EROSION OP THE KANSAN DRIFT SHEET. 121 



of a buried soil in the district west of the limits of the Illinoian, either under- 

 neath or within the Kansan drift, This feature makes it necessary to leave 

 open the question of the age of buried soils within' the Illinoian area which 

 occur at a level below the general elevation of the outlying Kansan drift. 



EROSION OF THE KANSAN DRIFT SHEET. 



The pre- Illinoian erosion of the Kansan till sheet is a third evidence of 

 the great length of the Yarmouth interglacial stage. Such erosion, as just 

 noted, is suggested within the limits of the Illinoian by the occurrence of a 

 buried soil below the general level of the Kansan drift surface. But 

 evidence may be found in the portion of the Kansan drift lying outside the 

 limits of the Illinoian which is not at all open to question. The evidence 

 first to attract notice was that of the relative degrees of erosion displayed 

 by the Illinoian and Kansan drift sheets. Prior to the discovery of the 

 extension of the Illinois lobe into southeastern Iowa it had been noted by 

 Charnberlin, as well as by the writer, that southern Iowa presents a more 

 eroded appearance than western Illinois and the southeastern counties of 

 Iowa, and the matter was discussed as a remarkable feature. In the district 

 outside the limits of the Illinoian the original drift plain is preserved only 

 in narrow strips along divides, estimated to comprise scarcely one-fourth of 

 the surface, while in the district covered by the Illinoian drift the remnants 

 are far more extensive, comprising apparently more than half the surface. 

 The branching of drainage lines is also carried to markedly greater maturity 

 in the Kansan than in the Illinoian drift. 



Definite means for determining the amount of pre-Illinoian erosion of 

 the Kansan is afforded by a study of the valleys in Kansan drift which 

 connect with the abandoned valley of the Mississippi, which was occupied 

 at the Illinoian invasion. These embrace the valleys of "West Crooked 

 Creek, Skunk River, and Big Cedar Creek. Valleys farther north have 

 generally been greatly modified by the Iowan invasion, and hence do not 

 furnish good illustrations. It is found that the three valleys just noted have 

 been cut at their points of connection with the abandoned valley to a depth 

 of about 50 feet below neighboring uplands and to widths of about three- 

 fourths of a mile, 1£ miles, and 1 mile, respectively, at the time the Missis- 

 sippi was occupying this abandoned channel, i. e., at the Illinoian stage of 

 glaciation. This width is two or three times that of the inner valleys, which 



