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PHYSICAL GEOLOGY 



vounger ones (Fig. 25 8 C) . The result of thrust faulting in the Selkirks 

 of Canada and in many other regions has been to move strata over and 

 upon others that were formerly hundreds of feet higher. Occasion- 

 ally, recumbent folds can be traced into thrust faults (Fig. 258 A> B). 

 This is not surprising, since it is evident that both are due to lateral 

 pressure; the movement being so great in the latter that the strain 

 could not be relieved without breaking. Thrust or reverse faults 

 involve a shortening of the earth's crust, as has been said, and differ 

 in this respect from normal faults which are the result of a stretching 

 of the crust. The fault plane usually approaches the horizontal more 

 nearly in thrust faults than in normal faults; that is, the hade of 

 the former is greater than that of the latter. 



Examples of Thrust Faults. — Many examples of thrust faults 

 might be cited. A great thrust fault (Bannock overthrust) l extends 

 approximately 270 miles from northern Utah into Idaho, in which 

 the older strata have slid over the younger (horizontal displacement) 



Fig. 259. — Thrust faults and folds in the southern Appalachians. 



a distance of at least 12 miles. In the same region other faults with 

 a throw of 15,000 to 20,000 feet have been described. In Massa- 

 chusetts, 2 a thrust fault has been described in which the strata have 

 slid along a fault surface for 15 miles. The southern Appalachian 

 Mountains (Fig. 259) are broken by many faults that run parallel to 

 the system. In Virginia and Georgia 15 or more parallel thrust faults 

 occur, running from northeast to southwest, along which the older 

 strata have been pushed over the younger. One of these faults 

 has been traced for 375 miles, and its greatest horizontal displacement 

 is at least 11 miles. 



Vertical and Horizontal Faults. — If faulting has taken place 

 along a vertical (90 ) joint or other fracture, the arrangement of the 

 strata may give the appearance at the surface of either a normal 

 or a reverse fault, depending upon whether the right or left side 

 moved down (Fig. 260). In many cases, along the same side of a 

 given fault line the movement may have been upward in one place, 

 downward in another, and without evident movement at another. 

 In some cases, a fault occurs along bedding planes and is called a 



1 Richards and Mansfield, — Jour. Geol., Vol. 20, 191 2, pp. 681-709. 2 J. Barrell. 



