MOUNTAIN TOPOGRAPHY. 5 



disintegration and abrasion much Longer than that immediately adjoining 

 the foothills, and that its drainage systems are necessarily of more ancient 

 date. The mountain forms in general show the rounded features charac- 

 teristic of massive and easily disintegrated rocks, iii contrast to the more 

 rugged tonus produced by the weathering of steeply upturned sedimentary 

 beds of varying degree of hardness 



The contrast between these tonus and those of the foothill region is 

 everywhere so sharp and distinct that the line dividing the older from the 

 more I'ecent ideological formations of which they are respectively composed 

 could be quite accurately determined at a distance or drawn on a correctly 

 constructed topographical map with extremely small probable error. 



FOOTHILL TOPOGRAPHY. 



The foothill region is a belt of sedimentary beds upturned at steep 

 angles against the mountain slopes. The most characteristic feature of its 

 topography is formed by the hogback ridges of harder upturned beds, 

 which stand like a fringing reef at a little distance from the shore-line or 

 base of the mountain slope. Within these ridges are narrow longitudinal 

 valleys eroded out of the softer beds beneath the more resisting sandstones 

 or limestones which form the hogback ridges. Where the harder beds rest 

 directly upon the crystalline rocks of the mountain slopes, or the upturned 

 edges of the Mesozoic beds are still covered by overlapping horizontal beds 

 of Tertiary age, or where, again, through faulting or any other cause, the 

 Mesozoic beds themselves still lie in a nearly horizontal position in contact 

 with the underlying complex of crystalline rocks, the hogback ridges may 

 be wanting. Still, the breaks in their continuity are in general of so lim- 

 ited extent that the foothill licit can nowhere he followed tor any great 

 distance without meeting them, if not in typical development, at least in 

 some modified form of fringing reef. 



In the Denver Basin area the hogback is found to extend in most 

 perfect form fr the southern boundary of the area nearly to Table Moun- 

 tain, a continuous knife-edge ridge of 1 >akota sandstone or quartzite, broken 

 only by the narrow gorges of the mountain streams; with a valley behind, 

 separating it from the mountain slopes, as regular and continuous as any 



