710 C. R. KEYES MiD-CO'NTiNENTAL EOLATIOl^ 



from the west — from the side of moist climate conditions in place of 

 from the side of aridity. 



ROCK-FLOOR OF THE PLAINS ALONG ROCKY MOUNTAIN FRONT 



In its general features the planation of the arid region to bedrock has 

 been so recently considered^^ that little need be added here, except to 

 emphasize the extent of the rock-floor immediately in front of the Cor- 

 dillera. In the present connection attention should be called to the con- 

 trasts presented by this western belt of the Great Plains as an area of 

 deflation and the median and eastern belts as tracts of eolic deposition. 

 As an eolic product the even rock-floor worn out on the beveled strata of 

 the substructure has particular interest and significance. 



RULINa GRADIENT OF PLAINS SURFACE 



Penck^^ makes the observation that, provided the waters of the ocean 

 be excluded, wind may go on indefinitely excavating the desert below 

 sealevel. The inference is that for deflation there is no baselevel of 

 erosion comparable to that of stream planation. More recently it has 

 been suggested that in such case permanent groundwater-level seems soon 

 to present conditions on account of which wind-scour can no longer act.^^ 



In a desert of mountainous aspect — as the Great Basin, for example — 

 the inclination and smoothness of the intermont plains are determined 

 partly by the general tendency of wind-scour to form more or less even 

 surfaces and partly by action of the sheetflood. A region of flat-lying 

 strata — especially an area undergoing deflation in one part and aeroposi- 

 tion in another — is little influenced in its grading by the sheetflood. The 

 through-flowing rivers and other waterways doubtless constitute the 

 main factor in the determination of the gradient of the general surface. 

 These features are brought out in another connection. 



Objections to the Lacusteine Theory 



At this time and distance it is hardly necessary to state at length the 

 shortcomings of the hypothesis of a lake origin of the Great Plains de- 

 posits. Insuperable seem to be the vastness of the lake conditions re- 

 quired, the absence of any known eastern barriers, the want of distinct 

 shorelines, the presence of interbedded coarse formations, the vague 



88 Bull. Geol. Soc. America, vol. 19, 1908, p. 63. 

 8» American Jour. Sci. (4), vol. xix, 1905, p. 167. 

 »» Journal of Geology, vol. xvii, 1909, p. 659. 



