
1901] PHYSIOGRAPHIC ECOLOGY OF CHICAGO 161 
morainic hills that the first mesophytic forests were developed. 
And they have been developed for so long that almost no traces 
of their history are left behind; we have only the completed 
product, the mesophytic forest. 
Where these mesophytic forests are disturbed we may per- 
haps get some notion of what took place in the first postglacial 
centuries. On the clay banks along the drainage canal and also 
on recent river bluffs, one may follow in rapid succession a series 
of plant societies leading to the forest. There is here no pro- 
nounced lichen or moss stage as on rock hills, but the first vege- 
tation consists of xerophytic annuals and perennial herbs. 
Xerophytic shrubs, especially Salix and Populus, soon appear. 
It is not long before there is an extensive thicket formation with 
an herbaceous undergrowth. Humus accumulates with great 
rapidity and we soon have almost a mesophytic vegetation in 
which the dominant thicket species are likely to be the aspen 
(Populus tremuloides\, wild crab (Pyrus coronaria), red haw 
(Crataegus punctata, C. coccinea, etc.). Such a thicket is the 
immediate forerunner of the oak-hickory type of mesophytic 
forest. When a forest of oak and hickory is cut down or 
destroyed by fire it returns after a comparatively short interval, 
but the first stages in the clearing are thicket stages much like 
those just described. Of course it takes much longer to develop 
a forest from naked clay soil than from a forest land that has 
been cleared. Whether the stages that led up to the first post- 
glacial forests are such as have been described is very doubtful. 
It is much more likely that the first forests were of slow growth 
and were coniferous in character, such as are found farther 
north. Fig. 28 shows a typical morainic hill forest of the above 
type. Here the dominant tree is the red oak; a white oak is 
seen at the right. 
Among the shrubs of these morainic forests there may be 
mentioned, apart from the crabs and haws, the hazel (Corylus 
Americana) , and various species of Viburnum. Many herbaceous 
plants are found, among which are Podophyllum, Claytonia, 
various species of Aster, Trillium, Geranium maculatum, Viola 
