44 



EARTH FEATURES AND THEIR MEANING 



though generally less easily, in passing through the Appalachian 



Mountains. 



In regions which have been closely 

 folded the larger flexures of the strata 

 may be found with folds of a smaller 

 order of magnitude superimposed 

 upon them, and these in turn may 

 show crumplings of still lower orders. 

 It has been found that the folds of 

 the smaller orders of magnitude pos- 

 sess the shapes of the larger flexures, 

 and much is therefore to be learned 

 from their careful study (Fig. 25). 

 It is also quite generally discovered 

 that parallel planes of ready parting, 

 which are described as rock cleavage, 

 take their course parallel to the axial 

 plane within each minor fold. As 

 was long ago shown by the pioneer 

 British geologists, these planes of 

 cleavage are essentially parallel and 

 follow the fold axes throughout large 

 areas. 



The overthrust fold. Whenever 

 a stratum is bent, there is a tendency for its particles to be 

 separated upon the convex side of the bend, at the same time 

 that those upon the con- 

 cave side are crowded 

 closer together there 

 is tension in the former 

 case and compression 

 in the latter (Fig. 26). 

 Within an unsymmet- 

 rical or an overturned 

 fold, the peculiar .dis- 

 tortions in the different 

 sections of the stratum 

 are less simple and are 



, , ... JbiQ.25. Secondary and tertiary flexures supenm- 



best illustrated by posed upon the primary ones. 



Recumberrf- 



FIG. 24. Diagrams to illustrate 

 the different shapes of rock folds. 



