MINGLING OF EOLIC AND AQUEOUS EFFECTS 709 



are doubtless confined mainly to tlie arid tracts and only occur sparingly 

 in the semi-arid belt. Jn the latter and in tlie moist region the move- 

 ment of the coarser materials is accomplished almost entirely by water 

 action. 



The position of the coarser gravels along the paths of the through- 

 flowing rivers indicates clearly that they are all of fluviatile origin. 



MINOLING OF FOLIC AND AQUEOUS EFFECTS 



The eastern belt of the Great Plains is of special interest at this time, 

 because of an overlapping of both constructive and destructive effects 

 produced by the rains on the one hand and on the other hand by the 

 winds. This phase of the subject, however, requires especial elaboration 

 which can not be accorded it here. One point in particular may be men- 

 tioned — the relations of the eolic deposits to the glacial drift. In the 

 enthusiasm aroused during the past two decades in the consideration of 

 the last, there has resulted a misinteri)retation of the phenomena pre- 

 sented by the first. 



The southern boundary of glaciation is approximately the line of the 

 Missouri River from its headwaters to its mouth. Everywhere along the 

 glacial margin the drift-sheet is deeply covered by loess or loess-like de- 

 posits. Although it was generally known that the latter extended forward 

 from the edge of the drift, glacialists outside of Missouri^" made little 

 attempt to find out just how far such deposits occur. On the usually 

 accepted hypothesis of glacial origin the occurrence of the so-called mar- 

 ginal loess has never been satisfactorily explained. Viewed as the atten- 

 uated l)order of an eolic intrusion from the west, which, in spite of 

 vigorous corrasive activities, has accumulated since the retreat of the 

 Kansan ice-sheet, adequate reason is at once found for the long puzzling 

 peculiarities of its distribution near and without the drift area. 



The important feature to note is that although eolic deposits attain 

 vast development along the Missouri River, their true genetic character 

 is obscured by the vigorous action of the rains, this being within the 

 limits of moist climate conditions; it is confused by the presence of 

 extensive glacial formations ; it is easily misinterpreted because the typi- 

 cal deposits were never traced forward from the glacial boundary to their 

 limits, and it is overlooked for reason that continental terranes of eolic 

 origin have been little understood. That the real nature of the deposits 

 in question was not deductively inferred is due largely to the circum- 

 stance that they were invariably approached from the east instead of 



87 Todd : Missouri Geol. Surv., vol. vlll, 189G, p. 115. 



