124 ILLINOIS ACADEMY OF SCIENCE. 



THE LIMESTONE CLIFFS. 



There are certain peculiarities in the zonation of the vegetation 

 on the Hmestone cHffs which must be mentioned, since the same 

 phenomena seem not to have been noted in descriptions of similar 

 regions. The Galena formation on Pine Creek furnishes the 

 best examples. 



The course of Pine Creek is entrenched in a generally hori- 

 zontal upland. Vertical cliffs of considerable height are com- 

 mon. On the upland above one of these cliffs is commonly found 

 the usual oak forest, and this approaches to within a few rods of 

 the cliff. At about the point at which the surface begins to 

 round downward to meet the vertical face below, the trees are 

 replaced by a very xerophytic grass and shrub zone of which a 

 characteristic member is PJiysocarpus. Below the Physocarpus 

 zone, and immediately above the vertical face of the cliff, where 

 the slope is so steep that only a few handfuls of soil are able to 

 find lodgment in the crevices, is an assemblage of mesophytic 

 plants. The most characteristic forms are Tilia, Ostrya, Acer 

 saccharuin, Ta.viis canadensis and Cornus stolonifera. Pinus and 

 Juniperus may also be present. Ta.vus is practically restricted to 

 this sort of habitat. Tilia, Ostrya and Acer occur together here 

 as on the talus slopes and in the climax forest. Ulnius may also 

 be found. Beneath this zone there is the bare vertical rock face 

 bearing only the usual very limited vegetation. 



There are no experimental data upon which to base an expla- 

 nation of the occurrence of a mesophytic society at the top of 

 a cliff, but general observation leads to the following suggestion. 

 The water table must fall as it approaches the cliff, but owing 

 to the heavy bedded character of the underlying rocks, it does 

 not fall rapidly. The much larger number of horizontal chan- 

 nels than vertical ones leads the percolating waters in an almost 

 horizontal direction. The water table therefore falls far enough 

 below the surface near the edge of the hill to make its shoulder 

 rather xerophytic, but intersects the steeper lower slope near the 

 top of the vertical face. The steepest slope is therefore the damp- 

 est and the vertical face may in wet weather become a dripping 

 cliff. The argument is strengthened by the independence of this 

 plant formation of all accidents of exposure, or any other factor 

 of the sort and its persistent recurrence upon every cliff of this 

 description, large or small. Its complete correlation with this 

 peculiar physiographic situation would indicate that its explana- 

 tion is to be found within the substratum. 



