134 STRUCTURAL AND FIELD GEOLOGY 
tops of such folds and undulations having been gradually 
denuded away, the truncated ends of the strata now crop out 
at the surface. 
As might have been expected, folds present every degree 
of complexity. Some are broad and open, others are narrow 
and compressed ; in some the strata are but slightly disturbed 
—they simply rise and fall in gentle undulations—in others 
the beds may be twisted, contorted, and confused in the most 
extraordinary manner. 
Monoclinal Flexure.—The simplest kind of flexure is 
the monocline. This structure is met with chiefly in regions 
of horizontal or gently inclined strata. It may be shortly 
defined as a sudden dip or abrupt increase of dip followed 
by an equally abrupt return to the former horizontal or 
gently inclined position (see Fig. 19). Frequently the strata 

FIG. I9.—MONOCLINAL FLEXURE. 
in the limb of a monoclinal fold appear attenuated (see 
Fig. 20), as if they had either been laterally compressed or 

FIG. 20.—MONOCLINAL FOLD SHOWING THINNING OF BEDS IN THE FOLD, 
drawn out. As we shall see later on, this attenuation 
becomes still more pronounced until the limb of the flexure 
vanishes and is replaced by a fault or dislocation. 
Quaquaversal and Centroclinal Folds.—Now and again, 
