704 C. R. KEYES^ MID-COINTINENTAL EOLATION 



Its surface is rather higher than the country on either side. The hard 

 substructure is deeply covered by fine, soft materials. Its surface sus- 

 tains a peculiar matted grass growth, colloquially called "sod/' that not 

 only protects it from deflative action and the sporadic corrasion of the 

 rains, but serves to hold the dusts settling from the air. Desert-leveling 

 is mainly of the constructive sort. The region is one of notable depo- 

 sition. 



East of the "High Plains" belt is a third wide tract, still constituting 

 a part of the Great Plains region, but extending an indefinite distance 

 until it merges completely with the prairies. It is a region far beyond 

 the limits of the desert, but still subject to some of its effects. Aero- 

 position is going on at a rapid rate, yet removal of the materials thus 

 deposited by stream corrasion is progressing almost as fast. The moist 

 climate conditions permit the rivers to cut down into the hard pre- 

 Tertiary rocks beneath. All eolic deposits and effects are largely dis- 

 guised or obliterated. The region is one of moderately rapid degrada- 

 tion and the surface is now maturely dissected. Nevertheless eolic de- 

 posits are accumulating locally, the most conspicuous of which are known 

 as loess.'^'^ 



DISSONANCE OF STRATIFICATION 



Singularly enough, the feature of the Great Plains deposits that is 

 most characteristic of eolian deposition is the very one about which least 

 has been said. This is the irregular, imperfect, discordant, or discon- 

 tinuous sedimentation. Much as has been written concerning the geol- 

 ogy of this region, the utter dearth of critical data upon the stratigraphy 

 is indicated in the summary on the Neocene "Lake beds of the interior," 

 as given in the correlation papers by Dall and Harris. "^^ A single short 

 paragraph suffices to set forth all our knowledge on the subject. 



One of the most important points recently brought out concerning the 

 so-called lake beds is the fact that in geologic age they range from Mid- 

 Tertiary to the Present. The most complete sections of any of the 

 beds under consideration are exposed in the mauvaises terres of the Mis- 

 souri Eiver region in South Dakota. As already stated, they were early 

 noted by Evans, later described in some detail by Hayden, and named by 

 Meek and Hayden^^ the White Eiver group. In the subsequent tracing 

 southward from the original locality of the White Eiver formation its 



"American Jour. Sci. (4), vol. vi, 1898, p. 299. 

 ''SBun. 84, U. S. Geological Survey, 1802, p. 175. 

 '» Proc. Acad. Nat. Sci. Philadelphia, vol. xiii, 1862, p. 433. 



