214 GEORGE W. STOSE 



several miles. The limestones strike northeast, approximately 

 parallel with the ridges on the south side, but diagonally across 

 the trend of the sandstone ridges on the north, which terminate 

 abruptly. This condition is the result of a fault cutting off the 

 quartzite beds along the north side of the re-entrant valley and off- 

 setting them several miles to the east. The fault apparently marks 

 the axis of a fold, similar to that just described east of Waynesboro, 

 in which a sharp anticline, followed to the southeast by a deep syn- 

 cline, was compressed so closely that the anticline was entirely 

 sheared out, and the limestone in the syncline now rests against 

 the edges of the broken quartzite beds. 



Big Flat anticline. — Big Flat Ridge, north of the Conococheague, 

 is a broad, flat-topped anticline which to the north, beyond 

 the quadrangle, forms the major part of the mountain range. A 

 line of small knolls, composed of the Scolithus-he2inng Antietam 

 sandstone and dipping from 50° to 70° to the northwest, skirts the 

 western foot of the mountain. An inner row of higher knobs is 

 composed largely of vitreous non- Scolithus rock, the Montalto 

 quartzite member. The upper shale of the Harpers formation is 

 thin in this ridge, and the sandstone member more prominent. 

 Apparently the shale is gradually replaced to the north by sand- 

 stone, which becomes the major part of the formation. The 

 counterpart of the line of knobs is found on the eastern side of the 

 ridge, composed of similar rocks and with dips of 45° to the 

 southeast. At the south end of the mountain the same rocks bend 

 around parallel to the southern face and dip 20° to the southwest, 

 abruptly ending the anticline. On the crest of the ridge the 

 beds, where exposed, indicate gentle folds, and over the level surface 

 of the mountain top the rock fragments are chiefly Scolithus sand- 

 stone, apparently of the Montalto horizon. The deep ravines 

 intersecting the anticline may cut through the Montalto quartzite 

 member into the lower shale of the Harpers, but in no place was 

 the underlying Weverton sandstone observed. 



Beyond the end of this plunging anticline the valley rocks for 

 several miles are hidden by wash, but from exposures farther to 

 the southwest they are seen to be intricately folded. Whereas the 

 hard massive sandstone beds of the mountain form one bold broad 



