CHAPTER VII 



THE STRUCTURE OF THE EARTH 



Unless subsequently disturbed, the sedimentary rocks of the earth 

 are in the approximately horizontal position which they had when 

 they were first deposited. For example, we find the sedimentary 

 strata covering the vast territory between the Appalachian and Rocky 

 mountains, for the most part, in the horizontal position which they 

 had when they were outspread in sheets on the ocean floor. 



Structural Features of Rocks 



Dip and Strike. — In such regions as the Appalachians, New 

 England, eastern Canada, the Rocky Mountains, and the Sierra 

 Nevadas, however, the strata are often inclined at angles varying 

 from horizontal to vertical. The strata thus tilted are sedimentary, 



fen /v/cf 



Symmetrica/ fv/ct 



Is oc fine 



Fig. 238. — Section through a folded region showing a symmetrical fold, an 

 isocline, and a fan fold. 



and their present altitude is the result of compressional forces to 

 which they were subjected. As a result of weathering and erosion 

 their upper portions were denuded, leaving the inclined beds outcrop- 

 ping at the surface. In Fig. 238 the position of the strata with refer- 

 ence to the present surface is shown, and the portion carried away by 

 erosion is indicated by dotted lines. Two terms are used to describe 

 an inclined bed : dip and strike. The meaning of the terms can best 

 be understood from an illustration. If one side of the roof of a house 

 is taken to represent an inclined stratum, the downward inclination, 

 that is, the course which water poured on the roof would take, is the 

 direction of the dip, the angle of the dip being the departure from the 

 horizontal. Thus, if a roof is inclined at an angle of 30 to a level 



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