Appalachian Faulting. 261 



chief factor in producing many of the peculiar structural 

 features which characterize this region.* 



Considered with reference to their massiveness, strata in the 

 Appalachian province fall into three great groups : at the base 

 a series of thinly stratified Cambrian sandstones, shales and 

 limestones; in the middle a very massive Cambro-Silurian 

 limestone, and on top a series of thinly stratified sandstones 

 and shales. Locally, strata in the upper and lower series be- 

 come massive but none have the extent, uniformity and mas- 

 siveness of the great limestone, which is 2500 to 6000 feet 

 thick in the folded zone and covers more than 600,000 square 

 miles. On any line across the zone of deformation this stra- 

 tum would most efficiently transmit a pressure acting tangen- 

 tially to the earth's surface, and if before compression it were 

 anywhere bent upward or downward, then during compression 

 the preexisting bend would determine the point at which it 

 would yield to sufficient force ; it would act as a crooked strut. 



That such bends did exist in the great limestone, before the 

 action of compression which produced the Appalachian folds, 

 has been shown from a consideration of the process of sedi- 

 mentation and from the observed synclines of deposition men- 

 tioned above. In the Appalachian province in such original 

 synclines the steeper seaward dip was northwestward and the 

 gentler shoreward dip was southeastward. Now if strata occu- 

 pying the initial position represented in fig. 1, p 259, be 

 subjected to a sufficient compressive force they will yield at 

 the bends ; the original anticline, and the original syncline 

 will be exaggerated while the steeper and hence shorter 

 limb of the syncline will occupy the position of an arm be- 

 tween the forces of a couple. It will therefore be rotated so 

 that the original seaward dip will grow steeper and, if com- 

 pression continues long enough, may be overturned. Such a 

 fold may be compared to a step of broad tread and moderate 

 rise and hence may be called a step/old, fig. 2, p. 259. This 

 has long been recognized as the usual antecedent of an Appa- 

 lachian thrust-fault. 



Development of Thrust faults. — There are three mechanical 

 conditions by either one of which a thrust-fault may develop 

 from a stepfold. The pressure tending to exaggerate the fold 

 will be most efficiently transmitted by the most massive stra- 

 tum, in the Appalachians by the great Silurian limestone, and 

 any condition which weakens this stratum may lead to a fault. 



The first condition is erosion. The development of folds is 

 probably gradual, and although the original position may be 

 submarine the rise of an anticline will eventually lift its crest 



' : - The Overthrust Faults of the Southern Appalachians, C. W. Hayes; Bull. 

 Geol. Soc. Am., vol. ii, pp. 141-152. 



