96 ILLINOIS STATE ACADEMY OF SCIENCE 



average width is 1100 yards west of the stream and 2200 

 yards east, while again the coincidence of the soil and 

 the timber is very close. 



From these examinations of the contonrs of the stream 

 valleys it is evident that the timber soil varies in its dis- 

 tribution with the varying slopes of the valleys. Indeed, 

 a close examination of snch timber soils makes it entirely 

 clear that they are but the subsoils of the prairie silt 

 loams where the latter have been removed by stream 

 erosion. The comparative youth of the stream valleys 

 has permitted little modification of the soil upon their 

 slopes. It is therefore clear that if, as has been shown, 

 the edge of the slope of the valleys marks the limit of the 

 timber soil, and hence that of the original distribution 

 of forest associations, prairie fires cannot be accepted 

 as an adequate explanation of the narrowness of the 

 wooded strip on the western slope of the stream. The 

 cause must rather be sought in some factor that affects 

 the slope of the stream valley and thus indirectly the 

 distribution of timber soils and their forest cover. 



Two such explanations seem possible, either one of 

 which is more satisfactory than the fire theory. Geol- 

 ogists have found that on account of the rotation of the 

 earth the waters of the north and south streams have 

 been deflected somewhat to the west and have thus erod- 

 ed the western bank more rapidly and given rise to val- 

 leys of unequal slope. It seems rather doubtful, how- 

 ever, if the volume of water in these small streams is 

 sufficient to account for the very decided difference here 

 seen in the slopes of the two sides of the valley. Exam- 

 ining somewhat similar small north and south streams 

 on Long Island, Jennings (6) is inclined to reject such 

 an explanation 



The other, and in the opinion of the writers, the more 

 logical explanation is that the wind-borne soil material 

 transported by the prevailing westerlies has been con- 

 tinually sifting in upon the western slopes and tending 

 to fill up the western half of the stream valleys. This 

 has at one and the same time checked erosion and caused 

 the grassland to tend to invade the woodland. This 

 explanation has been accepted by Jennings as the most 



