SCIENTIFIC EXPEDITION TO NORTHWESTERN COLORADO I07 



below Meeker cut through the Mesa Verde escarpment, while the minor 

 drainage by intermittent streams is controlled by the relations of litho- 

 logical units and the displacement of strata by folds. Thus, with the 

 strata dipping uniformly in a given direction at a given locality, the 

 minor drainage follows yielding strata and produces valleys between 

 more resistant horizons, while the major streams cut through the ridges 

 which have developed from erosion of the lateral valleys, probably 

 continuing in pre-established courses. 



The so-called Dakota formation here, as along the foothills of the 

 Front Range, is likely not all of Dakota age. In at least a portion of the 

 region it consists of two sandstone members, with intervening shales. 

 On Little Beaver Creek we found in the top of the upper sandstone 

 fragments of Ostrea and Inoceramus not specifically determinable but 

 strongly suggesting Benton forms. 



The Mancos formation is believed to be equivalent, in point of time 

 of deposition, to the Benton and Niobrara formations and about the 

 lower half of the Pierre. At the base are black calcareous shales, con- 

 taining thin beds of gray limestone in the upper part, as the Benton 

 does east of the Front Range. Above this series at Newcastle is a 

 limestone in places weathering almost white, containing Inoceramus 

 and Ostrea congesta, overlaid by calcareous shales. That limestone is 

 not as massive as the basal limestone of the Niobrara east of the Front 

 Range, but otherwise its occurrence and appearance are quite similar. 

 It shows usually on the lower part of the Dakota slope. The greater 

 part of the Mancos throughout the region occupies valleys and is covered 

 by alluvium and debris from the steep valley sides. The top of the 

 formation is frequently found exposed at the base of the Mesa Verde 

 escarpment (Figs 7 and n). The entire formation is marine, as are 

 the Benton, Niobrara and Pierre equivalents elsewhere The line of 

 demarcation between this and the next higher formation is not always 

 sufficiently definite to remove its exact position altogether from the 

 realm of personal opinion, although the formations are quite distinct in 

 general character and conditions under which they were deposited. 



The Mesa Verde formation consists of alternating massive sand- 

 stones, clays and coal beds, and marks a radical change in conditions of 



