136 STRUCTURAL AND FIELD GEOLOGY 
away from such an axis on either side at approximately the 
same angle, the structure is known as a Symmetrical Anticline 
or Saddleback (Plate XXIX.). The converse structure, in 
which the strata dip in from either side at equal angles to a 
central axis, is termed a Symmetrical Syncline or Zvough 
(see Fig. 23). When the inclination of the strata is moderate, 
individual anticlines and synclines do not usually extend for 
any distance. A wide region of gently undulating strata 
often recalls the appearance presented by a slightly rumpled 
tablecloth—in which the individual wrinkles, sometimes short, 
sometimes long, succeed each other at inconstant intervals ; 
and while tending, perhaps, to run in a particular direction, 
are yet frequently straggling and irregular. But when the 
strata are inclined at higher angles, anticlinal and synclinal 

FIG. 23.—NORMAL OR SYMMETRICAL FOLDS. 
a, anticline; ss, synclines. 
folds are apt to extend for longer distances, and to preserve 
their parallelism more or less persistently. When folding is 
well developed, it is often possible to follow the axes of 
individual anticlines and synclines throughout their whole 
extent. Each fold begins at zero—forming, at first, a quite 
insignificant “lirk” or crease; little by little, as we follow 
the axis, the dip of the strata augments until in a longer or 
‘shorter distance the maximum inclination is reached, after 
which the dip usually begins to decrease, and finally the fold 
dies away. Not infrequently, however, folds increase and 
diminish in an irregular manner—a great system of parallel 
anticlines and synclines often consisting of a series of 
dovetailed and interlocked folds of variable width and 
extent. 
When folded strata have been for a long period exposed 
to denudation, the original crowns or crests of the anticlines 
have invariably disappeared—the ridges have been gradually 


