109] FAUNA OF BIG VERMILION RIVER—BAKER 11 
the north and the Champaign moraine on the south. In its course it cuts 
through the Bloomington moraine in several places. The territory drained 
includes a small part of the Bloomington till plain and a considerable part 
of the Champaign till plain (Leverett, Illinois Glacial Lobe, plate VI.) 
As there are no outcroppings of rock in this area the streams have cut well 
defined, though meandering, channels. The upper, small, creek-like tribu- 
taries have sunk their beds but a few feet below the general level of the 
country, but lower down, the stream, after receiving several large tributa- 
ries, has cut its bed to a depth of ten or fifteen feet. In Middle Fork and 
North Fork, and in the Big Vermilion from above Middle Fork to the 
Wabash, the river has cut deep canyons upwards of 200 feet in depth, 
which produce some of the most picturesque scenery in the State of Illinois. 
This river valley varies from half a mile to a mile in width and in one place, 
about four miles below Danville, it widens to form a large amphitheater 
two miles wide and a mile long, with cliffs and hills rising on all sides to a 
height of over 150 feet. 
Outside of the stream valleys the country is a flat till plain, largely 
devoted to crop purposes. Ridges, made up of the Champaign and Bloom- 
ington moraines and their branches, occur and have been largely instru- 
mental in directing the course of some of the stream drainage. The Salt 
Fork turns to the north after leaving Urbana, and passes around Yankee 
ridge (a branch of the Champaign moraine) near the Brownfield woods, 
and spurs from the Champaign moraine occur in several places on the west 
and south banks of the Salt Fork, which are relatively high and the stream 
skirts their bases. 
The bottom of the main stream and its tributaries varies greatly. 
The small tributaries mostly have mud bottoms. In Spoon River the 
bottom is of mud in most places and the water is of considerable depth, 
even in summer (three to four feet maximum in August). In places 
there are riffles where the water is very shallow (a foot or less) and the bot- 
tom here is of sand and fine gravel. The Salt Fork below St. Joseph is 
made up of stretches of stream where riffles with sand and gravel bottom 
alternate with deeper back water with mud bottom. Near Muncie there 
are outcrops of rock, a small tributary, Stony Creek, flowing over a stony 
bed. In the Middle Fork, shale rock outcrops in several places, notably 
below the interurban bridge, where the whole bottom is composed of a 
sheet of rock with a thin coating of sediment in spots. The same conditions : 
are found in parts of the North Fork and in the Big Vermilion. These 
varying conditions provide the most favorable environment for the growth 
of river mussels, a fact made evident by the large naiad fauna found in 
the stream despite the unfavorable effects of sewage and waste pollution. 
Natural dams occur in a few places in the Salt Fork, caused by the 
accumulation of débris which has lodged against the trunk of a tree that 
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