LAKE BLUFF, RAVINE, AND RIVER VALLEY 



261 



FIG. 390. Common 

 scouring rush, Equi- 

 setum hyemale, 



shares this hunting ground. Some clay-bank wasps hunt here, 

 and the Carolina locust is common. Wherever the cliff is stable 

 enough to develop shrubs and tree thickets the plants 

 and animals of the mesophytic forest margins appear 

 (see p. 230). 



The ravine in clay soil attains at its mouth much 

 the same appearance as does the lake shore clay bluff 

 (Fig. 392), and the animals and plants found are similar. 

 It is evident, however, that the ravine will vary in 

 character according to the sort of material in which it 

 is forming. In loose and friable soil it will develop 

 rapidly, its sides will wear by erosive action 

 with celerity, and it will in a relatively short 

 time be broad in comparison with its depth, 

 so its sides will have gradual slopes. In 

 clay soil, however, the stream cuts into the 

 more resistant material more rapidly than 

 the less vigorous agents of erosion, atmospheric disintegration, 

 rain, and wind affect the sides. The ravine, therefore, will be 

 deep in comparison with its width 

 and will have steep slopes (Fig. 6). 

 In rock only the stream is powerful 

 enough to erode rapidly. The sides 

 wear away very slowly, so the rock 

 ravine has very steep, often verti- 

 cal, sides. Ravines of the first type, 

 hardly ravines at all, but miniature 

 valleys, are wide open to the sun, 

 so that light, heat, and the rate of 

 evaporation is much as it is else- 

 where in the vicinity. The slopes 

 are dry, however, since water runs 

 off them well and seeps out into the 

 lower stream bed. But in ravines in clay soil the sun has 

 access only at certain times of the day; they are, therefore, cool 



FIG. 391. Clay-bank spider, Par- 

 dosa lapidicina. After Shelford. 



