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 ComMMuNITIES 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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