CHICAGO AND VICINITT. 51 



form dunes. The Chicago area gives splendid examples of these two 

 types of sea activity; to the north of the city is an eroding coast line 

 with its- bluffs, and to the south and southeast is a depositing coast 

 with extensive areas of beach and dune. 



The lake bluffs at Glencoe give an excellent opportunity for the 

 study, of the life history of a sea-cliff vegetation. There can be al- 

 most no other habitat in our climate which imposes such severe condi- 

 tions upon vegetation as an eroding clay bluff. The only possible 

 rival in this regard is a shifting dune, and even here the dune possesses 

 some points of advantage so far as the establishment of vegetation is 

 concerned. In the first place, the conditions as to exposure are 

 almost identical with those of a dune; the heat of midday and of 

 summer and the cold of night and winter are extremely pronounced; 

 the intensity of the light and the exposure to wind make the condi- 

 tions still more severe. In other words, the only plants that can grow 

 on these lake bluffs, at least in the earlier stages, are pronounced 

 xerophytes. Again, the character of the soil is unfavorable, for while 

 the clay is wet in the autumn, winter, and spring, it dries out in the 

 summer and becomes almost as hard as rock. In the heat 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 insta- 

 bility: 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 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 satu- 

 rated 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 con- 

 tinues. Not only this, but vegetation at the top of the bluff is soon 

 destroyed. Fig. 30 shows a naked cliff of this character ; at the top 

 there can be seen overhanging turf, giving evidence both of the 

 destructive action of the lake and also of the tenacity with which a 

 grass mat holds its place in the presence of adverse conditions. Near 

 the center of fig. ji may be seen a white oak which was almost over- 

 thrown by the erosive activities, but which has been preserved through 

 the cessation of erosion at this point. The gully shown near the cen- 



