IOWA ACADEMY OF SCIENCES. 97 



blue clay phase of the Kansan is unusual in the presence 

 of interstratified beds of fine sand and in the abundance of 

 woody material. It is dark and might readily be taken for a 

 buried soil, though there is some doubt whether this is the true 

 interpretation. The portion of the pit examined by McGee and 

 Chamberlin is not now oi3en to examination. It seems to have 

 then presented much better evidence of a soil above the gravel 

 than can now be found. The material seen is stated to have 

 given a clear impression that it was a mucky soil accumulated 

 on the lee slope of the gravel hill. It contained much vegetal 

 material, and while normally but three to four feet thick, was 

 at one place bunched up to a thickness of six or eight feet. 

 The material now found at that horizon some few feet farther 

 east is full of pebbles and, except for the darker color and 

 woody material, does not differ from blue bowlder clay. 



The question raised by the various Afton exposures are 

 numerous. The principal ones are (1), are there two distinct 

 drift sheets present? and (2), if two drift sheets be present is the 

 unconformity above or below the gravel beds'? The earlier inter- 

 pretation was that two drift sheets were present, that the gravels 

 represented kames connected with the retreat of the earlier ice, 

 and that the blue-black clay at the base of the Kansan as seen at 

 Thayer was in part, at least, a soil, and marked the Aftonian 

 horizon proper. An alternative hypothesis would consider the 

 evidence of two drifts, so far only as these particular exposures 

 are concerned, as perhaps not wholly unassailable, and would 

 place the dividing line below the gravels. In support of the 

 latter hypothesis, it may be urged that so far as the exposures 

 now show there is nothing comparable to a soil above the grav- 

 els except at Thayer, and that even here the beds may be 

 explained, though perhaps with some difficulty, as merely a 

 portion of the blue clay phase of the Kansan. The passing of 

 the gravels by lateral transition at three points into bowlder 

 clay undistinguishable from, and apparently connected with, the 

 overlying Kansan, would seem to argue a contemporaneity of 

 age. It is possible, however, that the effect of a later ice 

 sheet working against the edge of a loose gravel hill would be 

 to obscure the distinctness of the two deposits more than has 

 been thought. Perhaps the loess-like clay seen beneath the 

 stratified beds and proven by test pits to run beneath some, at 

 least, of the gravel, may be urged as evidence of an unconformity 

 below the gravel. As it is quite probable that the gravel pits 



7 [la. Acad. Sci., Vol. v.] [May 2, 1898.] 



