Twenty-fourth Annual Meeting. 



nels — whose source, course, and outlet — are on the plains; that is, there are moun- 

 tain rivers and there are rivers of the plains. Into these latter no drop of mountain 

 water is carried. In the region called semiarid they have their birth and take their 

 course. Of these are the Red River of the North and the Red river of Texas, the 

 Little Missouri, the Pecos, the White river of Nebraska, the Medicine river of Kan- 

 sas, the Republican, Smoky, and Blue, the Running Water, and the Loup. These 

 rivers have cut gashes in the plains and laid bare something of the geological 

 structure. And besides this, they have varied the scenery — they have modified the 

 climate. 



Examination of these gashes shows us that the smoothest surface of the plains 

 is caused by a fawn-colored subsoil, which we speak of as the plains marl. This is 

 found on high land, and sometimes in valleys. It extends from the Rio Grande to 

 the Mauvaises Terres of Dakota. It is a lake silt from 2 to 200 feet thick — a fertile 

 soil to its lowest layer. Under this, and also where the plains marl does not cover 

 it, is a formation of limy grit, sometimes white plaster, (the terra blanca of New 

 Mexico, the native lime of west Kansas,) sometimes coarse mortar, sometimes a 

 hard conglomerate or loose gravel, with little lime. It holds the water for all the 

 wells of all the plains. None of this class of rivers, from Dakota to southern Texas, 

 have permanent water in them till their channels have cut through this tertiary grit 

 and found a bed on less permeable strata of the old formations. 



In Texas there are some other surface deposits, and in the Dakotas the whole 

 region east of the Missouri river has been overlain by a glacial drift, and long lines of 

 boulders, ridges of gravel and shallow lakes testify to the presence of the ancient 

 ice. These lake beds are silted up and are level meadows, and till and loess have 

 made other parts level. 



Under the surface formations, in nearly the whole region of the plains, the sub- 

 jacent rocks are of mesozoic age. On the upper Missouri, at Fort Benton, and 

 stretching down to southern Kansas, they are cretaceous (Laramie to Dakota and 

 Trinity); further south the Trinity, again in the Canadian valley, and the red beds 

 of the trias. Further east, even in Kansas, the surface tertiaries rest on rocks of 

 carboniferous age. 



Those who visit the grand caiion of the Colorado, usually learn the immense 

 force of erosion. This lesson may be learned on the plains. All the valleys in 

 which rivers run have been corraded by running water. The White river has cut the 

 plains north of Pine Ridge down a thousand feet, and left the escarpment of that 

 height a wall a hundred miles long. The Red river of Texas comes out of a ravine 

 also a thousand feet deep, carved out of the Llano Estacado — a gash made by the 

 agencies of nature, a wound gnawed by the tooth of time. 



This erosion has been done since the surface formations were laid down, in the 

 last tertiary or pleistocene lakes, but many of the valleys are on the lines of depres- 

 sions made by erosion on the old cretaceous surface, which the later lakes filled up. 

 and which the modern age has since reopened. This is true of the Kaw valley, and 

 notably of those of the Smoky and Republican. 



On the other hand, there are old valleys of the ancient premiocene erosion that 

 have been filled up by the later submergences and not reopened. In western Ne- 

 braska there are large depressed areas without outlet for their drainage, except some 

 underground percolation through sands and gravels that now fill up old river beds 

 to the level of the highest prairies. In Kansas there are similar areas, the most no- 

 table being that which has its eastern terminus in the basin at Scott City, into which 

 merges the valley of the White W^oman Creek, 100 miles long. An underground 

 channel probably allows water thence to percolate to the Arkansas, in the neighbor- 

 hood of Garden City. 



