570 
depth and area of these postglacial lakes are attested today by the 
depth and extent of the rich humus soils which accumulated in them. 
The deeper peat deposits in some parts of the state evidently had their 
origin in the postglacial bogs noted above. 
As the postglacial lakes became filled and better drained through 
the decay of vegetation in them and partly by the vertical cutting of 
streams and the deposition of debris washed from the uplands, the pio- 
neer water-plants died and were replaced by plants characteristic of the 
better-drained soils. The series of this succession of plants leading to 
the gradual filling of the sloughs may be seen today in areas little dis- 
turbed by man. At first the bulrushes are the most abundant and promi- 
nent plants accompanied by fewer individuals of many other water plants 
which are unable to become abundant in competition with the luxuriant 
growth of the bulrushes. The bulrushes, therefore, may be called the 
dominant plants at this stage of filling of the sloughs. A miniature bul- 
rush slough is shown in Plate LIV. The bulrushes begin to die as the 
sloughs become drier through filling or drainage and are replaced by 
sedges which dominate during a second stage of deposition (Plate 
XLVIII). The sedges are followed by slough grass (Spartina Michau- 
wiana. Plates XLIX, L). Sometimes the slough grass follows the bul- 
rushes directly without an intervening sedge stage. At this stage the 
sloughs may become dry for a few months during summer, but are still 
too wet for cultivation. The slough grass is followed in order by blue 
joint-grass (Calamagrostis canadensis, Plate Ll), and tall Panicum 
(Panicum virgatum) or by, Panicum alone, which, in turn, is finally re- 
placed by tall bluestem (Plates LXV-LXX). Tall bluestem remains as 
the dominant grass of the true prairie. The early settlers found most of 
the bluestem prairie dry enough for cultivation. All gradations in the fill- 
ing of the sloughs were seen by the early settlers. Most’of the prairie area 
on the older glaciated part of the state south of the latitude of Mattoon 
had become covered by tall bluestem, but a large part of the prairie re- 
gion in the younger glaciated area north of this latitude was not dry 
enough for tall bluestem and abounded in numerous sloughs of bul- 
rush and slough grass. These drained sloughs and prairies with their 
deep deposits of humus are now the most productive lands of the state. 
Where the upland clay soils become covered with a prairie vegeta- 
tion instead of a forest, the first abundant grass to appear upon them is 
short bluestem (Andropogon scoparius). ‘This grass, in time, becomes 
crowded out and is replaced by the tall bluestem. 
Some of the sand deposits of the state became forested, but a large 
part of them became covered with a prairie vegetation. The first plants 
that grew upon the dry moving sand-dunes were not plants of the 
prairie, but short grasses more characteristic of the plants found upon 
the sand hills of Kansas and Nebraska (Plate LXXI). These pioneer 
plants, in time, developed a carpet of vegetation sufficient to hold the 
sand against the force of the wind. Through their death and decay 
