material that formed the railroad grade or ballast or that had earlier eroded from 
the sandstone cliff; apparently the fern was not growing on the cliff itself, The 
station in the north section of Jackson Hollow, the eighth for filmy fern in Pope 
County and in Illinois, contained more plants than all of the other seven stations 
combined, On March20, October 8, and December 9, Mr, Clark visited this station 
with me and we photographed the overhang and the fern, fig. 12 and cover, 
In all eight localities the fern is associated with Pounds sandstone, There 
appears to be no correlation, however, between the growth of filmy fern and a par- 
ticular stratum, In some sites the plants grow on massive sandstone without apparent 
bedding planes. In others they grow on sandstone composed of numerous horizontal 
layers, each of which measures only 1 centimeter in thickness, They also grow on 
thicker layers, some of which are tilted at steep angles, 
Direction of exposure of the plants is not constant, In the Steagall site in 
Jackson Hollow the exposure is northwestward; in the north section of Jackson Hollow 
the exposure is southeastward, In Hayes Creek Canyon it is west. In the Sand Cave 
Fig. 11.--Filmy fern growing on the sand-covered floor at the base of the overhang in the north 
section of Jackson Hollow, 
1 
