540, 
On the other hand, if we disregard the blue grass and red top in any 
given area or choose areas in which they have not gained a foothold 
the order of succession of the associations of native grasses is readily 
seen. With the exception of the presence of Andropogon scoparius 
and Sporobolus heterolepis in some places this succession of associations 
does not differ materially from that of the flood-plain prairies already 
described. 
Andropogon furcatus does not play a very important role on the 
Chicago prairie. Under natural conditions most of this prairie was 
too wet for Andropogon. As noted above, it is frequently found on the 
higher ground, and it is most abundant in the neighborhood of the 
artificial drainage ditches. One of the areas that shows clearly the suc- 
cession from Panicum virgatum to Andropogon furcatus on this prairie 
is about one half mile southwest of Oak Ridge (Plate LIX). The fact 
that this particular prairie is separated from the Wabash railway by a 
small stream is probably significant. The conditions on this area are 
more like those in other parts of the state. Coarse herbs are present — 
but they occur as scattered individuals of secondary importance. Red 
top and blue grasses are not abundant. Special attention should be 
called to the fact that owing to their much greater height the native 
grasses readily crowd out the cultivated grasses in regions undisturbed 
by man. Mowing checks but does not entirely prevent the encroachment 
of Andropogon furcatus in associations of Poa pratensis. This fact 
was obtained from observations on carefully mapped areas about one 
mile northwest of Ashburn just north of 87th Street on some of the 
best drained portions of this prairie. While the observations on this 
particular area were limited to a period of- only three years, a slight in- 
crease in the amount of Andropogon was evident. The data recorded 
in Figure 6 show still more conclusively that when man does not inter- 
fere too much, Andropogon furcatus will crowd out our cultivated 
grasses. 
Portions of the Chicago prairie have been cultivated from one to 
a few years and then abandoned. In all cases so far observed these 
abandoned areas have reverted again to prairie. Agrostis alba is usually 
the first grass to get a foothold. Coarse herbs likewise get an early 
start. Then blue grass and Andropogon follow. ithe reversion to prairie 
is not entirely due to the absence of “seed trees”. Seedlings of cotton- 
wood frequently make a start, but owing to subsequent mowing and burn- 
ing they are usually destroyed. 
The Hydrarch Succession (c) in Morainal Depressions 
The morainal depressions vary in size from the small ones found 
in terminal moraines to the much more extensive ones of the ground 
moraine. Many of these depressions were formerly post-glacial lakes 
which became partially drained by subsequent gradation. As would be 
expected, the greater area of these depressions today occurs in the less 
