CHAPTER XII 

 LAKE BLUFF, RAVINE, AND RIVER VALLEY 



RAIRIE and climax forest, once estab- 

 lished, are not assured of permanency. 

 Indeed, the forces that operate to des- 

 troy them are often at work contem- 

 poraneously with those that complete 

 their formation. 



Along the lake shore wave action is 

 in many places undercutting the bluffs 

 so that adjacent forest or prairie is sliced off gradually to 

 disappear in the insatiable maw of the lake. The plant 

 and animal life of this unstable bluff side is quite different 

 from that of the prairie or forest it replaces. Again, a runnel 

 starts in the spring freshets, courses through the prairie or 

 the forest, excavating a shght depression. Year after year this 

 deepens and widens so that the prairie or woodland is invaded 

 by a growing ravine that produces extensive changes in the flora 

 and fauna. In time the ravine widens to a miniature river 

 valley. It is interesting to trace these changes produced as 

 bluff and ravine and river valley develop. 



The annual winter storms cut away the underpinning of such 

 bluffs, the upper portions slide down, carrying trees and shrubs 

 from the higher level to destruction in the wave-washed base 

 (Fig. i). Naturally, few plants and animals can maintain exist- 

 ence in such an unstable environment. The surface of such a 

 bluff is often largely free from vegetation or scantily covered at 

 best with such forms as can maintain a temporary footing on the 

 insecure soil. 



Moreover, such nearly naked bluffs have a high rate of 

 evaporation. The upper portions especially are dry because the 



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