
1901 | PHYSIOGRAPHIC ECOLOGY OF CHICAGO 87 
The exposure to wind and to alternations of temperature and 
moisture is excessive. The lack of vegetation, however, is due 
chiefly to the instability of the soil; this instability is particularly 
great in the case of clay bluffs such as these, where the seepage 
of water causes extensive landslide action. No plants can get 
a foothold in such a place, unless it be a few species that may be 
able to make their appearance between periods of landslide 
action; among these plants annuals particularly predominate. 
The perennials that may be found in such places are almost 
entirely plants which have slid down the bank. Near the center 
of fig. zis a clump of shrubs that has slid down in this way. 
Ravines of a similar type may also be seen at many places 
inland, and wherever found the poverty of vegetation on the 
slopes is the most striking character. 
As a ravine extends itself inland the conditions outlined 
above may be always seen about its head, but toward the mouth 
of the ravine the slopes are less precipitous. Torrents cut down 
the bed of the ravine until a depth is reached approaching the 
water level at its mouth. From this time on the slopes become 
reduced and the ravine widens more than it deepens, by reason 
of lateral cutting, landslide action, and side gullies. After a 
time a sufficient stability is reached to permit a considerable 
growth of vegetation. If the erosion is slight enough to allowa 
vegetation carpet to develop, a high degree of luxuriance may 
be attained. In fact ravine conditions are usually extremely favor- 
able for plants, after the initial stages have passed. In a com- 
paratively few years the vegetation leaps as it were by bounds 
through the herbaceous and shrubby stages into a mesophytic 
forest, and that, too, a maple forest, the highest type found in 
our region. Nothing shows so well as this the brief period 
necessary for a vegetation cycle in a favored situation when 
compared with an erosion cycle. 
Of such interest are the facts just noted that it is worth while 
to mention some of the characteristic ravine plants. Perhaps the 
most characteristic trees of the Glencoe ravines are the bass- 
wood (Tilia Americana) and the sugar maple (Acer saccharinum), 
