


Igo! | PHYSIOGRAPHIC ECOLOGY OF CHICAGO 1o1 
Hence the upper part of the island is the oldest, as the vegeta- 
tion well shows. igs. 8 and zo show at the lower end the sand 
bar, which comes to a point and is so young or so exposed to 
submergence as to be barren of vegetation. Next comes the 
Ambrosia, then the willows, and finally a characteristic flood plain 

4 a 
— ae line 
Fic. 13.—Flood plain forest along Fraction run at Lockport, showing a rather 
striking collection of southern trees (see text). Coffee tree in the foreground. 
forest (background of fig. 8). The asymmetry of the river 
island vegetation is in striking contrast with the zonal symmetry 
of pond islands, as will be shown later (fig. 79). The cause is 
evident, viz., the relative lack of symmetry in river currents as 
compared with pond currents. 
The gradual encroachment of the land upon a stream through 
continuous deposition is well shown along the Desplaines river, 
and to a less complete degree along the Chicago river and Thorn 
