67 



reach the state coming down grade. Taken as a whole the land sur- 

 face is a tilted plain sloping from the extreme northern part — where a 

 few elevations exceed a thousand feet — toward the south, bow^d in 

 the central part by a broad crescentic undulation caused by a glacial 

 moraine, and then declining gradually to the lowland north of the 

 Ozark Ridge, near the extreme southern part of the state. This east 

 and west ridge occasionally exceeds i,ooo feet, but its average height 

 is between 700 and 800 feet. It is very narrow, only about 10 miles in 

 average width, and rises about 300 feet above the surrounding low- 

 land (Leverett, '96, '99). South of this ridge lie the bottoms of the 

 Ohio River. The largest river within the state is the Illinois. 



The soils of the state are largely of glacial origin. Even the un- 

 glaciated extreme northwestern part and the Ozark Ridge region have 

 a surface layer of wind-blown loess. In some places considerable sand 

 was assorted by glacial water, forming extensive tracts of sandy soil, 

 and locally dune areas are active. Along the larger streams there are 

 extensive strips of swamp and bottom-land soils. The remaining soils, 

 which characterize most of the state, were either produced mainly by 

 the lowan or Illinoian ice-sheets, as in the case of the relatively poorer 

 soils, or by the Wisconsin sheet, which formed the foundation for the 

 better soil. The dark-colored prairie soils are due to organic debris. 

 Coffey ('12:42) has said: "Whether this accumulation of humus is 

 due to lime alone or to the lack of leaching, of which its presence is an 

 indication, has not been definitely determined. Neither do we know 

 whether it is due to chemical or bacteriological action ; most probably 

 the latter, an alkaline medium being necessary for the growth of those 

 bacteria or other microorganism which cause this form of decomposi- 

 tion."* 



2. Climatic Conditions 



The climatic features of a region are generally conceded to have a 

 fundamental influence upon its life. The controlling influences upon 

 climate are elevation above sea-level, latitude, relation to large bodies 

 of water — generally the sea — and the prevailing winds. The eleva- 

 tion and relief of Illinois have but a slight influence. In latitude 

 Illinois is practically bisected by the parallel 39^'^° in the north tem- 

 perate zone. This position influences the seasons and the amount of 

 heat received from the sun. The sea is far distant, but the Great 

 Lakes are near by, and proximity to the interior of a large continent 



*Consult Hopkins and Pettit ('08) and the County Soil Eeports of the State 

 Soil Survey for a detailed account of the chemical conditions of Illinois soils. 

 The bacterial, algal, and animal population have hardly been noticed by stu- 

 dents of Illinois soils. 



