CHAPTER VII 



THE NEBRASKAN ICE INVASION AND THE AFTONIAN 

 INTERGLACIAL INTERVAL 



I. The Nebraskan Ice Invasion 



The extent of the Nebraskan invasion is not positively known, as it fell 

 short of the later Kansan invasion and is buried beneath the drift sheet of 

 this stage. It has been recognized in many places in Iowa and Nebraska and 

 is correlated with early drift deposits in Pennsylvania and New Jersey (Jer- 

 seyan). Between these two areas few deposits referable to this stage are known. 

 The ice radiated from the Keewatin center of accumulation and apparently 

 extended down the Missouri Valley to an unknown extent. 



The Nebraskan drift is described as "a dark blue-black joint clay, some- 

 times more or less ferruginous, which when dry is hard and brittle, and breaks 

 up into very small angular blocks (resembling lumps of ordinary- starch, as 

 has been suggested). It is almost impervious to water, and when wet is very 

 tough, tenaceous, 'rubber-like,' and so difficult to work that it is the abom'na- 

 tion of well-diggers and road-workers, being the despised of all 'gumbos.' "' 

 The clay contains a few usually dark colored pebbles and small boulders, many 

 of which are angular and exhibit planed and striated faces, indicating that the 

 Nebraskan is a true drift sheet. Prof. Upham* believes that the Nebraskan 

 ice invasion occurred in the latter part of the Lafayette formation, following 

 the Ozarkian epeirogenic uplift. 



II. The Aftonian Interglacial Stage 



Resting upon the Nebraskan drift are deposits of gravel, sand, and fine 

 silt "variously interbedded and cross-bedded, and evidently deposited by 

 currents of different velocities. " The gravel is variously disposed, being 

 at the bottom in one place, at the top in another, and in a few sections it is 

 irregularly interbedded with the sand (Shimek). 



A. Organic Remains 



The biota of the Aftonian Interglacial stage has been carefully investigated 

 and described by Prof. B. Shimek, 3 whose data form the basis for the discussion 



1 Shimek, Geol. Iowa, XX, p. 307. 



2 Amer. Geol., XXX, pp. 135-150, 1902. 



3 Geol. Iowa, XX, pp. 271-486. The name Aftonian was first used by Dr. Chamberlin 

 in Geike's "Great Ice Age," 1894, pp. 773-774; and in the Journal of Geology, III, p. 272, 



1895. 



