114 Robinson — Tertiary Peneplain of the Plateau District. 



lie between the lava and the limestone. The contact between 

 the limestone and shale may be traced around the north end 

 of the mesa for two miles, in which distance it gradually rises 

 270 feet, thus making; it certain that the mass of shale is wedge- 

 shaped. Here then is a smooth surface developed across such 

 widely different rocks as the very resistant Upper Aubrey 

 cherry limestone and the soft shales of the Moencopie forma- 

 tion. The section thus shows the cherry limestone to be 



Fig. 2. Anderson Mesa, north end. 



1 Upper Aubrey cross-bedded sandstone, 2 Upper Aubrey limestone, 



3 Moencopie, 4 Basalt. 



involved in the planation and furnishes the clue to the correct 

 interpretation of the surface underlying the basalts of the 

 Black and Mogollon mesas farther south. 



The presence of the Upper Aubrey formations at Slate 

 Mountain, 14 miles northwest of San Francisco Mountain 

 and six miles southwest of Cedar Ranch, makes it evident 

 that the red shales and marls at Cedar Ranch lie either as a 

 beveled wedge between the cherty limestone and capping 

 basalt, as at Anderson Mesa, or as a degraded down-faulted 

 block ; a point that could probably be made clear by tracing 

 a bed of conglomerate by its numerous pebbles westward 

 along the face of the mesa. Whichever be the case, it 

 is certain that the level contact surface between the basalt and 

 the underlying sedimentaries is here, as elsewhere, a peneplain. 



At the three localities that have been described the surface 

 of the peneplain, as judged by the line of contact between the 

 sedimentary rocks and the overlying basalt, is remarkably 

 smooth. Of equal smoothness is the contact between the 

 Moencopie shales and basalt at all points in the vicinity of 

 the San Francisco Mountains. Judged by the character of 

 this contact, of which many miles are exposed in the Little 

 Colorado Valley, it may be said with a high degree of certainty 

 that the surface of the peneplain was one of practically no 

 relief. The contact between the cherty limestone and basalt 

 was not so extensively seen as that between the Moencopie 

 shales and basalt. Where displayed on the west side of Ander- 

 son Mesa, on both sides of Oak Canyon and in the Aubrey 

 Cliff west of Williams, the contact shows the same degree of 

 smoothness that characterizes the shale contact and creates the 



