COMPLEX FOLDS 



24I 



the base and the synclinal broader below than above, a reversal 

 of the normal arrangement. 



The isoclinal and fan folds may be upright, inclined, inverted, 

 or recumbent. In the closed folds there has been such enor- 

 mous compression that the same strata are of different thickness 



Fig. 99. — Inclined isoclinal folds, eroded. (Willis.) 



in different parts of the flexure. This is especially marked in fan 

 folding, in which the beds are much thinner on the limbs than at 

 the summit, and sometimes the central beds in the folds have 

 been actually forced to flow upward or downward, forming iso- 

 lated masses, cut off from their original connections. 



Besides the simple folds above described, there are frequently 

 found complex systems of flexures, in which the compressing force 



Fig. 100. — Diagram of monoclinal fold. 



has acted simultaneously or successively in different directions, 

 producing highly complicated cross-folds. These are, however, 



