THE TOPOGRAPHY AND HYDROGRAPHY OP ILLINOIS Ixiii 



of Champaign county and into Vermilion county as far as 

 Potomac, where it turns abruptly southward and passes through 

 the outer ridge of the Bloomington moraine. A few miles farther 

 south it receives its larger western tributary, the Salt Fork, and 

 the united stream then flows east for about 6 miles to Danville, 

 takes again a southeast course, and follows this direction to its 

 mouth. 



Salt Fork rises in western Champaign county at an altitude 

 of 740 feet and flows south and then east for a distance of 50 

 miles. It drains a plain in eastern Champaign and western 

 Vermilion counties, lying between the Bloomington and Cham- 

 paign morainic systems. 



North Fork rises in northern Vermilion county at an ele- 

 vation of 720 feet and flows southward for a distance of 37 

 miles, emptying into the Vermilion at Danville. It drains only 

 a small area among the ridges of the Bloomington system. 



The entire drainage system of the Vermilion is independent 

 of preglacial lines, the drift over this region being so deep as to 

 cover completely the old rock divides. The river and its 

 branches have narrow valleys, and in the upper courses the 

 banks are only from 10 to 50 feet high, and generally bordered 

 by scattered patches of timber. In the lower parts the streams 

 are skirted with strips of woodland from one to four miles in 

 width, and the banks are steep and high. Bed-rock is not exposed 

 in the upper portions, but at and below Danville the river has 

 cut into the Pennsylvanian to a considerable depth. 



Generally speaking, the headwaters of all these streams 

 were originally prairie swales, lying in shallow valleys or in 

 broad depressions of an otherwise plain surface. Here they 

 were often choked with weeds in summer, and were very muddy 

 in times of flood, but in their lower courses they often cut deeply 

 into the drift, or even into the underlying rock, forming deep 

 and narrow valleys, sometimes with decidedly gorge-like effect. 

 In comparison with most Illinois streams, however, the waters 

 of the Big Vermilion are in general fairly clear, and the bot- 

 toms relatively clean, forming a transition from the typical 

 prairie streams to those characteristic of the adjacent Alleghany 

 plateau. 



LITTLE VERMILION RIVER 



The Little Vermilion River rises in the southeastern corner 

 of Champaign county and flows southeast, east, northeast, and 



