14 CHICAGO ACADEMY OF SCIENCES. 



KANSAN DEIFT SHEET OF THE IOWA GEOLOGISTS. 



This is a sheet which has much importance in the drift series, 

 for it constitutes a large part of the drift of Southern Iowa and 

 Missouri and probably remains in extensive sheets beneath the 

 later drift in districts to the north. It apparently extends into 

 Western Illinois a short distance, beneath the lUinoian drift slieet. 

 It is a sheet that has as yet received but little attention, the chief 

 study given it being in Southeastern and Southern Iowa. 



SECOND INTEKVAL OF BECESSION OR DEGLACIATION. 



This interval is known not by a recession and readvance of 

 the same ice sheet, nor a sheet with the same gathering ground, 

 but by the recession of one ice sheet and the advance of another 

 ice sheet into the region thus abandoned. It has been studied only 

 in Southeastern Iowa, w'here the Illinois lobe has encroached 

 upon the territory which had been occupied by a more western 

 lobe. 



Evidences of the lapse of a long period between the with- 

 drawal of one ice sheet and the invasion of the other are clear and 

 decisive. The drift of the earlier sheet is a very calcareous till, yet 

 its surface had been leached to a depth of several feet before the 

 Illinois lobe encroached upon it. 



Accompanying the leaching was the formation of a black or 

 humus stained soil, numerous exposures of which are to be seen 

 in Southeastern Iowa under the drift of the Illinois lobe. 



There was also much erosion of the surface by stream action. 

 Prior to the discovery of the extension of the Illinois lobe into 

 Southeastern Iowa it had been noted by Professor Chamberlin, 

 as well as by the writer, that Southern Iowa presents a more 

 eroded appearance than Illinois and the southeastern counties ot 

 Iowa. In the former district remnants of the original drift plain 

 are confined to narrow strips between water courses, while in 

 the latter district such remnants are far more extensive, compris- 

 ing much more than half the surface. It is planned to make a 

 careful measurement of the relative amounts of erosion in the 

 two districts, for erosion studies made with proper analytical 

 method promise to furnish the most satisfactoiy means available 

 for measuring such time intervals. 



ILLINOIAN DKIFT SHEET. 



Under this name is included the sheet of drift which covers 

 Western Illinois and extends a few miles into Southeastern Iowa. 



