2IO 



DRY AND MESOPHYTIC FOREST COMMUNITIES 



level is high, and gravitation overcomes the viscosity of the clay, and 

 great masses, whose consistency is that of thick mud, slump down in the 

 form of landslides. This process naturally decreases the angle of slope at 

 the points where the slumping takes place. Slumping does not occur 

 equally everywhere and the bank becomes very irregular. Under such 

 conditions the only animals present are the Collembola. In summer the 

 steep bank dries. No animals are present as actual residents. The 

 bank serves only as a casual alighting-place for tiger-beetles, butterflies, 

 bees, flies, and other insects. Few or no plants are present. 



Fig. 157. — Upper figure is a 

 diagram showing Lake Michigan 

 bluff as seen from the zenith. 

 U, level surface of upland; BL, 

 bluff; SB, sandy beach; M, 

 water of Lake Michigan; J, 

 piers; toward the left is north; 

 sand has lodged on the north side 

 of the piers. AB and CD indi- 

 cate positions of cross-sections 

 below. Middle figure is a cross- 

 section AB. Slumping bluff 

 stage. The adults of Cicindela 

 limbalis are distributed from A 

 toB; the larvae, sparingly, from 

 E to F. Other letters as in the 

 upper figure. Lower figure is a 

 cross-section CD; stage of some 

 bluff stability and bare clay 

 exposure. Adults of limbalis 

 between C and D; larvae plenti- 

 ful between G and H. Other 

 letters as above. Reprinted 

 from the Journal of Morphology. 



Unless something interferes with the action of the waves the same 

 series of events just described continues from year to year. If for some 

 reason the action of the waves is checked, the associated processes will 

 be checked also. At various points along the shore piers have been built 

 out into the water at right angles to the shore for a distance of a hundred 

 meters or more (Fig. 157). The currents in the lake are southerly in 

 direction along the west shore. Whenever water in motion, laden 

 with material picked up by its action against the bluff, strikes one of 

 these piers, its velocity is decreased and a part of the material is dropped 



