Robinson — Tertiary Peneplain of the Plateau District. 113 



into the Shinarump formation.* In the more distant views of 

 Black Point from the south the slope shows no outcrops east 

 of a heavy bed of sandstone, corresponding to the conglomer- 

 atic or coarse sandstone member situated near the base of the 

 Shinarump, which is located about one-third the distance 

 between the stripped surface of the Upper Aubrey cherty 

 limestone and the end of the point overlooking the valley, 

 measured from the cherty limestone. This lack of outcrops 

 is due to the fact that that portion of the Shinarump consists of 

 very soft and easily weathered marls. Between the sandstone 

 and the cherty limestone exposures are not infrequent and are 

 occasioned by the sandstone beds of the Moencopie, a forma- 

 tion composed principally of soft argillaceous and calcareous 

 shales. 



The feature that attracts attention, as soon as the inclined 

 position of the beds is recognized, is that an exceedingly 

 smooth surface, outlined by the black basalt, has been cut 

 across the basset edges of the strata. It is in fact a very per- 

 fect plane of erosion and it would be hardly possible for so 

 smooth a surface to be developed across strata varying in hard- 

 ness from a compact sandstone to a barely consolidated marl 

 except at a base-level of erosion. To the west of the steeply 

 flexed portion of the monocline the peneplain is located on a 

 small thickness of the nearly horizontal shales of the Moen- 

 copie formation, but whether the beveling finally reaches the 

 Upper Aubrey limestone cannot be said. It does not do so 

 within a distance of 10 miles, beyond which the peneplain is 

 hidden from view by a cover of recent basalt. 



The exposure at Black Point is of interest because it per- 

 mits the peneplain to be traced directly from a region of dis- 

 turbed into one of undisturbed strata where its surface is 

 very closely parallel to the plane of stratification. Evidence 

 is thus obtained which permits the peneplain to be extended 

 so as to include the large areas of horizontally bedded Moen- 

 copie shales protected by basalt that occur especially in the 

 Little Colorado Valley, and which substantiates the general 

 conclusion of Dutton. 



Another locality wdiere the planation of the strata may be 

 seen is at the north end of Anderson Mesa some eight miles 

 southeast of Flagstaff. 



On the west side of the mesa the basalt caps cherty lime- 

 stone ; on the east side 400 feet of red shale and conglomerate 



* The stratigraphic sequence in this region is as follows : 

 Shinarump formation = Triassic 

 Moencopie " = Permian (?) 



U. Aubrey cherty limestone ) 



" cross-bedded sandstone J- Upper Carboniferous. 

 L " red sandstone. ) 



