122 ELEMENTARY GEOLOGY 
ing is possible only where at least a part of the 
decayed rock particles can lodge near the place where 
they have been formed. On a steep mountain slope, 
the winds and the rain quickly remove every particle 
that is small enough to be so carried away. In such 
places are always found either bare rock ledges, or else 
blocks of rock strewn over the surface (Fig. 55). Even 
PARTLY 
21 DECAYED 
<1 ROCK 
Fig. 54. 
Diagram to illustrate formation of residual soil. 
in hilly regions, where the slope is not so steep, the’ 
action of rain water may prevent the accumulation of 
soil, at least to any great depth. 
Soil Protection. — When the soil becomes deep, it 
really protects the rock from rapid decay ; for it forms 
a blanket which shuts out the heat of the sun, the ac- 
tion of frost, and of those plants whose roots are not 
long enough to reach down to the rock. In such 
