142 X. H. winchp:ll — was max ix America ix glacial period 



loam passes below into a quicksand. We have here, then, a series of changes by 

 which the loess-loam is produced from the drift hard-pan (or till) by the slow 

 withdraw^al of the stones and gravel and the gradual predominance of water action 

 over ice action." * 



Since the initial observation the same transition has been observed 

 repeatedly in Iowa by McGee, and by him such transition is made a fun- 

 damental fact in an elaborate discussion of the drift of northeastern 

 Iowa ; and the later Iowa surve}^ has not called it in question, but, on 

 the contrary, has added much to the details of this relationship. If such 

 transition be a fact, it is impossible to find an}^ place for the agenc}^ of 

 wind in bringing it about; but the presence and action of water are 

 everywhere evident in the structure of the deposit as it merges from till 

 to loess. 



Following is one of the many statements of Mr McGee affirming this 

 relation between the till and the loess : 



"The southern loess of northeastern Iowa generally grades imperceptibly into 

 unmodified glacial drift. So the southern loess is a connecting link not only be- 

 tween divergent phases of the deposit, but between the deposit itself and one com- 

 monly regarded as distinct in genesis and in period of formation. It is a hybrid ; 

 there is no place for it in the accepted taxonomies ; and no name commonly ac- 

 cepted can be applied to it that is not a misnomer with respect to certain of its 

 parts. For convenience it may be styled loess-drift or drift-loess, according to the 

 local predominance of the respective characteristics of two generally distinct de- 

 posits. 



" The passage from loess to drift in the southern counties is commonly gradual, 

 and the transition in the different diagnostic features of the two deposits is seldom 

 coincident, so that the zone of intermediate character, or passage bed, may be many 

 feet in thickness. Usually the fine comminution and homogeneitj'' characteristic 

 of the loess fail first, and an exceptional number of sand grains, at first fine, but of 

 increasing coarseness, and finally pebbles (sometimes polished and striated) ap- 

 pear; next, the fossils diminish in number, and finally fail ; with the diminution 

 of fossils the cla}^ element becomes predominant, and at about the same horizon 

 the obscure horizontal banding fails in turn, and the characteristic promiscuity of 

 the glacial drift ensues; then the calcareous tubules disappear, and are sometimes 

 replaced by films of carbonate of lime coating pebbles and accumulated in cleavage 

 planes; at length the loess-kindchen lose their characteristic nodular form and 

 cavernous structure, and either disappear or become transformed into plates and 

 irregular masses of carbonate of lime accumulated in pockets or about pebbles and 

 boulders; and finally the limy segregations fail. These successive changes super- 

 vene in such manner that the fossils sometimes extend into the horizon of striated 

 pebbles; the obscure banding of the upper part of the deposit may reappear well 

 within the promiscuous mass of the drift; the tubules and pebbles are frequently 

 associated, and the horizon of loess-kindchen commonly overlaps far upon that of 

 pebbles and boulders, indeed in nearly every deep roadside gully in southwestern 



*Sixtli Annual Report Minnesota, p. 105. 



