POST-CRETACEOUS UNCONFORMITY 613 



In the Cerrillos and Hagan fields, which may be considered together, 

 inasmuch as they are separated only on account of faulting, the uncon- 

 formity is probably equivalent in time to that in the fields just men- 

 tioned. It separates rocks of Montana age — the Mesaverde — from those 

 which have been correlated with the Denver formation and which are 

 shown in this paper to be probably equivalent to the Tertiary formations 

 of the San Juan Basin. The interval of erosion was comparable in 

 duration to that of the Eaton and the Denver regions, inasmuch as the 

 older sedimentary rocks in the mountains were eroded down to the an- 

 cient crystallines, as is shown by the presence in beds immediately above 

 the unconformity of pebbles of all of these rocks. 



In the Rio Puerco field also the unconformity separates the Mesaverde- 

 formation from rocks of Tertiary age, and a few miles farther north, in 

 the vicinity of Cuba, Tertiary rocks that are lithologically the same as 

 those of the Rio Puerco field lie unconformably on Mesaverde and older 

 rocks and also on the Lewis shale and the "Laramie" formations. Far- 

 ther north at Dulce the Tertiary rocks lie unconformably on the "Lara- 

 mie." The relations of the Tertiary to the older rocks in the several 

 fields described in this paper are indicated in figure 2, and the relations 

 of the formations near Dulce to those of the Raton field and to the inter- 

 vening mountains are shown in the sketch section, figure 3. 



The possible objection to the postulate that the unconformities in all 

 these fields have not been proved to be exact time equivalents may be 

 met by the equally valid argument that there is no proof to the contrary. 

 In no place do rocks younger than Cretaceous occur below the uncon- 

 formity, nor do any rocks of undoubted Cretaceous age occur above it. 

 From a diastrophic viewpoint, this unconformity is the logical plane of 

 separation between the Cretaceous and the Tertiary in the Rocky Moun- 

 tain region. The same plain of separation is indicated by the fossil 

 plants. Those below the unconformity in all of the fields described 

 belong to the Montana flora, while those above it are of Tertiary types. 



The correlation of the unconformities in the various fields is based 

 largely on a broad conception of the physical conditions existing in the 

 Rocky Mountain region during late Cretaceous time. This is too large a 

 subject to be discussed here, but it is generally agreed by geologists that 

 in early Cretaceous time the area now occupied by the Rocky Mountains 

 was one of very low relief if not actually baseleveled. It is not improb- 

 able that the Cretaceous formations once extended over much if not the 

 entire area now occupied by the Rocky Mountains. This conception is 

 wholly in agreement with the observed relations in New Mexico. The 

 Cretaceous formations are comparable in thickness and in lithologic char- 



