G GEOLOGY OF THE DENVEK BASDT. 



coast lagoon or sound within a s;in<l bar or fringing red'. At Golden the 

 hogback is wanting for a few miles, by reason of structural causes to be 

 explained later. It is resumed again opposite North Table Mountain and 

 continues with a lew unimportant breaks, due in every instance to some 

 readily explainable geological cause, nearly to Boulder, where is again a 



o;i|> due to structural causes similar to those operative at Golden. 



In all this area the hogback ridge, when typically developed, is formed 

 \>\ hard sandstones or quartzites of the Dakota formation upturned at an 

 angle of t5 or more, which are underlain by clays and easily eroded argil- 

 laceous sandstones. 'The variations from this type and the causes therefor 

 ma\ lie explained in detail as follows: 



The disappearance of the hogback at Golden results from the non- 

 deposition of the more resisting sandstones and of the easily eroded sedi- 

 ments beneath, caused by an arching or bowing up of the sea bottom at 

 this point during the time of sedimentation. A similar cause, combined 

 with tlu' subsequent leveling oil' of the surface by Pleistocene detritus 

 ■\\ ashed down from the mountains, accounts for the disappearance of the hog- 

 back in the immediate vicinity of Boulder, near the northern edge of the 

 area mapped. At Coal Creek, on the other hand, the hard Dakota sand- 

 stone is present, but it rests directly on a projecting lioss of granite, with 

 no intervening softer beds by whose erosion the hogback valley might lie 

 carved out and thus separate it from the underlying Archcan. Immediately 

 north of this point the hogback valley reappears with the recurrence of the 

 softer clays below the Dakota. 



The prominence of the Boulder peaks, which is a striking feature in 

 tlie foothill topography, is due to a series of north-and-south thrust faults, 

 Combined with a tendency to the echelon structure before alluded to, or the 

 formation of minor folds oblique to the direction of the range and pitching 

 southeastward under the plains. A more evident instance of the echelon 

 fold and its influence on foothill topography is found immediately north of 

 Ralston ('reek, which has resulted in the offsel to the eastward of a portion 



of the hogback ridge. 



PI. *\ 1 1 is the reproduction of a photographic view, looking southeast- 

 ward from Ralston Peak, on a line with the axis of this echelon fold, which 



