


286 STRUCTURAL AND FIELD GEOLOGY 

It is matter of common knowledge, indeed, that hills and 
ridges are usually, or at least very often, built up of relatively 
harder or more resistant rocks than those that occupy con- 
tiguous, low-lying tracts. This, however, is not invariably the 
case, as will be shown later on. Not infrequently the hills of | 
a country consist of no harder or less readily disintegrated 
rocks than are found in the low grounds. In a great many 
cases this is due to the geological structure or arrangement of 
the rocks. There are certain structures that tend to resist 

FIG. 109. —SURFACE-FEATURES IN GENTLY-FOLDED SANDSTONES, 
while others favour denudation. Hence, a series of strata 
having the same consistency throughout, may in some places 
form hills, while elsewhere they may occupy depressions of 
the surface. In the above diagram (see . Fig, (ig@)auem 
example, it is cbvious that the position of the hills has been 
determined by the strong synclinal arrangement, while the 
weaker anticlinal structures have been more readily reduced. 
If the observer be geologising in a region where the rocks are 
inclined for long distances in the same direction, he will 
usually find that the outcrops of relatively harder beds tend 

FIG. 110.—FORM OF GROUND INFLUENCED BY GEOLOGICAL STRUCTURE. 
to project more than those of the less durable strata arnongst 
which they are intercalated. Hence, even when the naked 
rock is concealed by vegetation and soil, it nevertheless will 
form a feature. Thus, in the accompanying section (Fig. 110), 
we have a series of limestones and shales, the outcrops of 
which are not actually seen, and yet their position is indicated 
