THE STRUCTURE OF THE EARTH 



255 



other, they are called overturned folds (Fig. 243 C). Folds are called 

 recumbent when they are so far overturned that they lie on their 

 sides. The crests of anticlines are often thickened and the limbs 



*\ D 



Fig. 243. — A, upright isoclinal folds; B, inclined isoclinal folds; C, overturned 

 isoclinal folds; D, fan fold. (Ries and Watson, after Willis.) 



thinned. In severe folding this may result in a limb so thin as to be 

 scarcely recognizable. When the compression has been very great, 

 the sides of an anticline may be driven toward each other to such 

 an extent as to produce what is called a fan structure (Fig. 243 D). 

 The axis of a fold, or its crest line, is not horizontal for long 



Fig. 244. 



Fig. 245. 



Figs. 244 and 245. — Surface and sectional views of a plunging anticline (244) 

 and of a plunging syncline (245)0 The effect of resistant strata in forming ridges 

 and canoe valleys is shown. (After Willis.) 

 CLELAND GEOL. — 1 7 



