LAKE BLUFF, RAVINE, AND RIVER VALLEY 269 



is very conspicuous. It is a large dragon fly with lance-shaped 

 yellow spots along the back of the abdomen, and two con- 

 spicuous yellow bars on each side of the thorax. Snails are 

 fairly abundant also in the lower portion of 

 the clay ravine; the white-lipped snail, 

 Polygyra thyroides, Pyramidula alter nata, 

 Pyramidula solitaria, and Circinaria concava 

 are the most common ones. The green tiger 

 beetle so characteristic of the climax forest 

 is also abundant here. 



The broad, open ravine, which develops as 

 erosion proceeds and the sides of the steep FlG 4I0 . Toad 

 clay ravine wear away, is so like the margin bug, Gelastocoris 

 of the customary river valley that no special 

 mention need be made of it; the description below will fit it 

 fairly well. 



The valley of the river that is cutting through rock as is the 

 Illinois at Starved Rock, the Rock River at Oregon and Grand 

 Detour, the Wisconsin in the neighborhood 

 of the Dalles, is bordered by steep rock bluffs. 

 The vegetation and animal life on these differ 

 quite decidedly from that of the steep sides 

 of the ravines, for the river valley is wide 

 open to the sun's rays. Such rocky river 

 bluffs are dry and hot. The top of the bluff 

 (Fig. 7) has a tree and shrub population 

 much like that of the crest of the ravine, 

 FIG " r 7 Hooded name ly> white pine, red cedar, white cedar, 

 grouse locust, Para- June berry, spreading juniper, blueberry, 

 tettixcucullatus. After huckleberry, ninebark, red elderberry, etc. 

 Luggen In the rock crevices near the top of the bluff 



the rock polypody fern is common (Fig. 409). The sides of such 

 bluffs support columbine, bluebell, alumroot (Fig. 404), whitlow 

 grass (Fig. 405), rattlebox (Fig. 406), sarsaparilla, spiderwort, 

 Prenanthes, and occasionally walking fern (Fig. 407). 



