BIOLOGICAL PAPERS 121 



SO^IE NOTES ON THE FORESTS OF OGLE COUNTY. 



W. L. ElKENBERRY. 



The principal rocks of Ogle County are the loose and porous 

 St. Peter's sandstone, limited to the vicinit}' of Rock River, and 

 the Trenton and Galena limestones. The latter formations under- 

 lie most of the county and are covered with a thin layer of Illi- 

 noian and lowan drift, which is rarely so much as twenty feet 

 thick. 



It is shown by Leverett that before the Glacial Epoch the chan- 

 nel of Rock River lay in the eastern part of the county, but at 

 the retreat of the ice it adopted a new course, such as to occasion 

 a rearrangement of most of the drainage channels of the county. 

 The insignificant Mud Creek appears to be about the only pre- 

 glacial stream in the county. Kyte River occupies its ancient* 

 valley in part, but with direction of flow reversed. Under these 

 circumstances it will be understood that the topography adjacent 

 to the river is very new and immature. In general, the region 

 near the river and other important watercourses consists of a 

 gently rolling upland trenched by narrow and deep ravines. 



The soil is the alternation of loam and clay common to glaciated 

 regions, together with some limestone residual soil and the sand 

 along the river arising from the disintegration of the sandstone. 

 The most marked contrast is between the rich black soil of the 

 gently rolling prairies remote from the river and the humus-poor 

 clays of the rapidly eroding area nearer to the streams. 



Consideration is here given principally to that part of the 

 county lying west of Rock River and south of Mud Creek, includ- 

 ing the basins of ^Nlud Creek. Pine Creek, and adjacent parts of 

 the valley of the river. 



PLANT SOCIETIES. 



Any consideration of the prairie societies is excluded by the 

 scope of this paper. There may be distinguished four forest 

 associations: (i) Oak, (2) Maple, (3) Pine, (4) a characteristic 

 Bottom Association. Only the first and fourth are of general 

 occurrence. In the absence of suitable physiographic situations, 

 swamp forests are wholly wanting from the part of the region 

 under consideration. 

 I. Oak Association. 



The oak association is the characteristic one of the countrv — • 



