• SEDIMENTARY ROCKS OF SOUTH MOUNTAIN 213 



Northward a minor sandstone ridge forks at an acute angle from 

 Montalto Mountain in strike with the soft calcareous sandstones 

 at the foot of the ridge opposite Montalto. This seems to indicate 

 that the Antietam sandstone is variable in thickness and hardness, 

 as previously stated; but its failure to make a separate ridge at this 

 point may in part be due to shearing along this highly compressed 

 zone. The increased width of the shale valley to the north is due 

 largely to flatter dips. The occurrence of brown iron ore in the 

 residual clays along the mountain front, once so extensively mined 

 in the vicinity of Montalto, has been considered by some as evi- 

 dence of the existence of a great fault here. Similar ore deposits 

 occur all along the mountain front, however, even around the ends 

 of folds where faulting cannot have occurred, as will be demon- 

 strated below, and are evidently produced by the leaching of iron 

 from the ferruginous shales and sandstone of the mountain rocks 

 and its precipitation, either by humic acid in bogs along the foot of 

 the mountain, or by chemical action with the limestone with which • 

 the deposits are usually associated. 



The Waynesboro fold. — Northeast of Waynesboro, where the 

 mountain bends sharply to the east, the low hills of the valley are 

 seen to curve first to the east and then to the south, roughly parallel 

 to the offset in the mountain. The rocks of both the mountain 

 and the valley ridges are found to trend and curve in the same 

 direction as the ridges; i. e., the southwest strikes change abruptly 

 to east, and even northeast, and then to the south again. The dips 

 in this curved portion change from 60° W. to 20° S. Beyond, the 

 ridges and the rocks resume their straight southward course for 

 a short distance. It is thus seen that the offset in the mountains 

 at this point is due to a sharp anticline and flat syncline plunging 

 steeply to the southwest, the axis of the syncline passing through 

 Sandy Ridge and Snowy Mountain. The parallelism and low 

 accordant dip of the hmestones of the valley and the mountain rocks 

 throughout this fold, and the absence of any stratigraphic break, 

 are conclusive evidence that no fault exists along the front of the 

 mountain at this point. 



The East Branch fault. — The East Branch of Little Antietam 

 Creek flows in a limestone valley extending into the mountains 



