122 THE ILLINOIS GLACIAL LOBE. 



are now cut far below the level of the pre-Illinoiau valleys, but repre- 

 sents nearly as much removal of material, and the removal was probably 

 effected at a lower gradient, i. e., under less favorable conditions than are 

 now afforded. In this connection it should be noted that no evidence has 

 been found that the gradient of the streams was increased until after the 

 Iowan stage of glaciation. The inner valleys represent, therefore, post- 

 Iowan rather than post-Illinoian excavation. But this fact does not appar- 

 ently set aside the estimate given above, for if the pre-Illinoian valley 

 excavation had not reached the level of the channel opened by the Missis- 

 sippi at the Illinoian stage of glaciation, that valley would have furnished 

 a more direct and presumably more favorable line of discharge for all these 

 streams than their present line. The features along- the present line of dis- 

 charge for the three streams under consideration (eastward through Skunk 

 River) strongly support the view that a pre-Illinoian valley was formed in 

 the lower course of Skunk River. A broad terrace borders the portion of 

 the valley below the point where Skunk River crosses the abandoned valley 

 at Rome, which stands sufficiently low to have afforded a line of discharge 

 for the portion of the valley west of the abandoned channel. This would 

 have been blocked at the Illinoian ice invasion, but not necessarily con- 

 cealed or greatly filled by Illinoian drift. Upon the withdrawal of the ice, 

 if it chanced to stand slightly lower than the Illinoian course of drainage, it 

 would receive the post-Illinoian drainage and become the line of a reestab- 

 lished stream. 



There is near the mouth of Skunk River an exceptionally clear illustra- 

 tion of the work of that stream in the Yarmouth interglacial stage. By 

 reference to fig - . 4 it will be seen that a valley or depression turns away 

 from Skunk River at Augusta and leads southward to Lost Creek. This 

 valley is more than a mile in width and 30 feet or more in depth, and is 

 excavated in the Kansan till. Evidently it was opened by Skunk River 

 after the Kansan stage of glaciation and before the Illinoian stage. Whether 

 it carried the whole or only a part of the stream is not yet known. The 

 northern end has received only a thin coating of Illinoian drift. The 

 southern end received a sufficiently heavy deposit of that drift to prevent 

 its subsequent use as a drainage line. Since the Illinoian stage of g-lacia- 

 tion the entire drainage of the river has been directly eastward into the 

 Mississippi. Low swells of Illinoian drift occupy the south end of the 



