VASTNESS AND EVENNESS OF THE GREAT PLAINS 703 



EoLic Significance of certain Great Plains Features 



VA8TNE88 AND EVENNESS OF SURFACE 



Tn the region east of the Eocky Moimtains there is an expanse un- 

 hroken and a smoothness of surface that are unparalleled elsewhere on 

 this continent. The remarkable evenness of the surface is almost per- 

 fectly developed in the median north and south belt. Eastwardly, as the 

 region of moist climate is approached, it begins to be broken by normal 

 stream corrasion. Westwardly, to the foot of the Cordilleras, it also is 

 somewhat interrupted by the inequalities arising from deflative action. 



The broad median belt is practically untrenched by drainage-ways. 

 This fact is also one which Johnson'^^ emphasizes in his description of 

 the "High Plains," as he terms this belt. It seems smoother than any 

 water-carved plain possibly can be. Its evenness is more complete than 

 that of any known peneplain. It is more perfectly a plain than even the 

 ideal peneplain demands. In itself this fact suggests that the surface 

 may have been fashioned by erosional processes of a kind with which we 

 have had as yet little to do. 



With the median "High Plains" Johnson correlates the great Llano 

 Estacado, or "Staked Plains," of Texas. The latter, as shown by Hill,*^* 

 are a part of the broad Las Vegas plateau of New Mexico, which also 

 finds a vast southern extension in the general plains surface of the Mexi- 

 can tableland. The intermont plains of the Great Basin present, on a 

 smaller scale, the same general aspects. In the arid plains of South 

 Africa Passarge^^ also finds plains smoother than peneplains, astutely 

 noting that "Wasser ist nicht imstande solche Ebene zu erodieren." 



The general plains surface of the region under consideration has had a 

 complex origin. The western part along the Rocky Mountain front is 

 clearly a plain of deflation. It is worn out on the beveled edges of an- 

 cient hard strata. This belt is wider and the bevelment is more clearly 

 displayed in the south in Colorado and New Mexico^® than elsewhere. 

 Only locally is notable aeroposition taking place. Stream aggradation 

 is also local and unimportant. Desert-leveling is progressing as rapidly, 

 probably, as it ever does, but it is devoloping unequally in different 

 places. Plant growth is scant and nowhere holds down the soil. 



The broad median belt of the Great Plains is one of smoothest relief. 



■^3 Twenty-second Ann. Rept. V. S. Geological Survey, 1808, pt. iv. 

 '♦Topographical Atlas of the Fnlted States, folio 3, 1900. p. 2. 

 'sZeitsch. d. deut. geol. Gesellsch., Ivi Bd., 1904, Protokol, p. 196. 

 '« Journal of Geography, vol. v, 1906, p. 254. 



