THE STRUCTURE OF THE EARTH 



257 



strata which are being folded are composed of beds of firm sandstone 

 or limestone, they will be thrown into anticlines and synclines. 

 Such strata are called competent. If the strata, however, consist of 

 shale or mud, they may not be strong enough to form arches and will 

 be squeezed and crumpled together, and are called in consequence 

 incompetent strata. The effect of horizontal compression upon beds 

 of rock differs under different conditions. For example, if a bed is 

 near the surface, it may upon being subjected to lateral pressure 

 be able to form an anticline ; but if it is deeply buried, the pressure of 

 the overlying strata may be so great that it will be crumpled into 

 many small folds. No one has ever seen strata in the process of 

 folding, but through experimentation (p. 361) and a study of folded 

 strata the means by which it is produced have become known. 



How the Structure of a Region is Determined. — In order to 

 determine the structure of a region in which the strata have been 

 greatly folded and 

 eroded, it is necessary 

 that a geological map 

 of the region be made, 

 in which the areas 

 underlain by the vari- 

 ous rocks are indicated 

 and the dip and strike 

 of the outcrops re- 

 corded. When such 

 a map as that shown 

 in Fig. 249 is com- 

 pleted, we find that 



not only does the strike of the beds vary, but that the dip as shown 

 by the arrows varies. By combining all of the data attainable, 

 the cross section also shown on the side of the diagram can be 

 constructed. 



Origin of Folds. — The origin of folds will be discussed more fully 

 under mountains (p. 358). It will be sufficient at this time to state 

 that the various folds which have been discussed (with the exception 

 of monoclinal folds) are in general due to lateral compression (p. 364). 



Warping. — Besides the folding described above there are move- 

 ments of the earth's crust which result in the warping of its surface; 

 that is, there are vertical movements which raise the surface in 

 certain places and depress it in others. This is well shown along the 



Fig. 249. — Block diagram showing the manner in 

 which geological maps are prepared. The various rock 

 formations are mapped and the dip and strike of the 

 strata (shown by arrows) recorded, in order that the 

 geological section (shown on the sides of the diagram) 

 can be determined. 



