46 EARTH FEATURES AND THEIR MEANING 



below the earth's surface, it is quite obvious that the results of the 

 process can be studied only after some thousands of feet of super- 

 incumbent strata have been removed. We are a little later to see 

 by what processes this lowering of the surface is accomplished, 

 but for the present it may be sufficient to accept the fact, realizing 

 that before foldings in the strata can reach the surface, they must 

 have passed through the upper zone of fracture. 



It might perhaps be supposed that the anticlines would appear 

 as the mountains upon the surface, and occasionally this is true ; 

 as, for example, in the folded Jura Mountains of western Europe. 

 More generally, the mountains have a synclinal structure and the 

 valleys an anticlinal one; but as no general rule can be applied, 

 it is necessary to make a restoration of the truncated folds in each 

 district before their character can be known. 



The geological map and section. The earth's surface is in 

 most regions in large part covered with soil or with other inco- 

 herent rock material, so that over considerable areas the hard rocks 

 are hidden from view. Each locality at which the rock is found 

 at the earth's surface " in place " is described as an outcropping 

 or exposure. In a study of the region each such exposure must 

 be examined to determine the nature of the rock, especially for 

 the purpose of correlation with neighboring exposures, and, in 

 addition, both the probable direction in which it is continued along 

 the surface the strike and the inclination of its beds - 

 the dip. If the outcroppings are sufficiently numerous, and rock 

 type, strike and dip, may all be determined, the folds of the dis- 

 trict may be restored with almost as much accuracy as though 

 their curves were everywhere exposed to view. A cross section 

 through the surface which represents the observed outcrops with 

 their inclinations and the assumed intermediate strata in their 

 probable attitudes 'is described as a geological section (Fig. 27). A 

 map upon which the data have been entered in their correct loca- 

 tions, either with or without assumptions concerning the covered 

 areas, is known as a geological map. 



If the axes of folds are absolutely horizontal, and the surface 

 of the earth be represented as a plain, the lines of intersection of 

 the truncated strata with the ground, or with any horizontal sur- 

 face, will give the directions of continuation of the individual 

 strata. This strike direction is usually determined at each expo- 



