FOOTHILL TOPOGKAPHi'. 7 



well shows the typical features of foothill topography. In the immediate 

 foreground arc outcrops of Wyoming red sandstones resting upon the 

 Archean, and just beyond them, over the tops of the trees, can be seen a 

 rounded hill formed by the echelon fold in the Dakota sandstones. To the 

 left of this is a long, narrow ridge of basalt, known as the Ralston dike. 

 Ralston Creek, issuing from the Archean area in the lower right-hand corner 

 of the view, flows first southeast, then bends sharply east, cutting through 

 the Wyoming sandstone ridge, and, curving round the hill of Dakota sand- 

 stone, takes a northeasterly course to the plains around the north end of the 

 Ralston dike. In the middle distance an- the Table Mountains, with Green 

 Mountain beyond, seen over their western point. Between them and the 

 foothills of the range can be distinguished the long line of the main Dakota 

 hogback, whose continuity is broken at Golden, in the valley of Clear 

 Creek, and which in the foreground of the view is offset to the eastward 

 l>v the echelon fold. 



Instances of horizontal Tertiary beds overlapping the upturned edges 

 of the Mesozoic strata do not occur within the Denver Basin area, although 

 topographically the Table Mountains and Green Mountain somewhat 

 approach this type of structure. They are, however, no longer in actual 

 contact with the mountain slopes, but are separated from them by a valley 

 of erosion. 



PLAINS TOPOGRAPHY. 



The topography of the plains area is that characteristic ol a series of 

 recent and easily degraded horizontal beds long exposed to subaerial erosion 

 in a semiarid climate. 



It is a region of broad, shallow valleys with gently sloping sides, the 

 higher ridges being plateaus or mesas whose surface is formed by some 

 harder or more resisting stratum. The lower ridges within the basin areas 

 have also the mesa structure wherever they are capped by a harder stratum, 

 but when composed of homogeneous and comparatively yielding materia] 

 they have softly rounded outlines. The stream bottoms between are wide 

 and shallow, and sometimes are bordered l>v indistinct terraces. 



The whole area in Colorado is divided by the erosion of the Arkansas 

 and Platte rivers and their tributaries into two shallow basins, each nearly 



