118 



Oquawka and Hanover areas, there are large tracts of prairie. The 

 Winnebago area Hes protected on three sides by streams of con- 

 siderable size, and is almost entirely forested, except the cultivated 

 fields. 



In the Havana area, there is a belt of forest along the Illinois 

 river, and large forest masses at the south and north ends, particularly 

 near Forest City and Kilbourne. In other parts of the territoiy the 

 broader deposits of sand are usually prairie, and the forest is re- 

 stricted to the narrow ridges. These extend north and south and 

 mark the location of old sand-bars. In the Amboy area the dis- 

 tribution is similar, but the ridges run g-enerally east and west. They 

 have probably all been forested except those nearest the margin 

 of the deposits. In the Oquawka area there is a belt of forest along 

 the Mississippi river and another inland near the bluff line. These 

 are connected by broad bands of forest which separate several areas 

 of prairie. The Hanover area has a similar belt along the Mississippi, 

 and a number of transverse strips extend inland. These have been 

 partially cleared, but probably none of them crossed to the bluffs 

 except at the extreme northern end. The Winnebago area was en- 

 tirely forested except a few small areas of marsh and islands of 

 prairie. It is difficult to estimate the proportion of the area covered 

 with forest. It was probably considerably more than half in the 

 Oquawka area, about a third in the Hanover area, and about a half 

 in the Havana area. 



The regular belt of forest along the rivers in the last three areas 

 may be correlated with the effect of fire. The transverse bands 

 across the area in the Hanover and Oquawka areas follow the most 

 irregular portion of the surface, ^vhere the effect of fire was possibly 

 limited. The large grove northwest of Hanover station, in partic- 

 ular, follows a line of steep-sided irregular dunes totally unlike the 

 gently rolling prairie. 



The encroachment of the forest is caused by the slow migration 

 of the forest trees in eveiy direction. The open structure of the 

 bunch-grass does not prevent the proper germination of seeds or 

 growth of seedlings as does the close sod of a normal prairie. Few 

 species of trees, however, are able to withstand in their seedling 

 stages the extreme conditions of the physical environment. These 

 are especially the shifting nature of the sand, the hot surface layer, 

 which may be almost totally dry to a depth of more than a decimeter, 

 and the lack of protection against wind during the winter. Still 

 another restricting influence is the absence of ready means of dis- 

 persal. The trees composing the early stages of the forest are oaks. 

 Thei! heavy acorns have no means of dispersal except gravity and 



