PHYSICAL CHANGES OF THE OUTER SHELL 



67 



When many anticlines or synclines are compared, it is 

 found that they present a wide variety of forms. Thus there 

 are low, broad folds (Fig. 46), sharp folds (Fig. 47), tightly 



FIG. 48. Overturned folds. 



FIG. 47. Closely folded strata in the southern part of the Appalachian 

 Mountains. (U.S. Geol. Sure.) 



compressed folds, and even inclined or overturned folds (Fig. 

 48). A layer of rocks may be bent into any of these forms 



according to the condi- ^. , ( 



tions under which the 

 pressure was applied. 



Competent and in- 

 competent folds. It is 

 often advantageous to 

 classify folds according to their competency. In order to form 

 an anticline a layer must have a certain amount of strength. 

 This will be readily apprehended if we imagine several layers 

 of loose sand to be compressed on each side, they would be 

 mashed without definite folding. As compared with loose 

 sand or mud, we can well understand that firm beds of sand- 

 stone or limestone would be likely to buckle up in the form of 

 folds. Beds which are strong enough to hold themselves up 

 in arches have been called competent strata, while materials 

 which may be squeezed and crushed together are incompetent. 



In considering competency, however, it is necessary to 

 take into account something more than the character of the 

 rock. Sheets of paper lying free upon the table, when com- 

 pressed sidewise, will arch into a fold and, under those con- 

 ditions, are competent. Nevertheless if several books are 

 piled upon the sheets, the latter will not arch when com- 

 pressed, but will merely be crumpled into many little twisted 

 folds. Similarly any layer of rock, however strong, may be 

 so weighted down by overlying beds that it will be complexly 



