CHANGES IN THE STRATIFIED ROCKS 285 
broken. The simplest fold is the monocline: (Fig: 
165), in which, between two nearly horizontal areas, 
there is an inclination of the layers. It is therefore 
a fold with an inclination in one direction only.’ 
Where the folding results in a dip in two directions, 
we have either an anticline or a syncline (Fig. 166), 
the first being an 
upfold, the second 
a downfold.? The 
direction in which 
the fold extends 
is the axis. Very 
often anticlines 
and synclines are 
symmetrical (Fig. 
167); but much 
more commonly 
P Map showing symbols used to indicate dip and 
unsymmetrica l, strike. Cross indicates horizontal rock. Arrow 
, ‘ points in direction of dip. Line at right angles 
with one side uF to this shows strike. Length of arrow shows 
limb dipping more amount of dip. Section at bottom represents 
7 actual structure. 
rapidly than the 
1 The term monocline is sometimes used to define one part of a more com- 
plex fold, such as one-half of the anticline or the syncline. It is also used 
to indicate the single inclination of tilted rocks on one side of a fault plane. 
These are really distinct monoclines, but not monoclinal folds. 
2 There are few mistakes more common on the part of students of geology 
than to suppose that an anticline is synonymous with a hill, and a syncline 
with a valley. This is often so, especially when an anticline is complete ; 
but where denudation has been in progress, we often find thatthe anticlinal 
elevation has. been reduced to a valley, and the syncline to a hill (Fig. 182). 
