708 C. R. KEYES MID-COINTINENTAL EOLATION 



dance on the surface appears quite incongruous. Without considering 

 the real nature of the formations as a whole, judgment based upon sur- 

 face materials alone suggests river action as a possible explanation of the 

 presence of such coarse debris. In a moist climate the observations 

 would naturally be thus interpreted. 



The coarser materials of the Great Plains deposits are best considered 

 under the three groupings of sands, fine gravels, and pebbles including 

 boulders. Each of the three classes are susceptible of further sub- 

 division in treatment accordingly as they occur in the arid western belt, 

 the semi-arid median belt, or the moist eastern belt. 



Sands do not seem to play the important role which they are com- 

 monly assumed to do. In true desert country they are in the process of 

 being constantly ground into dusts, so that permanent accumulation is 

 rarely accomplished. Since the desert seldom appears as an area of 

 notable deposition, sands are only locally and temporarily preserved. In 

 the semi-arid belt in the so-called "sand-hills country" of Nebraska and 

 Kansas especially, the unusual accumulation of sands seems to be de- 

 pendent somewhat upon the local character . of the substructure or the 

 proximity to the through-flowing streams. At Kinsley, Kansas, for in- 

 stance, the rather remarkable area of sand-dunes appears at a point 

 where this material may be directly blown up from the Arkansas Eiver 

 channel. Farther eastward the eolic sands are known to extend well 

 into the region of moist climate, even to the Mississippi Eiver. It is a 

 noteworthy fact that these sand ridges, or dunes, are continually moving 

 forward, and the general advance is to the northeast. Many of them 

 have been long known in Iowa. The latter are often several miles in 

 length and half a mile or more in width. They destroy cultivated fields, 

 cover up fences and buildings, until in the course of two or three 3^ears 

 they have passed by a given spot. Bain^^ describes a rolling sand-dune of 

 this kind in Mahaska County, Iowa, that was 30 feet high. During its 

 passage across a certain farm it required three division fences to be built 

 in vertical succession. These dunes are on the upland prairie surface. 



The finer gravel materials which are often scattered through the 

 desert loams appear to be partly wind-blown and partly transported by 

 sporadic waters. On the desert the loose accumulations of small stones 

 temporarily form the pebble pavements or pebble mosaics described by 

 Blake,^* by Tolman,^^ and by me.^^ The strictly Avind-moved pebbles 



83 Iowa Geol. Surv., vol. iv, 1895, p. 343. 



^ Trans. American Inst. Mining Eng., vol. xxxiv, 1904, p. 161. 



85 Journal of Geology, vol. xvii, 1909, p. 149. 



saBull. Geol. Soc. America, vol. 19, 1908, p. 74. 



