WORKING GROUP 2 
GREENWOOD 
97 
igneous types, whilst metamorphic rocks are least susceptible. Of the latter, 
quartzite is most resistant. 
It is evident that the interrelation of thermal and chemical weathering is 
complex, but the study of their combined effects permits a clearer under 
standing of weathering phenomena and thus aids photogeological-interpreta- 
tion. The following examples illustrate this. 
On aerial photographs of arid terrain, crystalline limestone forms upstanding 
ridges and jagged projecting peaks. Where contiguous with basement quartzite 
Fig. 3. 
Differential weathering in arid areas 
Crystalline limestone (yl) forms dominant feature in quartzite (yq) and grit (yg). Jurassic 
limestone (g) rests unconformably upon these rocks. 
Inda Ad area, former Somaliland Protectorate 
R.A.F. Photograph. Crown Copyright Reserved