THE IOWAN DRIFT 601 



drift correlating with the Buchanan gravels. The Iowan loess is 

 related to the Iowan drift in much the same way that the gravels 

 are related to the Kansan. The earlier view was that the loess 

 was deposited at the time of. maximum development of the Iowan 

 glaciation, when the Iowan area was still covered with ice. The 

 only modification of that view at the present time is that loess 

 deposition took place after the Iowan ice had retreated to a greater 

 or less extent, after an interglacial interval had actually begun. 

 By such retreat extensive mud flats were left, and as these dried 

 before becoming covered with vegetation, strong winds coming, 

 probably, from the ice fields to the north, carried fine sand and dust 

 from the bare surfaces and deposited them beyond the edge of the 

 Iowan area, out upon the old, eroded Kansan. For the development 

 of loess three things seem to be necessary: (i) a gathering-ground 

 of extensive bare and dry surfaces, such as would be furnished 

 by the part of the Iowan area from which the ice had retreated; 

 (2) winds to transport the materials from the dried mud flats; (3) 

 anchorage such as would be furnished most extensively by the 

 vegetation of the extra-marginal Kansan surface. The bare Iowan 

 area afforded no anchorage, but it was an excellent source of supply. 

 Waters carried and sorted materials from the Kansan till and 

 deposited the interglacial formation called Buchanan gravels; winds 

 picked up from the Iowan till such materials as they could trans- 

 port, and deposited amidst the vegetation of the extra-marginal 

 territory the interglacial formation known as the Iowan loess. 

 The genetic relation of the loess to the Iowan drift is not so very 

 unlike the corresponding relation of the Buchanan gravels to the 

 Kansan; and so far as genetic relationships are concerned, there has 

 been no abandonment of the view originally proposed. 



The color, composition, and calcareous content of the Iowan 

 loess are in perfect accord with the hypothesis just expressed; its 

 geographic distribution around the lobed margin of the Iowan area 

 agrees also with the view; the great thickness of this loess at and 

 near its inner margin, and its thinning out with increasing distance 

 from the source of supply, corroborate all the other lines of evidence; 

 while the great amount of eolian sand associated with it in a narrow 

 belt surrounding the lobes of Iowan drift lends additional support. 



