ROBERT HAY — GEOLOGY OF THE PLAINS. 



521 



In the region of the Black hills the Cretaceous rocks are broughl to view again 

 in descending order northward. North of the hills they disappear in reverse order, 

 and northeastward they thicken considerably, the Laramie of the lower Yellow- 

 stone and the Little Missouri "bad lands" attaining great thickness. 



Two facts in the topography of the mid-Plains region are to be noted : ( 1) there 

 is a decided valley between the Plains and the mountains, the former having a steep 

 western escarpment from near Pueblo to near Cheyenne, Wyoming. This valley 

 has its Tertiary formations, which are not here treated of. We would emphasize 

 the fact that in the region above described the Plains formations are cut off from 

 contact with the mountains. Running westward from Cheyenne is a ridge which 

 constitutes the highest part of the Plains, running up to nearly 7,000 feet, and on 

 this ridge the Plains formations abut against the mountains, overlapping the tilted 

 Mesozoic and Paleozoic formations and resting on the granite. There are traces — 

 the merest fragmentary patches — of this overlap down all the line of the foothills 

 to Canyon City, but this ridge in south Wyoming is apparently the only place 

 where modern erosion has not cut it away. It is the water-shed between the North 

 Platte and South Platte drainage, and north of this, down the Chugwater to the 

 North Platte, the western escarpment of the Plains is 1,0(10 feet high. (2) The 

 other fact is that the streams between the Platte and the Arkansas and some both 

 south and north of those rivers have their sources and courses on the Plains. Their 



Km. i i:r. 7. — General Section on tin- 102d Meridian. 



valleys, from 200 to 500 feet helow the level of the interfluvial plains, have been cut 

 by the meteoric agencies of the region, unaided by the mountain snows, and they 

 owe their perrenia] supply of water to the springs that issue from the Tertiary grit. 

 We are not treating hereof the mauvaises terres of the Dakotas, but these Tertiary 

 terranes in their weathering have, in, the valleys of Nebraska and Kansas, been 

 carved into fantastic forms of castles and buttes and palisades which vary by a 

 local picturesqueness the intense monotony of the plains. 



We desire here to call attention to the lines of investigation that w ill aid in the 

 elucidation of the phenomena of the plains. We have mentioned that erosion has 

 nol proceeded -n Ear (in the mid-Plains region) on the I02dason the 100th meridian ; 



I ml near the former line there are outcrops in many of the valleys which show the 



formations subjacenl to the Tertiaries. Surveys on the lOlsl meridian would, in 

 the Panhandle of Texas, cross the gashes cul by the Canadian and Red rivers to 

 the depth of i.oiiii feel : across Nebraska the same line would shoM very little out- 

 crop of Cretaceous rocks. A survey on the limth meridian from Dakota to the Rio 

 Grande would reveal largely the structure of the plains, and shorter lines north. 

 and-south further westward would show the variations of structure that characterize 

 part in ilar regions and the varying amount of the forces thai have combined in the 

 modern era to give the presenl physical characteristics to the region of the < rreal 



Plain-. 



