40 DESCRIPTIVE GEOLOGY. 



marls and clays below and the overlying blue shales, clays, and soft 

 crumbling- rocks of the Colorado group above, the Dakota beds are usually 

 a conspicuous feature in the ridges which form the foot-hills of the main 

 range. Where the Palseozoic and Mesozoic rocks are found resting upon 

 the Archsean masses at a highly-inclined angle, the Dakota group may. 

 usually be traced, forming the uppermost member of the steeper ridges ; but 

 where the angle is low, they frequently pass under, and are concealed by 

 the overlying Tertiary beds. 



In approaching the mountains from the Great Plains, the Dakota beds 

 are especially prominent, as they form the outlying member of the series 

 of upturned sedimentary beds, which rise so abruptly above the plain; for, 

 although the overlying Colorado group is perfectly conformable, they never 

 occur high up on the long ridges, which form a sort of barrier between the 

 level country and the mountain region beyond. 



As already mentioned, the Dakota is a sandstone formation, and pre- 

 sents but little variety in its physical features and composition along the 

 base of the mountains. The bottom of the series is usually a fine conglom- 

 erate, passing into a hard brownish-yellow sandstone, distinctly bedded, and 

 in many localities showing the presence of considerable iron scattered 

 through the rock. Occasionally, the iron will appear in segregations, form- 

 ing rounded patches or blotches of iron stain on the yellow sandstone in a 

 very striking manner. Frequently, the upper members of the group are a 

 loose friable rock, many of the beds being nearly a white sandstone. Thin 

 seams of black clay, rich in iron and occasionally .some carbonaceous mat- 

 ter, although somewhat rare, occur all through the upper part of the group. 

 Along the Laramie Hills the Dakota formation in places develops a ten- 

 dency toward a shaly structure, but in general they are characterized by 

 massive beds. 



In thickness, the Dakota group may vary from 200 to 300 feet. 



The following section in Northern Colorado will serve to show the 

 general character of the lower member of the Cretaceous series, especially 

 as represented between the Big Thompson and Cache la Poudre: 



