406 B. SHIMEK AFTONIAN SANDS AXD GRAVELS IX IOWA 



several rods back from the bluff in which the pit is located. The gravel 

 in the well is probably Aftonian. 



Other well-borings, though less definite, also indicate that the Aftonian 

 gravels extend well into the bluffs. 



THE AFTONIAN INTERGLACIAL WITH MILD CLIMATE 



That these sands and gravels belong with neither the sub-xAftonian nor 

 Kansan drifts is shown by the following evidence : 



1. They are not sub- Aftonian, because in every case examined they lie 

 unconformably on the older drift, the old oxidized and weathered surface 

 of which sharply marks the line of division between the two deposits. 37 



2. They are not Kansan, for in nearly all the exposures Kansan is 

 shown clearly resting unconformably on them, with calcareous plates 

 (nodular), cemented sands and gravels, and strongly oxidized materials 

 sharply denning the line of division. 38 



Moreover, evidence is furnished by several exposures that the Kansan 

 passed over the Aftonian beds while the latter were frozen, and plowed 

 and tilted them in mass or disturbed and folded them in intricate fashion. 



Thus in the McGavern pit, south of Missouri Valley, the Kansan pre- 

 sents a sharp but very irregular line of contact with the Aftonian, and 

 cross-bedded and stratified masses of the latter are twisted and folded 

 until in some places they are nearly vertical. This is shown especially 

 well in plate 36, figure 2, which represents the south end of the section 

 shown in plate 36, figure 1. Here the line between the Kansan and 

 Aftonian is very sharp, and the former is shown projecting into and 

 against the latter, which has had its strata pushed into an almost vertical 

 position. 



The Murray Hill exposures 39 show that both the Aftonian and sub- 

 Aftonian were crowded and folded by the Kansan. The Aftonian gravels 

 have been here pushed up to an unusual height — as noted, more than 100 

 feet above the valley. In these exposures sub-Aftonian and Aftonian 

 masses ("boulders") are common in the lower part of the Kansan, which 

 had evidently not only moved great masses of frozen gravels (for the 

 latter, even when most folded and tilted, show the original stratification 

 and cross-bedding), but also plowed into the underlying sub-Aftonian, 

 which is sometimes folded, and shows cleavage, concentric with its face 

 or front, evidently due to the great pressure of the advancing Kansan. 



A pit below Woodbine also shows a tilted layer of Aftonian, with a 

 mass of mingled and folded sub-Aftonian, Aftonian, and Kansan in front 

 of it, as though the whole tilted Aftonian mass had been moved forward. 



37 See plate 34, fig'ire 1, 



3S See plate 33, figures 1 and 2 ; plate 34. figure 2, and plate 35, figure 1. 



39 See plate 37, figure 1. 



