798 PRINCIPLES OF STRATIGRAPHY 



surface in a concave or synclinal fold, bed 4 is the youngest of the 

 series, and formerly overlay all the others. When the actual type 

 of folding cannot be observed, an examination of the beds them- 

 selves will often reveal their relationships. Thus, if bed 3 shows 

 normal ripple marks, footprints or other markings (see Chapter 

 X\TI) on the surface of any layer facing bed 2, it is evident that 

 the surface of 3 next to bed 2 was the upper surface of that bed, 

 and that the strata are related, as in Fig. 179, b, bed 3 being older 

 than bed 2. If, however, bed 3 shows on the side of its layers 

 facing bed 2 the natural molds or reverse impressions of the mark- 

 ings named, it is evident that that is the lower side of the bed, and 

 that 3 therefore overlies 2 and is younger, as shown in Fig. 179. c. 

 (d) Fan folds. In regions of sharp folding a fan type of fold 



Fig. 180. Generalized section of the fan fold of the central massif of the 

 Alps. (After Heim.) 



may be produced, in which the limbs of the arch dip toward each 

 other for a certain distance. Here the lower or concave portions of 

 the fold are pressed inward with the result that the beds at the cen- 

 ter of the fold are squeezed out or pinched. This type of fold is 

 characteristic of the Alps and other strongly folded districts (Fig. 

 180). 



(e) Monoclines. Typically a monocline is a part of an anti- 

 cline, cut off by faulting or erosion. Simple monocHnes are those 

 in which the strata have no further continuation. Thus the Front 

 Range of the Rocky Mountains is flanked by a series of simple 

 monoclines, all of which face, with their erosion slopes, the crys- 

 talline axis of these mountains. In many cases the continuation of 

 these folds is, however, on the opposite side of the crystalline axis. 

 The Blue Ridge extending from New Jersey for the entire length 

 of the Appalachians is a variable series of eastward facing mono- 

 clines. The Appalachians themselves are for the most part com- 

 posed of complementary monoclines, these representing the oppo- 

 site limbs of anticlines with the axis opened by erosion. Simple 



