262 ILLINOIS STATE ACADEMY OF SCIENCE 



relics along the Illinois and Rock rivers. It is probable 

 also that they advanced along the steeper morainal 

 ridges, especially those that are sandy or stony. Pines 

 still remain on some of the sandy moraines in Ontario. 



As streams and gullies cut back into the prairie, their 

 slopes are invaded by shrubs and trees. Where the 

 stream bluffs are low and but little dissected the timber 

 belt is narrow but, where there is much erosion, the tim- 

 ber belt is correspondingly wider. Where streams cut 

 through moraines, the forests have spread out along the 

 morainal slopes. This is noticed along the Kaskaskia at 

 Shelbyville and along the Embarrass at Charleston. The 

 shores of small lakes probably have been invaded by 

 bottomland forests from nearby streams. These forests 

 gradually move inward as the lakes fill up or become 

 drained. Some depressions develop into bogs which may 

 be invaded by forests. Along the borders of forests, the 

 prairie grasses are killed out by the shade and the wind 

 velocity is checked so that tree seedlings are able to 

 grow. In this way the forests enlarge at the expense of 

 the prairie. 



As already mentioned, most of Illinois is a region with 

 low relief which probably was occupied by a tundra at 

 the time of the Late Wisconsin glaciation. This region 

 became a prairie at the close of the ice age and much of 

 it still remains prairie because post-glacial time has been 

 too short for the invasion of such large relatively flat 

 areas by forests. The Late Wisconsin drift has more 

 relief and can be invaded more rapidly by forests. For- 

 ests advancing up the Mississippi, the Rock, the Illinois, 

 and the Wabash rivers invaded the Late Wisconsin drift 

 and spread rapidly over its surface. Farther east the 

 tundra belt was narrow and easily crossed by the forests 

 so that invasion was rapid. These differences in topog- 

 raphy and consequent differences in rate of forest 

 invasion explain why Illinois still has large areas while 

 the states to the north and east are almost completely 

 timbered. It also explains why forests are found on the 

 moraines in northern Illinois but only along the streams 

 in southern Illinois, 



