FOURTH INTERGLACIAL INTERVAL 149 



stained Kansan drift; (2) fossiliferous loess containing Succinea, Pupa, 

 and other terrestrial mollusks belonging to species still living in the local- 

 ity, and (3) Wisconsin drift, with boulders 3 to 4 feet in diameter. 

 Many of the exposures were made in grading streets or excavating for 

 foundations, but a section that will not be concealed for some time may 

 be studied at the tile works in East Des Moines. Still better sections 

 were seen a few years ago in the fresh cuts made by the Great Western 

 railway in the western edge of the Wisconsin lobe, near Carroll. These 

 were studied in detail by Shimek, and showed (1) typical Kansan ox- 

 idized and otherwise weathered for some feet below the original surface; 

 (2) an old blue, fossiliferous loess, with a weather-stained band at the 

 top and many ferruginous cylindrical concretions throughout its whole 

 thickness; (3) a much younger, unaltered, yellow post-Iowan loess, and 

 (4) Wisconsin drift. There are no known sections showing direct con- 

 tact of the Wisconsin with the Iowan, but the yellow loess on which the 

 Wisconsin rests at Carroll and at so many other points in Illinois and 

 Iowa has, on good grounds, been correlated with events which followed 

 the disappearance of the Iowan ice. The loess of certain parts of the 

 Mississippi valley is a deposit belonging to the Peorian interglacial in- 

 terval, and the Wisconsin lies on top of it. The changes whch took place 

 in the Iowan drift during this entire interval are too small for numerical 

 or relative expression. The interval, compared with the Yarmouth or 

 the Sangamon, was very short. The Iowan is probably not more than 

 twice as old as the Wisconsin. 



Wisconsin Drift 



CHARACTERISTICS OF THE WISCONSIN COMPARED WITH THOSE OF THE 



IOWA A 



The Wisconsin drift sheet is something very distinct from the Iowan. 

 It differs in composition, being excessively calcareous, while the amount 

 of lime carbonate in the Iowan, in any form, is very small. The calca- 

 reous constituent of the Wisconsin drift takes the form of fine limestone 

 flour mixed with the clay, together with great numbers of limestone peb- 

 bles. It differs also in color ; the color is a lighter yellow. Furthermore, 

 the Wisconsin differs 'from the Iowan in habit. The Iowan drift is thin 

 and meager, and fails completely over large areas toward the margin of 

 the lobe. From areas even in the interior of the lobe it may be absent. 

 The Wisconsin drift is more abundant; it completely disguises the pre- 

 Wisconsin topography; in most cases it becomes thicker toward the 

 margin. The ice of this stage seems to have been heavier and more 

 energetic; it scoured down to bedrock in the limestone areas north of 



XI — Bull. Geol. Soc. Am., Vol. 20, 190S 



