210 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. 
Fic. 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 
to B; the larvae, sparingly, from 
Eto 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 
