CURVATURE OF STRATA 137 
lowered by denudation. On the other hand, the synclinal 
structure has evidently offered greater resistance to the forces 
of decay, for not infrequently we find that hills are built up 
of trough-shaped strata. But although the original tops of 
all anticlinal ridges have, as a rule, disappeared, and synclinal 
troughs have also been reduced, geologists still speak of these 
Geametures as if they were~periect folds. . Fig. 24 represents 

Fic. 24.—MODEL OF DENUDED SYNCLINAL AND ANTICLINAL FOLDS. 
a—b, a—b, axes of folds. 
the model of two synclines with intervening anticline—the 
arrows indicating the direction of the dip. The dotted lines 
(2 are the axes of the folds. The section shows the 
geological structure, and the relation of that structure to the 
surface features. 
Unsymmetrical Folds.—When the axial plane of a fold 
is inclined at any angle from the vertical, the fold is said to 
be unsymmetrical. The inclination may be very slight—so 
slight that the strata on either side of the axis may have 
much the same angle of dip; or the axial plane may depart 
so far from the vertical as to be actually horizontal, and the 
fold then lies on its side: Between these extremes every 
degree is encountered. As a rule, unsymmetrical folds are 
closely compressed, the limbs flattened against each other, 
and the crowns of the arches usually somewhat pointed. 
Some typical forms are represented in the accompanying 
illustrations (Figs. 25-30). When the axial plane is so much 
inclined that one limb of the fold becomes doubled under the 
other, we have the structure known as an Overfold (see Fig. 


