536 
The Hydrarch Succession (b) on the Old Lake Bed of Lake Chicago 
The geological history of this region has been described in detail 
by Leverett (17) and Salisbury and Alden (20). The following sum- 
mary is given to bring out more clearly the relation of the vegetation to 
the present physiography. During the retreat of the Wisconsin ice sheet, 
a temporary balance between the melting back of the glacial front and 
its forward movement resulted in the formation of the Valparaiso 
moraine. With further recession of the glacier this moraine served as 
a dam which hemmed in the water from the melting ice, forming a lake 
known as Lake Chicago. The surface of this lake was at one time 60 
feet above the present level of Lake Michigan. An outlet through the 
Desplaines River gradually lowered the level of the lake until the glacier 
receded far enough to open an outlet to the north which resulted in the 
withdrawal of the water from the Chicago plain. Three old beach 
lines, sixty, forty, and twenty feet above the present level of Lake 
Michigan, show that the outlet to the north was interfered with a second 
and third time, and that a lake again occupied the area in question. 
With the final retreat of the glacier and subsequent drainage through 
the St. Lawrence System the water was gradually drained from the region 
between the Valparaiso moraine and the present beach of Lake Michigan. 
The area thus exposed was a flat, level plain, poorly drained, and 
abounding in lakes and swamps which have been gradually filling with 
vegetation. 
The old beaches and sand-dunes areas are forested, but the re- 
mainder is largely prairie or swamp land. Much of the present un- 
disturbed prairie is considered too wet for agricultural purposes except 
in dry seasons. In some of the drier portions, however, cultivated 
plants ecologically equivalent to Panicum virgatum and red top could 
be grown. Roadways and ditches have recently been established along 
the main section lines, and in some cases city blocks have been laid 
out. This artificial drainage has reacted to some extent upon the natural 
vegetation, and it is undoubtedly a partial cause of some of the dis- | 
turbances noted in the natural plant associations of this area. 
The drainage ditches are marked by a line of cottonwoods and a few 
willows, mostly Salix amygdaloides, though in some places swamp 
grasses are found refilling the ditches. These trees are growing in the 
bottom of the ditches where the prairie turf has been completely de- 
stroyed. 
This prairie is exceptionally rich in organic matter, peat sometimes oc- 
curring. The subsoil is mostly glacial till, forming poor subsoil drainage, 
but local areas of sand and rock also occur.. Owing to the flat surface and 
the impervious clay subsoil, much of the surface soil is usually saturated 
during the wet spring months. Later in the season the water content 
under present conditions of drainage may drop below the wilting co- 
efficient, Harvey (12) has shown that during the summer of 1911 the 
