PLAINS STEUCTUEE. 45 



low ridges or sand bars on the ocean or lake bottoms, inure or less parallel 

 to and at some little distance from the shore-line. The waters pouring 

 out from the mountains at various periods of erosion would have come in 

 greatest volume from the present general upper drainage system of the 

 Platte River; these ridges would have tended to give an initial northern 

 direction to their course, which, once determined, would have influenced 

 all drainage courses of subsequent erosion systems. Thus, as is shown 

 in a general way on the profiles, the hollowing out of the bottoms of the 

 Denver and Arapahoe lakes to form the Denver I'>asin would have had 

 at first a northerly direction, and would bend more and more rapidly 

 eastward with the general slope of the country as its distance from the 

 ancient shore-line increased. 



FOOTHILL STRUCTURE. 



At first glance the geological structure of the foothill regions seems 

 extremely simple, being merelj the upturning of the ends of the sedi- 

 mentary beds along the mountain Banks where they were most nearly in 

 contact with the crystalline core of the range. The first explanation to 

 suggest itself tor such upturning is evidently a vertical upward movement 

 of that core, which carried up with it the l>eds immediately resting upon it. 

 It seems hardly necessary to mention the theory maintained by early 

 explorers in the region, when the study of mountain structure was in its 

 infancy, that this upturning was merely the remaining limb of a great 

 anticline whose crest had been eroded off and planed down, and that the 

 whole series of upturned strata once arched entirely over the mountain 

 crest. This theory has long been disproved by a demonstration of the 

 rarity in nature of such conditions, once held to be typical of mountain 

 ranges, and of the impossibility of explaining by it the actual phenomena 

 in this field. 1 



The idea that mountain elevation was produced by a vertical upthrust 

 or force acting directly upward under the center of the range was one of 

 the primitive theories held when the field of geological observation was 

 very limited and theories were based more on meditation in the office than 

 on actual exploration in the field. As field study advanced it was entirely 

 'Mon. V. s. Geol. Survey, Vol. XII, p. 20. 



