so. The lower portion curves around to meet the talus 
slope, which is very steep at the top; or there may "be a 
series of lower escarpments wi th shorter talus slopes be¬ 
tween as shown at the left (fig. 24). Owing to the dip 
of the strata, the face of the cliff is alv;ays very uneven, 
and the seams of the rock, on account of the direction of 
dip, often retain a sufficient degree of moisture to 
support the more xerophytic tree types. The unusual sever¬ 
ity of the situation so far as vegetation is concerned, is 
due to the continual splitting off of rook fragments from 
the parent rock. 
The cliff is often "broken by ledges, due to 
different resistances of the various kind of rock to 
weathering; these may vary in width from a few decimeters 
to several meters, with corresponding accumulations of 
soil. On the vertical rook faces, a few species of lichens 
may be found, such as Locidoa lucida A oh., Gyrophora vellea 
(L.) ach. and Ainphlloma lanuginosum (Hoffm. ) Uyl. The 
vortical face of sandstone represented in A, fig. 26, is 
marked by bands of the yellow lichen, Placodium elegans . 
Where ledges overhang, as shown in fig. 33, the conditions 
are more moist on account of the shade. Amphiloma lanupin- 
osum and the leafy liverwort, Radula complanata , aro usually 
abundant; also various foliose and frutiooso lichens, such 
