28 THE ILLINOIS GLACIAL LOBE. 



the drift becomes more variable in its constitution than at points remote 

 from the boundary. This variableness is set forth in the detailed discussion 

 of the drift border below. 



The till which forms this Illinoian sheet is usually of a yellowish-brown 

 color to a depth of 15 feet or more, beneath which it assumes a gray or 

 blue-grav color. In many places there is a transition from the brown to 

 the gray, in which gray streaks remain in the brown till, or cracks stained 

 a brown color extend down some distance into the gray till. In such places 

 it is probable that the brown is simply an altered gray till, the oxidation of 

 the iron having produced the change in color. In places a thin bed of sand 

 or gravel occurs at the junction of the brown and gray till, which gives 

 them the appearance of being originally distinct. But it is not certain that 

 the brown till in such places was not originally gray in color. The points 

 at which there is a transition from till of one color to that of the other are so 

 numerous that it seems highly probable that the brown till is generally but 

 an altered phase of the gray. At least nothing decisive has been discovered 

 to indicate that the brown and gray tills are referable to distinct invasions 

 or to different modes of deposition by the ice. 



In portions of central Illinois, especially in the Sangamon and Kaskaskia 

 and Embarras drainage basins, the well diggers and drillers report a marked 

 change in the texture of the drift in passing from the brown to the gray 

 tills. The gray till is said to be much harder to penetrate than the brown. 

 "Where wells are dug, they may be spaded without difficulty through the 

 brown till, while in the gray till a pick is usually required to remove the 

 material. This difference may be due to the effect of ground water or to 

 some secondary change in the brown till which does not affect the underly- 

 ing gray till, a change which is coextensive with the change in color. It 

 may, however, prove to be an original difference and may be of significance 

 in determining the glacial history. Possibly the gray till in these districts 

 is sufficiently older than the overlying brown till, or sufficiently distinct from 

 it in deposition, to be referred to a separate stage and considered pre-Illi- 

 noian. But few exposures of this hard till were found, and in these no sign 

 of a distinct interval between the brown and the gray tills was recognized. 

 The available evidence is, therefore, of an inconclusive nature. 



Gumbo (>). — The Illinoian till sheet, and also portions of the Kansan till 

 sheet in southeastern Iowa and northern Missouri, are extensively covered 



