2o8 GEORGE W. STOSE 



deposition in that area, instead of to infolding in the Harpers shale 

 and to faulting, as previously supposed. Irregularity of Antietam 

 sedimentation in the Pennsylvania area also is indicated by the 

 absence of ridge-making character east and southeast of Montalto, 

 where the bed is thin, disintegrated, and inconspicuous. 



Shenandoah limestone. — The rocks of the Valley are chiefly light- 

 and dark-gray, massively bedded, magnesian limestones, described 

 and mapped in the northern Appalachian folios of the United States 

 Geological Survey atlas as the Shenandoah limestone. They are so 

 intricately folded, and have so few easily recognized horizons, 

 that the details of the stratigraphy cannot be determined nor the 

 thicknesses accurately measured. The series is estimated to be about 

 6,800 feet thick, and is here divided into six formations. The lowest, 

 composed largely of drab to white, impure limestones, has near its 

 base beds of purer, mottled, dark- and light-gray limestone, which is 

 frequently quarried and burned for hme, and near its top a massive 

 bed of cherty limestone. The formation is limited above by ferru- 

 ginous sandstone and purple shale. It is approximately 800 feet 

 thick. The name "Tomstown Umestone," here applied to it, is from 

 a village at the foot of South Mountain, where the formation out- 

 crops. 



The next succeeding formation is composed largely of hard, sili- 

 ceous, ripple-marked, purple shale and flaggy, calcareous sandstones, 

 about 600 feet thick. At the base is a siliceous rock weathering into 

 large slabs and masses, stained yellow by iron, and usually banded 

 and contorted. These masses, together with fragments of vein quartz 

 and white porous chert, strew the surface and in places produce 

 a low ridge. Scattered deposits of limonite occur on the slopes of 

 these ridges, produced by the leaching of iron from the ferruginous 

 shale and its precipitation in the soil and wash at the surface. Flaggy 

 sandstone and sandy shale, forming the top of the formation, make 

 a rather continuous low ridge in the limestone lowland, thus afford- 

 ing a marker in the otherwise monotonous series of beds. Shattered 

 portions of this sandstone are veined with barite, and in the soils 

 at the base of the hillslopes the weathered product has in places 

 been concentrated and mined on a small scale. At Waynesboro, 

 in the upper sandy shales and in the immediately overlying lime- 



