such as Physcia obscura (Schaer.) Hyl., Parmelia caperata 
5 Ach., Puiaullna calicaris (L.) Fr., and Qyrophora vallea 
(L.) Ach* In the crevices of such ledges occur two character¬ 
istic fern species, namely. Polypodium vulgare L* and Woodsia 
ilvensls (L.) R. Br. 
The flora of the exposed ledges is very variable* 
On the smaller, newer lodges, a few lichen epecioa, such as 
Lecidea lactea (Plot.) Sohaer. and Parmolia conspersa (rlhrh.} 
Ach* nay gain a footing* On larger ledges, the higher plants 
are usually well represented, from small herbaceous species 
to trees whose roots are anchored in some of the bigger cre¬ 
vices (fig. 34)* Typical species are shown in the figure* 
Among others nay be mentioned Potentilla tridentata . Panicue; 
xanthophyaum . CeanOthus oner icanus , ??ubus parviflorus . Amel - 
anchier canadensis . Polystichum Lonehitis , Thuja occidentalis . 
and Juniperus co;munfta L* vur. mentana Ait* 
When there is a marked difference in rate of 
weathering between the sandstone and trap, a bread ledge 10 - 
16 m* aoross mey be formed (fig* 26A). On account of the greater 
deposit of soil and consequent storage of moisture, the con¬ 
ditions here are much moro stable* Such an area may develop 
a comparatively thicx stand of pine, thriving under the pro¬ 
tection of the cliff. 
The destiny of the escarpment is the condition seen 
