CHAPTER XII 



ANIMAL COMMUNITIES OF DRY AND MESOPHYTIC FORESTS 



I. Introduction 



The forest communities discussed in the preceding chapters are those 

 displacing aquatic communities. In a climate suitable for forests, trees 

 spring up on high, well-drained surface materials of all kinds. Forest 

 appears on rock, sand, clay, etc., first as shrubs or scattered trees, later 

 as dense mesophytic forest. In the region about Chicago we have forest 

 in all stages of development and on several kinds of material. 



The bluffs of the lake and artificial exposures of clay along the drain- 

 age canal and the till uplands afford examples of development peculiar 

 to this type of soil. The few outcrops of Niagara limestone and the 

 quarries and rock dumps present scattered data on the history of forests 

 on rock. The extensive sand areas afford examples of all stages of 

 development peculiar to sand. From all these situations, we find 

 forests leading toward some type related to climate, either the typical 

 forest of the forest climate, or the forest of the savanna climate. 



II. Forest Communities on Clay 

 (Fig. 157) (55) 



The chief areas of more or less active erosion are along the west side 

 of the lake, from Waukegan to Winnetka, and on the east side of the 

 lake from South Haven to Benton Harbor. The old bluffs of the Tolles- 

 ton and Calumet stages as represented north of Waukegan and at 

 various other points offer valuable areas for comparison. There are 

 also similar bluffs along many of our streams, some of those in Michigan 

 being very old. 



When the ice sheet receded entirely and left the outline of Lake 

 Michigan much as it is now, doubtless the shore presented a more or less 

 rounded profile. However, since that time waves have gradually 

 changed the shore profile. By washing away the clay at the base of 

 such a shore, a bluff has been developed (62). 



I. STEEP BLUFF ASSOCIATION 



(Station 56; Table XLIX) 

 a) Ground stratum (55) (Fig. 157). — In spring, when the frost goes out 

 of the ground, leaving the clay somewhat loosened, the ground-water 



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