66 PHYSICAL GEOLOGY 



down by thousands of feet of rock lying upon it, a bend may 

 result instead of a break. Since both of these conditions 

 exist in the crust, we actually find the rocks broken in some 

 places and bent or folded in others. In fact, different kinds of 

 rock may show both types of structure in the same place, 

 the stronger rocks being broken, while the weaker are folded. 



FOLDS 



Kinds of folds. On examining the layers (or strata) of 

 rock over a large area we may find them flat in one place, 

 wavy or rolling in another, and intricately twisted and crum- 

 pled in a third, with all gradations between. Thus we may 

 describe folding in general as simple or complex ; as mild or 

 intense. 



The individual folds may be classified from a variety of 

 points of view. Simplest of all would be a grouping accord- 

 ing to their attitude. Thus, all folds are either down folds 

 (syndines), up folds (anticlines}, or stepfolds (monoclines). 

 Usually anticlines and synclines are combined in a series of 

 undulations, the former making the crests and the latter the 

 troughs of the waves. 



FIG. 46. Gently folded sedimentary rocks in the central part of the Appa- 

 lachian Mountains. (U.S. Geol. Surv.) 



Before going further into the consideration of folds we may 

 stop to examine the parts of a single simple fold : Each 

 consists of two limbs, rising to a crest in the anticline and 

 descending to a trough in the syncline. The inclination of the 

 limb of a fold is called the dip. In field studies the angle 

 and the direction of the dip are of much importance. The dip 

 is always measured downward and from a horizontal plane. 

 Thus a limb having a dip of 5 would be nearly level, while 

 one with a dip of 90 would be vertical. 



