1901] PHYSIOGRAPHIC ECOLOGY OF CHICAGO 165 
becomes almost as hard as rock. In the heart of summer the 
conditions for vegetation are no better on the hard dry slopes 
of a clay bluff than on the hot, dry sands of a dune. Finally as 
to instability: it is doubtless the constant shifting of the sand 
which in the last analysis accounts for most of the poverty of 
the dune vegetation. It is similar on clay bluffs, for when the 


Fic, 30.—Sea cliff along the eroding shore at Glencoe, exposing the morainic 
clay. Vegetation almost entirely absent. Projecting turf mats at the top show the 
tenacity with which the vegetation holds its ground in the face of the erosive forces 
Waves undermine the cliff at its base, the action of gravity 
Causes great masses of material to fall down from the entire cliff 
face. Furthermore, when the clay is saturated with water, great 
portions of the cliff face slide down, entirely apart from the 
action of the sea or lake. At no time, then, is an eroding bluff 
any more stable than a naked dune. 
It becomes evident from a survey of the bluff conditions 
that all vegetation is impossible so long as active erosion by the 
lake continues. Not only this, but vegetation at the top of the 
bluff is soon destroyed. Fig. 30 shows a naked cliff of this 
