18 THE ILLINOIS GLACIAL LOBE. 



Occasional basin-like expansions of river valleys appear outside the 

 Coal Measures. One formed in the Hudson River or Maquoketa shales 

 and underlying formations along the Pecatonica River near Freeport has 

 a breadth of about -4 miles where widest, though usually it is but 2 or 3 

 miles. This basin has been discussed recently by Mr. Oscar Hershey as a 

 peneplain, 1 but to the writer it appears too immature to be thus classed. A 

 similar basin borders Elkhorn Creek in Carroll and Whiteside counties. 

 The Mississippi Valley also has an expansion where it crosses these shales 

 on the borders of Carroll County, Illinois, and Jackson County, Iowa, 

 being fully twice as wide as it is where cut in the Galena or in the Lockport 

 (Niagara) limestone. These contrasts in width are well shown in the 

 Clinton, Savannah, and other topographic sheets covering this part of the 

 Mississippi Valley. The Clinton sheet appears as PI. XVIII of this report. 



Most of the streams in this region have courses independent of the 

 preglacial drainage lines. It is only in the western half and southern 

 third of Illinois and in southwestern Indiana, Avhere the drift is compara- 

 tively thin, that the course of preglacial drainage can be confidently indi- 

 cated. Even here the larger valleys only are traceable, for the valleys of 

 the smaller streams have usually been completely filled, and deep borings 

 are too few to supply data to ma]) out their position and connections. The 

 effect of glaciation on the drainage will appear in the progress of the 

 discussion. 



1 Am. Geologist, August, 1896. 



