366 E. B. MATHEWS AND W. J. MILLER — COUKEYSVILLE MAKBLE 



eastern termination of the western anticline as a fault complementary to 

 the fault recognized by him and the authors on the eastern side of the 

 valley. It was supposed by Professor Williams that this valley repre- 

 sented a faulted block of limestone. That such is not the case is well 

 shown by observations recorded in the accompanying map, where the 

 strikes and dips may be traced with constantly changing azimuth about 

 the nose of the anticline from Rogers station to the southwestern corner 

 of the sketch map. The structural details in the center of the valley in 

 the limestone indicating a southward plunging syncline also corroborate 

 the later interpretation. The original view evidently arose from the fail- 

 ure to recognize the difference between the overlying Wissahickon and 

 the underlying Baltimore gneiss. 



Although no exposures are found along the actual fault line between 

 Sherwood and Brightside, the sharp divergence in strike and dip as well 

 as the difference in character of the rock leaves no doubt as to the occur- 

 rence of a fault at this point. The ends of the various beds of the Balti- 

 more gneiss strike northwesterly against the limestone and Wissahickon, 

 which have a more southerly trend and a dip to the westward. 



The westward thrust of the eastern anticline widens very perceptibly 

 (see general map, plate 65), the distance between the quartzite areas 

 representing the limbs of the anticline. They are fully twice as far apart 

 as in the corresponding anticlinal dome on the western side of the val- 

 ley. If the foregoing parts of what appears to be a single fault zone be 

 regarded as separate faults, they may be characterized as follows : The 

 northernmost fault differs from each of the others in some particulars 

 and in other particulars is like them. Like the Warren fault, it occurs 

 parallel to the strike of the major anticlinal dome, but, unlike both the 

 Warren and Ruxton faults, the thrust is toward the southeast. This dif- 

 ference in the direction of the thrust within the distance of a few miles 

 would be a serious matter to explain without a knowledge of the conti- 

 nental structure to the westward. With these facts at hand the relation- 

 ships become clearer. All three of the faults show the older, more crystal- 

 line, and more competent Baltimore gneiss thrust over the younger and 

 more yielding marbles and Wissahickon schist. In the faulting the 

 quartzite is associated with the underlying gneiss, with which it is litho- 

 logically very similar. The difference in thrust at the different points 

 may be produced by the slightly divergent lines of force which have pro- 

 duced compression and local glancing blows resulting in a small amount 

 of contortion. Thus on the north the Baltimore gneiss is shoved south- 

 ward and on the south it is shoved northward, while in between is a less 

 marked faulting, which partakes of the westerly thrust from the south, 

 but is here not as strongly marked. 



