531 
spring or skated for many miles across country in winter with but short 
walks between the swamps. Artificial drainage has not only reduced 
the swamps, but it has also caused a general lowering of the water- 
table, so that wells in this region must now be dug 5-10 feet deeper than 
formerly. A few of these undrained morainal depressions surrounded 
by definite zones of native vegetation remain today, but most of them 
have been disturbed by grazing and mowing. The swamp prairies of 
the Chicago plain with their intervening lines of beaches may be in- 
cluded in this group. 
Postglacial drainage at first favorable for prairie formation through 
the drainage of swamps ultimately became destructive through erosion 
of the soil in the development of the natural drainage systems. The 
invading heads of the ravines destroying the prairie turf were usually 
accompanied by the development of a forest vegetation. This fact is 
strikingly illustrated in Brendel’s map, which shows a fringe of forest 
along all the principal water courses. These belts of forest are much 
narrower and the intervening prairies much larger on the youthful 
topography of the Wisconsin glaciation than on the older Illinoisan 
glaciation. This condition is a result partly of the difference in age of 
the drainage systems of the two regions and also of the greater amount 
of forest encroachment on the pre-erosion areas of the prairie in the 
southern Illinoisan glaciation. Flood-plain formation has resulted in 
both forest and prairie; the prairies arising from the lagoons and swampy 
regions have much in common with those arising from morainal depres- 
sions. Outwash from the Wisconsin glaciers gave rise to enormous 
sand areas and sand dunes in the northern half of the state, some of 
which were forested, the remainder being covered with a prairie vege- 
tation and plants of the Great Plains. Rock outcrops and eroding clay 
bluffs were generally forested, but the slopes of lower gradient were 
frequently covered with a xerophytic prairie flora. Local areas of this 
type are still to be found in the vicinity of forest borders, though many 
of them must be regarded as secondary successions following deforesta- 
tion. 
In addition to the prairies arising from postglacial lakes and sub- 
sequent erosion, there is some evidence that the sand prairies and some 
of the upland prairie region of the state date back to an arid postglacial 
period during which there was an eastward extension of the prairie and 
plains flora. The conifer zone following the retreat of the glacier is 
thought to have been stfeceeded immediately by a xerophytic prairie 
flora extending as far eastward as Ohio. Subsequent increase in humid- 
ity in the Mississippi valley was then followed by a westward migration 
of a more mesophytic prairie flora and the deciduous forest of the east 
which have their present tension zone in Illinois. As evidence of this 
view Gleason (8) calls attention to the presence of scattered colonies 
of prairie species beyond the eastern limit of the present prairie province 
and to the isolated occurrence of such western plants as Cristatella 
