Article I. — Third Report on a Forest Siin'ev of Illinois. By 

 Clarence J. Telford. Forester, Illinois State Natural History Survey. 



Part I. Description of the Forests 



Physiographic Features 



In preglacial times Illinois was not a prairie state, but was character- 

 ized by hills and valleys such as are found in the unglaciated area of 

 Jo Daviess county today. When the ice sheets retreated, Illinois, except 

 for a few places, was extensively covered by a deep mantle of soil, the 

 old valleys were filled and the preglacial eminences modified and buried 

 until the surface was a flat to rolling upland. During the period when 

 the glaciers were melting great quantities of water were released, found 

 outlets across this mantle of raw soil, and quickly sluiced out wide chan- 

 nels. The flooding was apparently intermittent, and during the periods 

 of restricted stream-flow the exposed deposits of finely ground glacial 

 debris, drying, were carried widely by winds and deposited extensively 

 over Illinois. 



Thus the topography of the state is that of an elevated plain having 

 a slight slope to the south. Large streams have cut wide channels through 

 the deep soils, and the boundary between the uplands and the large stream 

 valleys is characterized by the abrupt bluff condition of an upland region 

 geologically young. Lesser streams cut through the bluffs to the main 

 rivers, forming often a hilly topography near the blufi^s, but the relief 

 becomes less pronounced as the distance from the larger streams increases. 

 The level expanse of the glaciated area is dissected by innumerable 

 streams and further broken by moraines and partially buried preglacial 

 eminences rising above the general level. 



Geologists recognize at least four periods of ice invasion in the state ; 

 but for a distinction of forest from prairie soils, two divisions suffice — 

 (1) the Lower-Middle and Lower Illinoisan and (2) all others. Forests 

 were the prevailing type of vegetation over the first of these divisions and 

 grassy prairie the prevailing type over the second. 



The Lower-Middle and Lower Illinoisan glaciation extended farther 

 south than any of the other ice sheets, the southern limit reaching north- 

 ern Johnson county, or approximately latitude 37 degrees 45 minutes N. 

 The northern boundary conforms to the moraines of the Wisconsin glaci- 

 ation from Paris to Shelbyville, and to the division between the Middle 

 Illinoisan and the Lower-Middle Illinoisan from Shelbyville to Carlinville, 

 or approximately latitude 39 degrees 20 minutes N. The subsoils of ex- 

 tensive areas of this region are but slightly pervious and the black surface 

 soils, whose color is due to a partial decay of grass and prairie vegetation, 



