42 



Certain extensive regions of the state manifest a tendency toward 

 either the oak-hickory extreme or the rich mixture, and the general up- 

 land hardwood type will be described under two subtypes, (a) upland 

 mixed hardwoods, less than 90 per cent oak-hickory, and (b) the oak- 

 hickory, 90 per cent or more oak-hickory. From seventy samples taken 

 in this type in twenty-eight counties, the oaks and hickories make up 90 

 per cent of the stand in thirty-six. 



Subtype (a) Upland Mixed Hardzvoods 



The regions where this subtype commonly prevails are the entire non- 

 glaciated part of southern Illinois, the deeply eroded section along the 

 bluffs of the Mississippi River as far north as the Wisconsin line, the 

 eroded bluff's of the Illinois River, and the modified uplands of the Wa- 

 bash as far north as Vermilion county. This mixed hardwood associa- 

 tion occurs locally in many counties of the state on moraines, well-drained 

 slopes, and similar sites favorable to variety. 



In the Ozark upland region this subtype extends completely across 

 the state ; but elsewhere the general areas where it is found are restricted 

 to a strip, bounded on the river side by a very definite hne where the 

 uplands break to the river plain by precipitous slopes or rock ledges, often 

 with a relief of several hundred feet. The interior boundary of this strip 

 is not clearly demarked, as mixed hardwoods here merge with the oak- 

 hickory extreme ; but, in general, the mixed hardwood subtype is associ- 

 ated with deposits of deep and medium loess, and varies in width from 

 2 to 12 miles. The depth of the soil varies greatly in this bluff area, as 

 it is a region where wind-carried soils built up deep deposits and where 

 erosion has been very active. Rock outcrops are frequent along the 

 outer rim of the bluff's and where the lesser streams have cut through 

 the heavy soil mantle ; but generally soils are deep. In texture these 

 loessal soils are very fine-grained and may approach sands or clays, but 

 they are characteristically porous, friable, and fertile. They readily ab- 

 sorb moisture, and slopes which on heavier soils will gully disastrously, 

 are safely cleared in this bluff region. 



The Ozark uplands extend from the Mississippi to the Ohio, and 

 from the Big Muddy and Saline rivers to the Cache as an upthrust with 

 an axis running east and west. The highest points, which are among the 

 highest of the state, are near each end and close to the rivers. This 

 results in a pronounced relief along the eastern and western parts, which 

 together with the series of cliffs marking old faults along the southern 

 part, make this a region of rugged topography, characterized by more or 

 less gentle northern and more or less abrupt southern slopes. The older 

 residual soils were buried under a loessal deposit of varying depth. Sub- 

 sequent erosion and weathering have altered these deposits, but they 

 form the main soils of the region. The soils of the interior section are 

 shallower and less porous than the loessal deposits of the bluffs, hence 

 unprotected slopes erode seriously. (Plate II.) 



