522 THE ILLINOIS GLACIAL LOBE. 



have been determined by the slope of a drift plain and is apparently inde- 

 pendent of preglacial lines. 



Big Sandy Creek, the next succeeding eastern tributary of the Illinois, 

 has a drainage area of about 190 square miles. Its watershed is broader 

 than either of the two preceding, but is much shorter, its extreme length 

 from east to west being scarcely more than 18 miles. It comprises a dis- 

 trict lying somewhat below the general level of the neighboring watershed. 

 This fact, together with the absence of rock outcrops in the portion of the 

 Illinois bluffs immediately north of its mouth, renders it probable that a 

 preglacial drainage line traversed the midst of its watershed, though in a 

 course not precisely coincident with that of the present stream. 



APPLE CREEK. 



Apple Creek has a drainage area of about 500 square miles, which 

 includes southeastern Morgan, northern Greene, and northwestern Macoupin 

 counties. Its course is southwestward, and the watershed has a length of 

 about 40 miles. The greatest breadth is about 15 miles. Its lower course 

 appears to be along the line of a preglacial valley, but the headwater 

 portion and also the majority of the tributaries show little dependence upon 

 preglacial lines. The drift is comparatively thin over much of the water- 

 shed, and streams have cut down into the underlying rocks at many points. 



MACOUPIN CREEK. 



Macoupin Creek is the most important eastern tributary south of the 

 Sangamon River, its drainage area being nearly 1,000 square miles. It 

 drains the greater part of Macoupin County and a portion of the neighbor- 

 ing counties on the east and west. Its watershed is widely branching in 

 the middle portion and tapers toward either end, giving a broadly ovate 

 outline. With the exception of the headwater portion above Carlinville, 

 the main stream apparently has its course determined by a preglacial line, 

 there being a broad depression, deeply filled with drift, through which the 

 creek takes its course. The tributary streams appeal' to be largely inde- 

 pendent of preglacial lines. The extent of the watershed on the north and 

 south appears to be determined in large part by preglacial divides, but the 

 influence of preglacial divides is less apparent at the eastern border of the 

 watershed. 



