108 UNIVERSITY OF COLORADO STUDIES 



sedimentation. It is believed to include strata representing upper 

 Pierre and most of Fox Hills time. 



I am not aware of any Lewis shale or Laramie in the region traversed 

 by us. Neither the overlying Tertiary formation nor those underlying 

 the Dakota were examined particularly by us. Portions of the Wasatch 

 between Rifle Gap and Piceance Creek afford excellent examples of 

 "bad land" topography. 



Mr. Richardson 1 says: "In the Grand Hogback, northwest of New- 

 castle, Colo. , T. W. Stanton and H. S. Gale collected fossil plants which 

 F. H. Knowlton refers to the Fort Union, but Gale did not find it prac- 

 ticable to map the rocks containing these fossil leaves distinct from the 

 Wasatch." 



The table on the next page (not drawn to scale) shows the relation 

 to each other of the Cretaceous formations of eastern Colorado, the 

 Yampa River Region, and the Grand Hogback near Rifle Gap. 



The coal of northeastern Colorado is confined to the Laramie, while 

 in the Yampa Region it is found in both Laramie and Mesa Verde, and 

 in the Grand Hogback it is in the Mesa Verde. 



A comparison of the several faunas discussed in a subsequent part 

 of this report, and the associated lithological evidence, is instructive. 

 The Mancos and lowest Mesa Verde faunas are strictly marine. The 

 later Mesa Verde faunas are of brackish- water types. The fossil plants 

 of the Mesa Verde are terrestrial. The evidence as a whole indicates 

 the following post-Dakota geological history of the region : 



Throughout Mancos time marine conditions prevailed, with a slowly 

 subsiding sea-floor and low-lying or rather remote shore, so that only 

 fine sediments reached this region. It is likely that deposition was 

 approximately equal to subsidence until several thousand feet of fine 

 lime and clay shales had been deposited. Then the rate of subsidence 

 decreased, or deposition was accelerated, or both, resulting in the 

 shallowing of the sea, the sediments becoming more sandy. Marine 

 conditions, however, still continued until the lower sandstones assigned 

 to Mesa Verde age had been deposited. Then the sea retreated and 

 there followed a period of low-lying shores, swamps and lagoons, with 



' U. S. Geol. Surv., Bull. No. 371, p. 21. 



