1917] GANO— ECOLOGY OF FLORIDA 363 



I 



in 



illustrations of erosion topography. The conditions are unusual, 

 however, since the presence of limestone so near to the surface has 

 brought about the development of extensive subterranean as well 

 as surface erosion, and the topographic features are thus modified 



)licate ecological analysis. In considering 



com 



may 



largely due to the underground erosion. The lakes, sinkholes, and 

 enclosed valleys seem evidence of this. 



Surface erosion 



Branches and creeks. — The trough of almost every valley 

 has a waterway marked by an aggregation of trees and shrubs. 

 The stream is usually an insignificant affair so far as the amount of 

 movement of the water is concerned, and consequently the erosive 

 work accomplished by such a stream is slight. Its course may be 

 found to lead, by a slight rise, to a bayhead where the water is 

 seeping from the base of a slope; or it may issue from a spring 

 whence the water may flow across the ground, spreading out into 

 a miry tract; or, as in the clayey soil of the hills, a definite channel 

 will be cut or gullied down the slope; or the spring will eat back 

 into the hill as a narrow ravine and a small clay canyon thus be 

 cut along the steeper part of the grade. The erosion work lessens 

 as the level is reached, the washing and gullying of the steep banks 

 grade and widen them, and in this way the little streams are 

 gradually bringing the soils of the hills to the valleys. 



Ravines. — In the shady and moist ravines there grow numerous 

 liverworts and mosses, with soil lichens in the upper zone and with 

 ferns along the edges and in the niches. Of the ferns, Paly podium 

 polypdioides Hitch, grows on the moist clay banks, also Asplenium 

 platynearon Oakes, A. resiliens Kunze., Polystichum acrostic hoides 

 Schott., Aspidium Thelypteris Sw., and A. patens Sw. As the 

 stream broadens and shallows and the banks are lowered, reeds, 

 canes, and marsh grasses border the edges, while trees and shrubs 

 develop to form a meadow hammock. 



Rivers.— The Ocklocknee River is an example of an extended 

 stream, rising in Georgia and cutting its way across the latest 



