348 BOTANICAL GAZETTE [DECEMBER 
habitats. of disjunct groups of various kinds: glacial relicts left 
hind by the northern retreat of the first post-Pleistocene flora, 
outliers from the mesophytic southeastern forests, forerunners of 
western and southwestern plains, and desert types. This contact 
of outposts of such different plant hosts is in itself an argument for 
the existence of notably different environmental complexes in close 
juxtaposition, and hence for the advantageousness of such locations 
as critical points in the study of the physical factors of ecology. 
The data for the present study were obtained at the Illinois. 
State Park at Starved Rock, in La Salle County, Illinois. Here, 
during early post-glacial times, the Illinois River cut a steep-sided 
trench through the St. Peter sandstone (which at this point is thrust 
to the surface by the La Salle anticline), and through its overlying 
strata of Pottsville shales and clays and blanket of glacial till. 
The geography of this region has been treated in detail by SAUER, 
and the geology by Capy (6). The sides of the trench still remain 
as lines of steep cliff, about a mile apart, facing each other across the 
floodplain of the now much shrunken river. The cliff on the south 
side of the river, from a point opposite the village of Utica eastward 
for about seven miles, is unusually precipitous and high, reaching 
a maximum of 157 feet from crest to mean low water level at the 
Starved Rock itself. It is furthermore cut into by a series of remark- 
able box canyons made by small tributary streams. Their sides 
are as precipitous as those of the cliff itself, and for the most part 
their bottoms are either at or near base level. Since the recession 
of the river (which now washes the base of the cliff only in a few 
limited spots, notably Starved Rock, Lovers’ Leap, and Pulpit 
Rock) erosional débris has collected in places as talus slopes at the 
foot of the cliff, both within and outside the canyons. In addition, 
there are at the top of the cliffs the steep slopes from the top of the 
sandstone caprock to the general level of the upland till, and at the 
base a number of fragmentary river terraces of varying age, includ- 
ing oxbows in various stages of filling, and finally the present juvenile 
(and largely treeless) floodplain. A cross-section through a typical 
location either over the cliff side or into a canyon, therefore, would 
reveal the following types of terrain: (1) level upland of glacial 
or Pottsville clay; (2) more or less steep slope toward the edge of 
