248 ILLINOIS ACADEMY OF SCIENCE 



has probably the largest area of tillable residual soil to be 

 found in Illinois. These soils come from the disintegra- 

 tion of shale and sandstone and vary from sandy loam to 

 sand. Their small area is due not only to the wide extent 

 of the ice sheet and to its action in removing disintegrated 

 rock or in covering it with glacial drift or boulder clay, 

 but also in the southern part of the state to the large de- 

 posits of wind borne loess burying the products of rock 

 decay. 



Stream Development. 



The water courses of the county differ greatly in age 

 and present many stages of development from small inter- 

 mittent streams, just beginning to entrench their channels 

 in the prairie soil, to great rivers flowing in broad valleys 

 deeply cut in soil and rock even before the coming of the 

 ice sheets. The forest distribution is so closely associated 

 with the diversities of stream development that it seems 

 advisable for our purposes to recognize four distinct 

 stages of erosional development. 



1. Small streams, often intermittent. — These are shal- 

 lowly entrenched in the comparatively level surface of the 

 prairie and have done little or no cutting except to exca- 

 vate the channels through which their waters flow. All 

 the streams of Meriden and Mendota townships belong to 

 this class, 



2. Creeks with shallow valleys. — These have cut through 

 and removed portions of the prairie soil exposing strips of 

 the underlying subsoil which become the "upland timber 

 soils." Ill these subsoils their channels are entrenched 

 but their valleys show little lateral cutting and almost no 

 deposition. The Little Vermilion furnishes a typical ex- 

 ample of a stream of this class. It may be noted that the 

 soil maps show many streams of this class with a border of 

 timber soil, or in other words a valley slope, distinctly 

 broader upon its left or eastern bank. The cause of this 

 unequal cutting and its significance in forest development 

 will be reserved for discussion in subsequent paragraphs. 



3. Small rivers. — To this class are to be referred 

 streams that have cut through the soil mantle almost or 

 quite to the underlying rock and are now doing compara- 

 tively little vertical erosion. They are engaged in widen- 



