262 A NATURALIST IN THE GREAT LAKES REGION 



and moist. Evaporation goes on very slowly. These same 

 conditions prevail even in a more emphatic manner in the 

 rock ravine. In the latter type especially, springs emanating 

 through rock cracks and crevices often keep the sides drip- 

 ping wet. 



The ravine is naturally most completely developed at or 

 near its mouth, for it was at this end that it began cutting back 

 into the highland. At its head it is still a young ravine, pushing 



-%^ 





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-«*« 



i 



Fig. 392. — Mouth of ravine at the lake shore 



back gradually into the prairie or forest. As one follows down 

 the ravine older and older stages are encountered. At first it 

 is a mere gully with a tiny trickle in it, except during spring 

 freshets. Then it deepens and widens into a young V-shaped 

 ravine with a growing stream as tributary guUies add their 

 contributions. In clay and rock the sides are very steep — so 

 steep that the rock or clay slides down in landslides into the 

 bottom, often producing bare areas so unstable that plants can- 

 not get a foothold. Still farther down the growing brook begins 

 to cut into one side, then the other, as it assumes a tortuous 

 course due to varying obstructions. So the stream meanders 



