224 ILLINOIS ACADEMY OF SCIENCE 



ing this sequence are (1) the Rhus association, (2) the 

 Populus-shrub association, (3) the Juniperus-shrub as- 

 sociation, and (4) the Upland-climax forest association. 



a. The Rhus association. — Since the topography and 

 the vegetation are so closely associated it will be well to 

 discuss the topography of the bluff first. With the excep- 

 tion of a very small bit of lake bluff in front of the Mc- 

 Leish property in N. T. 8, S 10, the lake bluff from Park 

 Avenue to a point just south of Dell Place was slumping 

 badly. 



The amount, or even the presence of vegetation, de- 

 pended entirely upon the degree of soil stability. In 

 places there was no vegetation on any part of the bluff; 

 sometimes it was scattered sparsely over its face; then 

 again, it was confined to the upper part of the bluff. Aside 

 from Rhus glabra and Rhus typhina, which are found in 

 the most xerophytic parts of this association, the vegeta- 

 tion consists primarily of plants from the Populus-shrub 

 and the Juniperus-shrub associations. Among the scat- 

 tered plants found here in addition to the above were 

 Ostrya virginiana, Juniperus communis, Salix syrticola, 

 Corylus americana, Cornus paniculata, Populus grandi- 

 dentata, Populus tremuloidcs, and saplings of Fraxinus 

 americana and Tilia americana. 



A picture looking south from a point just north of 

 Ravine 5 ( see Fig. 7 ) shows the steep angle of the actively 

 eroding clay bluff as it appeared eighteen years ago (1900). 

 The photograph taken this year (1918) shows how 

 through continued erosion and the development of a 

 slightly wider beach in places, owing to the building of 

 piers, the angle of slope has been lessened and the vegeta- 

 tion of the Rhus association type has come in (see Fig. 8). 



b. The Populus-shrub association. — The Populus- 

 shrub association occurred where the soil of the clay 

 bluff was in a more stable condition than that of the above. 

 This association was best represented from a little over an 

 eighth of a mile north of Ravine 4 (Newhall) (see map, 

 N. T. 6, C 31) to the Cook-Lake County line. An idea of 

 the vegetation and its more or less uniform, though rather 

 sparse distribution over the face of the bluff may be seen 

 in Fig. 9. The trees were small, and the shrubs formed 

 the most conspicuous part of the vegetation. 



