directly in 

 the saddle the junction of the Cretaceous and Jura-Trias 

 appears, but it is exposed in a well-developed monocline dip- 

 ping eastward. The angle of dip is about 16° to 18°. The 

 monocline, before the eruptions of lava,f was beveled off 

 smoothly by erosion, so as to form a nearly horizontal surface 

 across the basset edges of the upturned strata. The course of 

 the monocline is a little east of south, and it just touches the 

 sharp southwestern angle of the Mount Taylor mesa, where 

 the edges of the Cretaceous beds beneath the lava cap are 

 flexed up by it." 



Gilbert states,;}: in regard to this same fold, that it may be 

 seen in the Acoma mesa, south of Mount Taylor, which is 

 capped by lava. Dutton also describes the San Mateo mono- 

 cline, north of Mount Taylor, which carries Cretaceous strata, 

 dipping 15° east, under the Mount Taylor plateau. On account 



*Op. cit. (b), p. 151 and pi. xiv. 

 f The italics are Dutton's. 

 J Op. cit., p. 559. 



