120 GEOLOGY OF THE DENVER BASIN. 



end, both of which arc, however, concealed by debris from the foothills. 

 The inclination of the fracture plane can not be seen, btrl the fault is 



probably of the same class as that at the South Boulder Peaks anil the one 

 lo the south — that is, a compression fault. Though without direct connec- 

 tion with the dome-shaped fold in Deer ('reek, a little farther to the south, 



it is likely that the same or a closely contemporaneous force was accountable 

 tor both. 



THE PLAINS. 



Tilt; HOULDER VALLEY REGION. 



INI LtODI i HON. 



Surface features. — This is a region of more or less disturbed Cretaceous 

 strata and embraces about 100 square miles of prairie in the northwestern 

 portion of the Denver field, confined to the drainage slopes of Boulder 

 Creek and its important tributaries to the south — South Boulder, Coal, and 

 Rock creeks. The surface is gently rolling, hut in the southwestern part 

 and along the southern border are mesadike remnants of'the earlier bench 

 lands, which characteristically project at intervals from the foothills, not 

 only in tin- Denver field but throughout the whole extent of the Colorado 



Range. The stream depressions, which are all comparatively shallow, have 

 a course between north and east, and their profiles show long, even slopes 

 broken at the water's edge by low bluffs of the Quaternary or by the clays 

 and light-colored sandstones peculiar to the Laramie and Fox Hills. At 

 the confluences of the larger streams occur broad, low, rich, alluvial plains, 

 the extent of which is indicated on the general map by the Quaternary 

 symbol. .Much of the area is under high cultivation, and its streams and 

 ditch sections afford inanv details of geological structure. The geologic 

 and topographic confines of the area are coincident. Beyond the area 

 there is no evidence of the dynamic forces which were so active within, as 

 expressed in existing folds and faults. 



Geological formations involved. TllCSC illclllde tile 1'ielTC, FdX Hills, Laramie, 



Arapahoe, and (Quaternary, of which the Fox Hills and Laramie arc 

 the most important, from the presence of coal near their line of union. 

 The Pierre is confined to the northwestern and western portions of the 

 area. The Fox Hills appears entire along the western edge of the more 



