SEDIMENTARY ROCKS OF SOUTH MOUNTAIN 205 



feldspathic and vitreous sandstones above. The purplish conglom- 

 erate bed is composed of small pebbles and grains of quartz, feldspar, 

 and purplish slate or tuff, the flat slaty fragments often having a 

 diameter of 2 inches. This grades almost imperceptibly through soft 

 purplish arkose into the reddish rhyoHtic eruptives below, demonstrat- 

 ing their derivation largely from similar volcanic rocks exposed along 

 the nearby shore of the Georgian sea. In Maryland, and also at 

 Mount Holly at the north end of South Mountain, basal conglomer- 

 ates contain numerous large quartz pebbles, probably in part derived 

 from the granitic basement complex of the Piedmont. 



This basal sandstone, on account of its hardness, forms high, 

 rugged ridges in the heart of the range, such as Rocky Mountain 

 and Snowy Mountain. It is continuous with the Weverton sand- 

 stone of the Catoctin and South mountains, Maryland, as mapped 

 by Keith^ and the name is therefore used here. The underlying 

 Loudoun formation, which is described by Keith as variable in com- 

 position and thickness in Maryland, was not recognized as a distinct 

 formation in this area, but may be represented in the soft arkose at 

 the very base of the sedimentary series. 



Upper shales and sandstones, — Above the Weverton sandstone 

 there are about 3,200 feet of shale and soft sandstone in which are 

 two horizons of hard, ridge-making sandstone. The softer beds 

 are poorly exposed, being everywhere covered by the debris from 

 the adjacent sandstones. Their presence is inferred from the fact 

 that their outcrop' is always occupied by valleys and depressions. 

 Their character is indicated in part by occasional fragments of thin 

 shaly sandstone and black banded slate or red ferruginous shale. 

 The hard sandstone beds form the ridges along the mountain front 

 and cap the high, flat-topped Sandy Ridge, as well as Big Flat Ridge 

 north of Fayette ville. The lower of the two sandstones is the more 

 massive, and is composed of a hard quartzitic stratum, usually of 

 dark-gray color and veined with quartz, and a softer, granular, white 

 layer containing long, slender Scolithus tubes. The upper hard bed 

 at the top of the shale, is a milk-white or slightly pinkish, granular, 

 calcareous sandstone, frequently disintegrating by the removal 



^ "Geology of the Catoctin Belt," Fourteenth Annual Report of the U. S. Geo- 

 logical Survey, pp. 285-395. 



