ILLINOIS RIVER DRAINAGE BASIN. 51 1 



about 3 miles below the mouth of Fox River. The basin drained by this 

 stream, as previously described, carries sand ridges, Avliich were; perhaps 

 formed by a lake-like expansion of the river prior to the opening- of the 

 channel to the west. 



VERMILION RIVER. 



This southern tributary of the Illinois should not be confused with a 

 stream of the same name which leads into Wabash River from eastern 

 Illinois. To distinguish these streams, the names Wabash- Vermilion and 

 Illinois- Vermilion have come into use. Both streams have their sources in 

 the Bloomington morainic system at the reentrant angle in southeastern 

 Livingston and western Ford counties. The course of the Illinois- Vermilion 

 is northwestward, while that of the Wabash- Vermilion is southeastward, 

 from the elevated district in which they have their common source. 



The Illinois- Vermilion has a drainage area of 1,300 square miles, and 

 drains the district immediately south and west of the Marseilles moraine in 

 Ford, Livingston, and southern Lasalle counties. The main stream follows 

 the west or outer border of the inner ridge of the Bloomington system from 

 source to mouth, but an east fork leads through this moraine. The other 

 eastern tributaries find their sources in these morainic belts. A southern 

 tributary — Rooks Creek — heads near Lexington in Cropsey Ridge, another 

 weak moraine of the Bloomington system. The western tributaries drain a 

 plain which slopes gradually eastward nearly to the border of the moraine. 

 The western side of the watershed is much more extensive than the eastern, 

 yet no streams worthy of note lead across it to the Vermilion. Its drain- 

 age is through small streams which pass directly down the slope to the 

 river in somewhat parallel courses. 



The unfavorable conditions for drainage along the Vermilion have been 

 discussed on preceding pages. The plain through which it passes, as there 

 noted, has little descent in the lower 40 miles of the river, and was appar- 

 ently occupied by a marsh, if not by a shallow lake, until a stream had 

 been given time to open a channel from the Illinois back several miles into 

 the plain. Sandy deposits on the south border of the plain are thought to 

 be due to the existence of a lake in the portion to the north. The narrow- 

 ness of the channel of the Vermilion River near its mouth is well shown on 

 the Lasalle topographic sheet. 



