CHARACTERISTICS OP THE GREAT PLAINS 689 



Concerning both the stratigraphy and the relief of the region, there are 

 few details to add to those contained in the already quite voluminous 

 literature on the subject. The published observations of geologists who 

 have traversed the area are acceptable without dispute. The tectonic 

 descriptions given by them abundantly corroborate one another. Only 

 the interpretation of some of the recorded facts are open to question. 

 The foundation of the interpretation here offered lies mainly in the 

 records of personal observations of a rather comprehensive character. 

 The generalizations derived from the observed facts are soon stated. 



DoiiiiNANT Characteristics of the Great Plains 



The origin of the most striking and characteristic features of the Great 

 Plains has never received adequate explanation. Although these several 

 characters have repeatedly been the subject of extended notice, there has 

 been little attempt to refer them directly to the geologic processes to which 

 they owe their origin. Five features in particular attract first attention : 

 (1) The great extent and uniformity of the substructural terranes; (2) 

 the remarkable evenness of the surface relief; (3) the peculiar loamy or 

 marly nature of the deposits; (4) the general vagueness of stratification, 

 and (5) the manifest recency of deposition. 



It is indeed a suggestive fact concerning their genesis that the Great 

 Plains deposits have been traced so continuously southward from the 

 Dakotas to New ^Fexico, where, at the southern end of the Pocky Moun- 

 tains, under conditions of arid climate, they are correlated with the 

 Galisteo sands and the Santa Fe marls of Hayden^ and desert adobe — 

 typical soils and formations of the desert. Still farther southward in- 

 distinguishable deposits continue far into Mexico. From this it may be 

 inferred that the "lake beds" of the Great Plains are of wide distribution ; 

 that they apparently have in great part a similar origin ; that they merge 

 imperceptibly into other formations, and that different parts are of 

 quite different geologic age. 



Long before the Great Plains region had come under the critical sur- 

 veillance of the physiograplicr its most distinguishing feature was accu- 

 rately described wlicn it was cliaracterized as an illimitable expanse of 

 flat, treeless country almost untrenehed by streams. This most striking 

 feature is, then, the remarkal)ly even plain of such vast extent. Only in 

 the northern part, where traversed by the Missouri River, and in the 

 immediate vicinity of that stream, are the Great Plains notably trenched. 

 There, on account of certain ])eculiarities of texture which tlie substruct- 



5 Third Ann. Rept., IT. S. Geological Survey of the Territories, 1873, p. 167. 



