242 A NATURALIST IN THE GREAT LAKES REGION 



then the dragon flies that breed in the adjacent waters are busy 

 hunting, flying with quick, darting movements along the open 

 pathways. 



The ash-elm swamp forest is inhabited by many animals, few 

 of which are, however, peculiar to it. Most of them are equally 

 or even more prevalent in the elm-maple forest of the flood plain. 



The green heron often 

 nests in the ash trees 

 along the swamp mar- 

 gin and may be taken 

 as the typical animal 

 of this zone. 



Not always does 

 the filling of a low area 

 in the Chicago region 

 ultimately lead to a 

 forest area. There is 

 another association of 

 plant and animal 

 forms that seems to 

 be the end result 

 of such a process, 

 namely, the prairie as- 

 sociation. Extensive 

 prairie areas have de- 

 veloped from the 



Fig. 345. — Nest of marsh wren, woven of marsh i^iarshes that formerlv 

 grass. ^ 



occupied, and m part 



still occupy, the beds of the old lakes that formed back of the 



terminal moraines, such as the Morris Basin and the Kankakee 



Basin. They have developed also on extensive areas of sand 



that outwashed from the glacier and on upland regions in the 



moraines. Possibly the prairie finally transforms to forest, 



but if so the prairie stage in the development is a protracted 



one. It is difficult to decide just what it is that determines 



