146 B. SHIMEK PLEISTOCE^'E OF SIOUX FALLS AND VICINITY 



tion is frequently separated from the overlying Aftonian l)y a thin layer 

 of laminated, silty material of similar toughness, the latter heing gray 

 (as if weathered) or sometimes ferruginous. A similar laminated layer 

 sometimes separates the Aftonian from the underlying Cretaceous shales 

 when the Nebraskan is missing, as in the exposures near Chatsworth and 

 at other points just south of our territory. It is probable that much of 

 the finer Nebraskan drift material was derived from the dark Cretaceous 

 shales. In exceptional cases numerous boulders, large and small, many 

 of them planed, are scattered throughout the formation, as in the ex- 

 posure in section 33, township 99 north, range 48 west, in Lyon County, 

 Iowa. The ver}' tough whitish or light gray stratum which appears 

 under sand and gravel in several sections probably also belongs with the 

 Nebraskan. 



While the jSTebraskan of this region sometimes presents the gray phase, 

 the greater part of it is dark and tough. It is also somewhat variable in 

 the coarseness of fracture, but not more so than the Xebraskan (sub- 

 Aftonian) of Aft on Junction, Council Bluifs, and other points where the 

 older drift has been recognized; indeed, it seems to be identical with the 

 latter in all respects. 



The Nebraskan of this region is very variable in thickness. As noted, 

 the finest exposure which the writer has thus far found here or elsewhere 

 is that which was formed by a land-slide in the Big Sioux Eiver bluffs in 

 section 33, township 99 north, range 48 west, in Lyon County, Iowa. 

 Here fully 60 feet of typical blue-black Xebraskan, containing, however, 

 an unusual number of boulders, are exposed. In the Beloit section about 

 the same thickness is somewhat less clearly exposed. In section 8, in the 

 Big Sioux bluffs, Xebraskan was found at a height of 125 feet above the 

 river, but both the Xebraskan and the Aftonian at this point show evi- 

 dence of disturbance, and were probably plowed up by the Kansan ice- 

 sheet, which left its burden of drift on the summit and northeast slope of 

 the ridge. Evidence of such plowing may also be seen in some of the 

 sections near Sioux Falls. 



Other sections present smaller exposures of this drift, but it is evident 

 that the sheet is still quite thick. 



Aftonian intcrglacial deposits. — The Aftonian as interpreted in this 

 region is exposed in the bluff sections and consists of silts (usually gray), 

 sand, gravel, and boulders. While the silt when present usually forms 

 the uppermost member of the formation, an examination of the section 

 records herein presented will show that there is no fixed relative position 

 of the silt, sand, gravel, and boulders, but that they are variously disposed. 

 The coarser materials are usually more or less distinctly cross-bedded, 



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