32 



In certain parts of Boone, McHenry, and Lake counties, where 

 stream erosion has not developed sufficiently to properly drain the recent- 

 ly glaciated region, a marsh or meadow type of vegetation prevails on 

 the bottomlands, and forests are on the elevations. In Lake and Mc- 

 Henry counties some of these poorly drained bottoms have the tamarack* 

 bog association common to Wisconsin. This is of ecological interest as 

 representing one phase of the initial period of forest development, just 

 as the few beeches in the ravines of Lake county are of interest as repre- 

 senting the climax type or final state of forest development for the region. 

 However, neither is important as a producer of wood, since there are 

 only a few beeches, and since the tamarack, covering but 157 acres, is 

 rarely more than 12" D. B. H. (Waterman, '21.) 



Upland Type 

 (1) Post Oak 



The area included in this type lies largely between southern Shelby 

 and southern Williamson counties in those regions drained by the Kas- 

 kaskia. Big Muddy, Saline, and Little Wabash rivers. Thus it extends 

 from within ten miles of the Mississippi on the west across the interior 

 of the state to within twenty miles of the Wabash on the east. It is 

 somewhat less than, but almost entirely within, the area covered by the 

 Lower and Lower Middle Illinoisan glacial invasion. (See Map II, fac- 

 ing p. 1.) Isolated areas of small extent are found in Knox, Massac, 

 Hardin, Pike, Union, and other counties. 



During the ice invasion, preglacial eminences were ground down and 

 valleys were filled. The retreating ice left a deep deposit of unstratified 

 boulders, gravel, sand, silt, and clay similar to the glacial till of northern 

 Illinois. Following a later ice invasion (the lowan), which was limited 

 to the northern part of the state, a very fine soil was carried by the wind 

 and deposited extensively over the entire state. Later ice invasions buried 

 and modified this loessal deposit in the central and northern parts of the 

 state, but throughout the south-central region it averages from four to 

 ten feet in depth and forms the very fine, poorly drained soils of this post 

 oak region. These fine-textured, gray, surface soils are generally under- 

 laid by a stratum of silty clay. The resultant poor drainage renders these 

 soils of low agricultural value. 



The general flatness of the region is broken by occasional glacial mo- 

 raines or preglacial eminences, rarely more than one hundred and fifty 

 feet above the plain level, and by the valleys of the intersecting streams. 

 The larger stream valleys have a wide level floor but a few feet below 

 the general plain-level. Gradients are low and extensive bottomlands are 

 common. About 12% of this region is bottomland, whereas the average 

 for the entire state is 8%. Where the layer of loess has been eroded, as 

 along the stream courses, the soil is a yellow-gray silt loam, changing to 

 yellow silt loam as erosion progresses deeper. 



