150 S. CALVIN PRESENT PHASE OF PLEISTOCENE IN IOWA 



Iowa: it carried quite a load out to its margin; it constructed morainic 

 knobs and ridges, such as Pilot Knob and its associates in Hancock 

 county, 100 feet in height in some cases, with material actually thrust 

 out and heaped up around the edge of the ice. Besides being a moraine- 

 forming ice-sheet, the AVisconsin is responsible for the largest amount of 

 Pleistocene gravels to be found in the state. The streams and surface 

 sheets of water were large in volume and were loaded to their full capac- 

 ity. Whole townships are covered with coarse outwash gravels in con- 

 tinuous sheets, and gravel trains occur along all the drainage courses 

 leading out from the edge of the Wisconsin drift. In a wide belt along 

 the western border of the lobe, in Osceola, Dickinson, Palo Alto, and 

 Clay counties, the surface is marked by hundreds of knobs and ridges of 

 gravel having the form of kames or eskers. This feature may be said 

 to culminate in Ocheyedan mound in Osceola county, a noted and con- 

 spicuous gravel kame so prominent that it commands attention from 

 every part of the area surrounding it within a radius of 25 miles. The 

 many scores of other gravel ridges along this western border of the Wis- 

 consin are only less interesting than Ocheyedan mound because they are 

 smaller in size. 



In marked contrast with all this evidence of kame-forming and general 

 gravel-depositing habit of the Wisconsin are the meager indications of 

 outwash from the Iowan. The Iowan ice was certainly very thin and in- 

 efficient near its margin, and most of the water produced by melting may 

 have been disposed of by evaporation. Along some of the streams which 

 drain the Iowan lobe and flow beyond its margin there are a few sand 

 terraces, usually small : but on the upper Iowa river, near Decorab. tbere 

 are extensive deposits of fresh sand which have been correlated with the 

 melting of the Iowan ice. Down in the valley of the Iowa river at Iowa 

 City, in the relatively young gorge which has been excavated since the 

 Kansan. there are sand terraces of Iowan age beneath river silt and ordi- 

 nary loess. On the whole, however, the evidence of floods of any force 

 or volume, flowing from the wasting Iowan ice, is almost zero. All the 

 known deposits referable to the melting phase of the Iowan are sands : 

 never do they assume the character of gravels such as would require the 

 agency of energetic currents. 



Comparing the surface features in the larger way, the differences in 

 these two drift sheets in Iowa are still obvious and decisive. Apart from 

 the gravel kames and morainic knobs already noted, there are differences 

 due to the larger quantity of material carried to the outmost limit of its 

 territrry by the Wisconsin ice. Owing to greater thickness of the glacial 

 deposit, there are, as a rule, no signs of the old Kansan topography ex- 



