516 THE ILLINOIS GLACIAL LOBE. 



COPPERAS CREEK. 



This small western tributary of the Illinois drains an area of about 

 150 square miles in southwestern Peoria County. Several of the tribu- 

 taries, as well as the creek itself, have their courses largely determined by 

 preglacial drainage lines which are only partially filled by glacial deposits. 

 There are other small western tributaries of the Illinois between Copperas 

 Creek and the mouth of Spoon River, which also have their courses largely 

 determined by preglacial drainage lines. 



SPOON RIVER. 



This large western tributary of the Illinois has an estimated drainage 

 area of 1,870 square miles. It enters the river opposite the city of Havana. 

 The sources of the stream are in southwestern Bureau County, a few miles 

 west of the great bend of the Illinois, and the course of the stream for 

 nearly 100 miles is west of south, nearly parallel with that of the Illinois 

 River. It then turns abruptly southeastward, and in a distance of about 25 

 miles joins the Illinois. 



In its headwater portion Spoon River receives the drainage of the 

 western slope of a portion of the Bloomington and Shelbyville morainic 

 svstems. With this exception the entire watershed lies outside the limits of 

 the Wisconsin drift, occupying a region covered by Illinoian drift, upon 

 which there is a capping of loess. 



The course of the main stream, and also of several tributaries, appears 

 to have been determined largely by preglacial drainage lines which have 

 been but partially filled by the glacial deposits. The filling, however, has 

 been so great that the present streams conform only in a general way to 

 the course of the preglacial lines, and thus at a few places they trench upon 

 the projecting rock points in the bluffs of the preglacial valleys, and give 

 the appearance of being in courses independent of preglacial lines. 



The river receives several tributaries, both on the east and on the west, 

 which have a length each of 15 to 20 miles or more. These tributaries are 

 usually widely branching, and the entire watershed displays a perfection of 

 drainage such as does not occur within the limits of the Wisconsin drift. 

 There appears to be on the main stream, and also on many of the tribu- 

 taries, a more rapid descent in the headwater portions than in the lower 



