38 THE ILLINOIS GLACIAL LOBE. 



Wright. As indicated above, its position in Monroe, Owen, and Greene 

 counties has been mapped in detail by Mr. C. E. Siebenthal, of the Indiana 

 Survey. The portion between Greene County and the southwest corner of 

 Indiana has been traced by the present writer. The portion mapped by 

 Siebenthal has been reconnoitered by the present writer, and also most of 

 the border in southern and southwestern Illinois. 



TOPOGRAPHIC EXPRESSION. 



The drift border in the portion examined by the writer, both in south- 

 eastern Iowa and in western Illinois, is generally marked by a low ridge, 

 seldom rising more than 60 feet above the outer border district, and 

 averaging perhaps 40 feet, In Adams and Pike counties there are a series 

 of ridges shown on the glacial map (PL VI), which have nearly parallel 

 trend, but which are broken by wide gaps, and represent imperfectly the 

 successive positions of the ice margin in these counties. The ridge forming 

 the border seldom exceeds 2 miles, and is usually but a mile or less in width. 

 On the eastern slope. there are low swells, 10 to 20 feet in height, extend- 

 ing out in places to a distance of several miles from the drift border, but 

 seldom showing a disposition to form connected chains or ridges. 



The portion of the drift border along the east side of the Driftless 

 Area in northern Illinois and southern Wisconsin is in places slightly thick- 

 ened beyond the usual depth of drift in districts to the east, but is in other 

 places very thin. It may be possible to trace an ill-defined ridging more 

 or less successfully near this border. The writer's observations touch it at 

 only a few points, and are not sufficiently full or continuous to justify an 

 opinion on this matter. 



From the point where the border crosses the Illinois River in south- 

 eastern Pike County, Illinois, southward to the Mississippi, there are occa- 

 sional knolls, 20 to 40 and occasionally 60 feet in height, the majority of 

 which are elliptical, with the longer axis trending ENE.-WSW., or about 

 at right angles to the trend of the drift border. These knolls do not lie at 

 the extreme border, but are situated 5 to 10 miles or more back from it. 

 Their form is drumlinoid, but seldom assumes the regularity of the typical 

 drumlin. 



The portion of the drift border touching the State of Missouri displays 

 only patchy deposits of drift, usually in the valleys or depressions, and 



