CONTORTIONS OP THE STRATA 



43 



zontal, but in more complexly folded regions this is seldom true. 

 The departure of the axis from the horizontal is called the pitch, 

 and folds of this type are described as pitching folds or plunging 



FIG. 23. Anticlinal and synclinal folds in strata (after Willis) . 



folds. The axis is in reality in these cases thrown into a series 

 of undulations or " longitudinal folds," and hence pitch will 

 vary along the axis. 



The shapes of rock folds. By the axial plane each fold is 

 divided into two parts which are called its limbs, which may have 

 either the same or different average inclinations. To describe 

 now the shapes of rock folds and not the degree of compression of 

 the district, some additional terms are necessary. Anticlines 

 or synclines whose limbs have about the same inclinations are 

 known as upright or symmetrical folds. The axial plane of the 

 symmetrical fold is vertical (Fig. 24). If this plane is inclined to 

 the vertical, the folds are unsymmetrical. So soon as the steeper 

 of the two limbs has passed the vertical position and inclines in 

 the same direction as the flatter limb, the fold is said to be over- 

 turned. The departure from symmetry may go so far that the 

 axial plane of the fold lies at a very flat angle, and the fold is then 

 said to be recumbent. The observant traveler by train along any 

 of the routes which enter the Alps may from his car window find 

 illustrations of most of these types of rock folds, as he may also, 



