569 
NON-TECHNICAL SUMMARY 
The foregoing technical report is written, in the main, for trained 
botanists and agriculturists and needs no further summary. Since the 
prairie region of Illinois is the greatest natural asset of the state and, 
therefore, of considerable interest to the general public, it may not be 
amiss to end this report with a non-technical summary of its outstanding 
features. 
The variations in the native fertility of the different soils of the 
state are a result mainly of the kinds of native plants that have grown 
upon these soils since the recession of the glaciers that formerly covered 
nearly the entire area of the state (map page 530). When the glaciers 
melted, the surface of the state was left covered with a deposit of clay, 
pulverized rock, and bowlders which the glaciers had picked up and 
carried southward from the Hudson and Labrador regions. The un- 
equal deposition of this glacial drift left the state with a gently rolling 
topography. The depressions became shallow lakes, many of which were 
several square miles in area. Streams from the melting ice left rather 
large deposits of sand in some parts of the state. Thus the early post- 
glacial soil, aside from these sand deposits, consisted of clay, pulverized 
rock, and bowlders, practically devoid of organic matter and of general 
uniformity throughout most of the state. The varying amounts of 
humus in these soils today are the results of the growth and decay of 
vegetation upon them during the postglacial period. 
Botanists are reasonably certain that the first plants which grew 
upon the glacial drift were the same as those found growing today just 
south of the present glaciers in the north. The first zone of vegetation 
south of these glaciers is a tundra on which mosses, lichens, grasses, 
sedges, and low heath-like shrubs are the prominent plants; the second 
zone is the northern evergreen forest of spruce and fir on the uplands 
and bogs in many of the smaller lakes; the third zone is the deciduous 
hardwood forest with trees on the uplands and sedges in many of the 
shallow lakes. 
As the glaciers receded from Illinois the tundra was replaced by 
the northern spruce-fir forest, and finally, the spruce-fir forest by the 
deciduous hardwood forest. This forest, however, did not cover all of 
the upland area of the state, a part of which was covered by tall grasses 
characteristic of the prairie, chiefly tall bluestem (Andropogon furcatus). 
Only about one third of the state became forested in contrast to the much 
greater proportion of forest to prairie in Indiana and Ohio. There is evi- 
dence that the prairie on the upland areas originated during a postglacial 
dry period when the prairie extended farther eastward than at present. 
The shallow postglacial lakes did not become forested but supported a 
luxuriant growth of herbaceous water plants which decayed, but slowly, 
beneath the surface water and gradually filled the lakes. In this way 
enormous sloughs arose and a deep deposit of humus accumulated. The 
