626 THE ILLINOIS GLACIAL LOBE. 



BUREAU COUNTY. 

 GENERAL STATEMENT. 



Bureau County is situated east of Henry, in the north-central part of 

 the State, its southeast border being at the Illinois River, near the great 

 bend. It has an area of 870 square miles, and Princeton is the county 

 seat. The northwest part of the county is a marshy and sandy district, 

 drained westward by Green River; the central and northeastern portions of 

 the county are drained by Bureau Creek, a tributary of the Illinois; the 

 southwestern portion, which is an elevated district, divides its waters 

 between Green River on the north and Spoon River on the south. 



The bulky morainic system forming- the outer border of the Wisconsin 

 drift passes southward through the central portion of this county, leaving 

 a belt 10 or 12 miles in average width on the west border of the county, 

 which is outside the limits of the Wisconsin drift. This morainic system 

 constitutes the highest portion of the county, much of its main crest being 

 above 900 feet. Yet it apparently crosses and occupies for a few miles the 

 deep preglacial valley through which Rock River is supposed to have dis- 

 charged to the Illinois. The moraine is so bulky as to completely conceal 

 the course of this valley, whose presence is known only by well borings. 

 It is probable that the drift in places exceeds 600 feet in depth where the 

 moraine occupies this old valley. The sections given below will set forth 

 the basis for this opinion. 



With the exception of a narrow strip on the southern border of the 

 county, and the elevated southwestern portion outside the Wisconsin drift 

 sheet, the drift of this county is a very heavy deposit. The average of 29 

 well sections which reach rock, the majority of which are in the southwest 

 part, show a thickness of 155 feet; but this is evidently much below the 

 average for the county, since 82 records of deep wells which do not reach 

 rock and which are scattered widely over the county are found to average 

 190 feet in depth. It is probable that the average thickness for the county 

 exceeds 200 feet, and it may possibly reach 300 feet. It is thought that 

 the thickness is greater in this county than in any other within the State. 



The upper portion of the drift, to a depth of 100 feet or more, in the 

 portion of the county covered by the Wisconsin sheet is composed in the 

 main of a blue till. The well sections indicate that the lower portion of 



