EXPLANATIONS OF PLAINS FORMATION 693 



recently degraded on account of the heavy precipitation of a humid cli- 

 mate, while the desert portion to the west is a degraded surface because of 

 the light precipitation of an arid climate. 



FLU VI ATI LE AGGRADATION 



The necessary conclusion that after issuing from the mountains heav- 

 ily loaded streams must, on account of the abrupt dirainution of gradient, 

 drop a considerable part of tlieir burden doubtless has given rise to the 

 deduction that in the case of the Great Plains deposits they also are en- 

 tirely of aggradative origin. 



So far as 1 have been able to ascertain, Gilbert^- appears to be the first 

 to call into question the generally accepted notion that the Great Plains 

 deposits were strictly lacustrine formations. In eastern Colorado he re- 

 gards the Tertiary deposits as mainl3^ due to stream-work, but aided some- 

 what by wind-borne dust and sand and some slight augmentations of lake 

 silts. Haworth,^^ from his study of the deposits as displayed in the west- 

 ern and central portions of Kansas, concludes that lake beds are few in 

 number and small in extent, but tliat the major part of the formations 

 are essentially river deposits. 



Farther north, Matthew^"^ sets forth considerable evidence to show that 

 the White River Tertiary beds may be of eolian origin. Davis,^^ after a 

 visit to the Great Plains in company with Penck, formulates his reasons 

 for assigning to the deposits in question a strictly fluviatile origin. This 

 particular aspect of the problem is considered in detail elsewhere. 



WIND-SCOUR AND ITS EFFECTS 



Although specific work of the winds on the Great Plains has never been 

 critically discussed, there has been occasional reference to eolic activities. 

 A dozen or more years ago-'' it was pointed out that the wind should be 

 regarded as a potent factor in the formations of the great loess deposits 

 along the ]\Iissouri River in the prairie region of the Great Plains." This 

 view was further emphasized by Bain, Leverett,^^ and others. WinchelP® 

 graphically describes the powerful effects of a dust storm in the Dakotas. 

 Farther south, in the New Mexico part of the plains region, Tarr-^ men- 



22 Seventeenth Ann. Rept. T^. S. Geological Survey, 1896, pt. ii, p. 575. 



23 Univ. Geol. Surv. Kansas, vol. ii, 1807, p. 281. 

 2* American Naturalist, vol. xxxiii, 1809, p. 403. 



25 Proc. American Acad. Arts and Sciences, vol. xxxv, 1900, p. 345. 

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

 27Zeitsch. f. Gletscherkunde, iv Bd., 1910, p. 229. 

 28 American Geologist, vol. ill, 1889, p. 397. 

 2» American Naturalist, vol. xxiv, 1890, p. 457. 



