of a pool of water which gradually filters into the lake as more water 

 is brought down the ravine; (6) after heavy rains the channel is 

 invariably reopened by sweeping the materials which form the barrier 

 into the lake. This gives some conception and appreciation of the 

 force of the current as an active agent in modifying the physiography 

 of the ravine. These statements indicate the dynamic character of 

 the McLeish ravine. 



Succession of Vegetation 



After this brief consideration of the geographical and physio- 

 graphical features, we may now turn our attention to the vegetation. 

 Cowles ('01), in his "Plant Societies of Chicago and Vicinity," calls 

 attention to the fact that the slopes of the embryonic V-shaped ravine, 

 after they attain sufficient stability, develop a carpet of luxuriant 

 vegetation. He says: "In a comparatively few years the vegetation 

 leaps, as it were, by bounds through the herbaceous and shrubby 

 stages into a mesophytic forest, and that, too, a maple forest, the 

 highest type found in our region." This quotation describes admi- 

 rably what has taken place in the greater portion of this ravine and its 

 tributaries. A somewhat detailed examination of the summer and 

 fall flora of the ravine showed that the slopes of the gullies leading 

 into the ravine (PI. I, Fig. 2) were nearly or quite devoid of vege- 

 tation, with the exception of some mosses on the north-facing slopes. 

 A wash or gully near the mouth of the ravine, comparable to the 

 head of a ravine working inland, had rather severe conditions for 

 plant growth in the upper half of its slope. Owing to the extreme 

 exposure to light, wind, and heat, and to the absence of humus in the 

 soil, only a few of the xerophytic pioneers succeeded in establishing 

 themselves. Near the foot of this wash a more or less stable minia- 

 ture plateau (PI. II, Fig. 3) was formed. On this, during the latter 

 part of the summer, there was a great variety and profusion of 

 asters, — one of the most pleasing sights in the ravine at that time. The 

 slopes with mesophytic forest vegetation, with its usual undergrowth, 

 constitute the most general feature of the ravine. This mesophytism 

 in a ravine is regarded by ecologists as only temporary. After the 

 vertical cutting has reached base-level, and as lateral cutting reduces 

 the inclination of the slope, removing the humus and increasing the 

 exposure, there is retrogression from mesophytism to xerophytism. 

 This tendency towards xerophytism is checked through the develop- 

 ment of vegetation, which finally results in the establishment of the 

 permanent mesophytic climax of the region. In the ravine under con- 

 sideration one of the strongest features showing the significance of 

 exposure in the gradual retrogression from mesophytism to xero- 



