> St eertetene a 258 Ba 6 kt ee Bee 
1912] GLEASON—PRAIRIE GROVE 47 
of the more advanced stages, toward the southwest, the present 
location of the pioneer black oak and shingle oak. Of all the 
possibilities, development of the grove by continuous immigration 
from the northeast seems the only plausible explanation, and is 
accepted as the correct conclusion. 
This idea postulates the existence in the past of a large tract of 
forest farther to the northeast, from which immigration into the 
grove took place. A few miles beyond the grove a moraine extends 
from northwest to southeast, perpendicular to the general direction 
of the forest migration, and beyond the moraine and parallel to it 
is the Vermillion River, bordered with a narrow belt of forest. 
The original source of Bur Oak Grove must be looked for at the 
river or along the moraine. 
Several reasons lead to the belief that the moraine was the site 
_ of the ancient forest from which Bur Oak Grove was populated. 
In the first place, the scanty forests along the river are entirely 
incommensurate in size, and the distance is too great. Secondly, 
moraines in northeastern Illinois and parts of central Illinois are | 
regularly forested, and other moraines in Champaign County 
have even now small groves upon them. Most important of all, 
various moraines in central Illinois have upon them forest relics 
which point indubitably toward a former forest covering. Thickets 
of hazel, an immobile forest plant not seriously injured by forest 
fires, are known from several places. On the moraine north of 
Bur Oak Grove, Erythronium albidum, Trillium recurvatum, and 
Claytonia virginica occur. These forest mesophytes produce 
seed in this region so seldom and propagate by vegetative means 
so regularly, that they cannot be considered recent invaders from 
the forest upon the prairie. They die to the ground in the summer, 
before the season of prairie fires, and their persistence on the prairie 
is probably due to this habit, together with their ability to with- 
stand exposure to the full sunlight. Because of these three reasons, 
it seems probable that the moraine was originally covered with a 
forest of some luxuriance, and that from it as a center invasion 
of the surrounding prairies took place. Other moraines must have 
been similarly forested, so that in some prehistoric time a vastly 
larger proportion of the state was covered with forest than at pres- 
