50 THE PLANT SOCIETIES OF 



hills. The beech (Fagus ferrugined) is much rarer than the sugar 

 maple, though it is a rather important constituent of the mesophytic 

 forests about Chesterton. Why the beech-maple forest has lagged so 

 far behind in the region about Chicago is a question not yet settled. 

 If these forests elsewhere have had an oak stage it indicates that the 

 development here is very slow. 



Though the forests just described, whether of the oak-hickory or 

 the maple-beech type, are of a high degree of permanence, it can be 

 seen that this permanence is but relative. Sooner or later stream 

 action will enter these districts and base leveling processes will begin 

 on a more rapid scale. But for these activities the lowering of hills 

 would be very slow indeed, so slow as hardly to interfere at any point 

 with a luxuriant development of the vegetation. The destruction of 

 these morainic forests by stream erosion is well shown near the shore 

 north of Evanston, and also along Thorn creek. Fig. i8 shows a 

 morainic island in a flood plain, the sole remnant of an extensive 

 stretch of upland mesophytic forest. We must therefore regard upland 

 forests as temporary also, though they endure for a much longer time 

 than do the temporary mesophytic forests of the ravines. 



C. The sand hill. — A third type of upland is found in the sand 

 hills, but since most of these in our district are of dune origin, their 

 treatment will be deferred until later. 



B. THE COASTAL GROUP. 

 I. THE LAKE BLUFF SERIES. 



The plant societies that have been discussed hitherto may be found 

 in many if not in most inland districts. The societies that follow, on 

 the other hand, are best worked out only in connection with the coasts 

 of oceans or great lakes. The^etically a bluff may be composed of 

 any kind of rock or soil, but those of our area are composed of mo- 

 rainic clays, and the life histories that follow will not hold good in 

 other conditions. It may be noted here that there is a short stretch 

 of rocky shore with lithophytic algae at Cheltenham, but there is 

 nothing that in any way approaches a rock cliff. 



Wherever a sea or lake erodes rather than deposits, there is com- 

 monly developed a sea cliff of greater or less dimensions. The mate- 

 rial which is thus gathered may be deposited elsewhere in the form of 

 beaches, and later the wind may take up the sands from the beach and 



