690 C. R. KEYES' MID-CO'NITINENTAL EOLATION 



lire presents, the so-called mauvaises terres result. The topographic 

 modifications of the eastern border, for reason of the nearness to moist 

 climate conditions and the prevalence of normal streams, are described 

 in another connection. 



A remarkable characteristic of the Great Plains formations, and one 

 which has always been a fertile theme of discussion, is its peculiar loamy 

 texture. In this respect they closely resemble the loess. Their homo- 

 geneity of texture over wide areas and through great thicknesses is a 

 feature which has long been puzzling. 



The stratigraphy of the Great Plains formations is singularly obscure. 

 In the relatively few tracts and small areas in which outcrops are well 

 displayed the finer materials are disposed in layers so thin and even that 

 they might be easily mistaken for lamination produced by deposition of 

 sediments in quiet waters. On the other hand, certain gravel trains near 

 the Eocky Mountains point at once to fiuviatile action. 



The title Fresh-water Tertiaries, long applied to the Great Plains de- 

 posits, fairly indicates general opinion regarding their genesis and geo- 

 logic age. Later investigations show conclusively that under this term 

 are included deposits of all ages from early Tertiary to the present. The 

 most noteworthy determination is that of the extreme recency of the 

 major portion of the exposed terranes. 



Explanations of Plains Formations 

 general statement 



Unlike the case of the majority of geological formations, the sub- 

 structure of the Great Plains has had ascribed to it every known method 

 of sedimentary origin. Its surface relief has been also variously ac- 

 counted for. From the time when the deposits were first brought to pub- 

 lic notice and the entire region thought to be the upraised bottom of the 

 sea, until the present day, no less than half a dozen hypotheses of origin 

 have been advocated. 



In this one formation and in this single topographic feature is reflected 

 in all its phases the trend of sedimentative and physiographic thought in 

 America. For this reason alone the theme merits more than passing 

 note. Critical criteria may be evaluated and compared as is nowhere else 

 possible in this country. 



MARINE DEPOSITION 



From the first the great interest in the geology of the region unde^ 

 consideration centered in its post-Paleozoic history. The early expedi- 



