FORMATIONS OF THE NACIMIENTO GROUP 721 



Along the Arroyo Torrejon, the Cretaceous and Tertiary formations 

 are very slightly disturbed; the dips in this region are probably 

 initial slopes with the exception of local variations produced by settling 

 of the strata; the latter are noticeable in the Torrejon beds on the 

 east fork of the Arroyo Torrejon near the Escavada road. 



The Nacimiento group is limited by very evident unconformities 

 at the top and bottom as shown on the accompanying map. The 

 more evident of these two unconformities is the one at the top, 

 or at the base of the Wasatch formation. Between Gallina and Naci- 

 miento the Wasatch covers the outcrops of sedimentary rocks, includ- 

 ing the Nacimiento, "Laramie," Lewis, Mesaverde, Mancos, Dakota, 

 and older formations. Four miles northeast of Nacimiento, where 

 the Wasatch covers the upturned edges of older rocks, and from there 

 northward along the foot of the mountains, the unconformable 

 relationships are strikingly shown. This overlap of the Wasatch 

 on older beds is equally evident four miles northwest of Gallina, 

 where the soft, variegated shales swing across nearly to the Mesaverde 

 hog-back. This unconformity at the base of the Wasatch is not 

 easily recognized away from the mountains. The contact with the 

 underlying formation of the Nacimiento group on the Arroyo Torrejon 

 would not ordinarily be considered unconformable; for there is no 

 discordance in dip in that section and there is no contrast in either 

 composition or color of the sediments on either side of the contact. 

 The variegated shale and soft sandstone of the Torrejon and Wasatch 

 on respective sides of the unconformity are very similar. These 

 facts, together with the variability in color and lenticular character 

 of deposits in each of the formations, make this unconformity very 

 difficult of detection except where the stratigraphic break is recorded 

 in the dynamic structure'. The same holds true for the unconformity 

 at the base of the Puerco. 



The unconformity at the base of the Puerco formation is evident at 

 the point eleven miles southwest of Nacimiento, where the "Laramie" 

 sandstone, shale, and accompanying coal beds appear from beneath 

 the Puerco striking nearly at right angles with it. Westward from 

 this locality, where the outcrops are nearly parallel and there is no 

 inconsistency in direction of dip, the unconformity is not so easily 

 noted. 



