Illinois, and these differences are the combined effects of physical and 

 historical factors. 



These sand areas are extensive deposits made by the ancient Illi- 

 nois River, which was once many times its present size, and by cer- 

 tain of its tributaries. The old Illinois was the outlet of Lake Chi- 

 cago, of which Lake Michigan is the present-day successor, and it also 

 carried away an immense volume of water from the melting Wiscon- 

 sin ice-sheet, the terminal moraine of which crosses the river in the 

 vicinity of Peoria. Below the moraine the valley suddenly widens, 

 and the entire left bank of the glacial flood-plain is buried beneath 

 many feet of sand and gravel. The surface of these sand deposits 

 consists of several low, broad ridges of almost pure sand, separated 

 by extensive sandy loam flats. The sand-plain reaches its maximum 

 width, about 14 miles, in Mason county; toward the south it gradu- 

 ally becomes narrower, ending near Meredosia, in Morgan county. It 

 is about 75 miles in length. The total area of sand deposits is about 

 179,200 acres (Hart and Gleason, '07: 145). 



The original vegetation was of three types. The first was the 

 prairie or meadow of the level flats, merging into swamps and bogs 

 in the poorly drained areas; these are now drained, and all of the 

 level country is under cultivation. The second was the sand prairie, 

 which occupied the pure sand of the ridges and dunes. The sand 

 prairie is of two formations : the prairie or bunch-grass formation, in 

 which vegetational reaction upon the environment results in a domi- 

 nating tendency toward stabilization of the sand, and the establish- 

 ment of more mesophytic conditions"; and the blowsand formation, in 

 which the wind is the controlling factor, resulting in a dominating 

 tendency tow-ard continual shifting of the sand, and the establishment 

 of desert conditions. The third type, the forest, is of comparatively 

 recent development in the sand region, and is confined to the pure sand 

 of the ridges, having replaced the sand prairie over a large part of the 

 dune country. The early stage of the forest is very xerophytic, being- 

 composed of black-jack oak, black oak, and hickory. The black oak 

 soon becomes the dominant tree, and a more mesophytic forest is de- 

 veloped — best seen in the dunes along the river (see PI. V) — com- 

 posed of black oak, hickory, walnut, hackberry, and a few other 

 trees. Most of the forest area is uncleared. 



The original fauna has been greatly modified. Most of the prairie- 

 swamp animals, such as marsh birds, muskrats, frogs, and inverte- 

 brates, have disappeared, while the animals favored by cultivation have 

 taken their place. In the sand prairie, the small mammals, the birds, 

 and the insects, have undergone little change. The animals of the 



