34 THE ILLINOIS GLACIAL LOBE. 



penetrated by the wells in the various counties of the State. The structure 

 of the portion of the Illinoian drift in southeastern Iowa and western Illinois 

 is set forth in the detailed discussion of the border of the Illinoian drift sheet 

 which follows. 



THE DRIFT BORDER. 

 DISTRIBUTION. 



The border of an old drift sheet, tentatively referred to the Illinoian, 

 emerges from beneath the Wisconsin drift in southern Wisconsin a few 

 miles southwest of the city of Madison, and from that point southward to 

 the Mississippi River it forms the eastern border of the Driftless Area of 

 southwestern Wisconsin and northwestern Illinois. The border of this drift 

 sheet probably crosses the Mississippi a few miles below Savanna, but, as 

 shown in Pis. VI and XII, the presence of the Iowan drift sheet, together 

 with the heavy loess deposit, both of which are later than this drift, has 

 so obscured it that its limits can not well be determined in Clinton and 

 Scott counties, Iowa. In Muscatine County, Iowa, the slightly ridged west- 

 ern border of the Illinoian drift sheet becomes visible, and is distinctly devel- 

 oped from that county southward. It crosses the Iowa River just below 

 Columbus Junction and leads southward through western Louisa County, the 

 village of Cairo being within a mile and the village of Morning Sun within 

 3 or 4 miles east of the border. It passes thence in a course west of south 

 across northwestern Des Moines County and southeastern Hemy County, the 

 villages of Yarmouth and New London being situated on the ridged border 

 and the village of Lowell, in Skunk River Valley, being situated where the 

 border crosses that valley. In Lee County, as shown in fig - . 4, the course of 

 the ridge is southward past the village of West Point to the vicinity of the 

 Mississippi bluff, about 5 miles, below Fort Madison. From this point south- 

 ward to Keokuk the border apparently is nearly coincident with the course of 

 the Mississippi River, though there may be slight deposits of the Illinoian 

 drift on the west side of the valley. Immediately south of Keokuk, on the 

 Illinois side of the river, a distinct ridge of drift appears which marks the 

 border; and this may be traced southward along the east bluff of the Mis- 

 sissippi across Hancock and Adams counties, though in the southern portion 

 of Adams County it lies back a mile or two east from the river bluff. In 

 Pike County the border bears gradually away from the Mississippi bluff in 



