68 



PHYSICAL GEOLOGY 



crumpled instead of being folded in a series of simple waves. 



We therefore see that a given stratum may be competent if 



not much weighted, but incompetent if heavily loaded by 



reason of its burial deep beneath the surface. 



In accordance with these facts there are two types of folds, 



one characteristic of the surface layers and the other of the 



great depths. When a 

 given region is subjected 

 to compressive horizontal 

 forces, the layers at the top 

 may arch and buckle into 

 open folds; while those 

 thousands of feet beneath 

 may be mashed and crum- 

 pled into many little broken 

 parallel crenulations (Fig. 

 49). Between these there 

 is, of course, a transition 

 zone wherein the weak 



rocks such as shale Will be 

 , , ,., ,, 



crushed, while the stronger 



Compare the thickness of the beds near limestones and sandstones 

 the crests and troughs of the folds with , , , , 



the thickness along the limbs. Why the ma Y Cie merely Dent. 

 difference ? Would the same be true of Folds considered in 

 competent folds ? mi, f 



ground plan. Thus far 



we have been viewing folds in cross section. In order to see 

 them as they really are we must add the third dimension and 

 regard them also in ground plan. 



One of the simplest types of fold is the dome, in which the 

 strata dip away equally in all directions from a central point. 

 But the majority of folds are more or less elongated in one 

 direction. If unaltered by erosion, they would form long 

 ridges and troughs, gradually decreasing in relief toward either 

 end. But since almost all the folds we have in nature have 

 been eroded, many of them having been completely planed 

 down, it is better to consider them in their truncated con- 



FIG. 49. Incompetentfoldsinjaspercon- 

 taining streaks of iron ore. (U.S. Geol. 



