57 



are now spread out in all the lowland areas of the Hagerstown Valley in a 

 strip one to two miles wide paralleling the mountain. 



Fossils have not been found in this sandstone, but as it is a part of the 

 siliceous series terminated by the Antietam sandstone, which contains 

 a Lower Cambrian fauna, the age of the Weverton sandstone also is very 

 probably Lower Cambrian. 



THE HARPEKS SHALE 



The bluish gray slate or schist exposed so prominently in the vicinity 

 of Harper's Ferry, West Virginia, in the gorges of the Potomac and 

 Shenandoah rivers and known as the Harpers shale, follows the Weverton 

 sandstone in the geological column, although its outcrops are almost 

 everywhere included between faults. In southern Maryland the Harpers 

 shale is composed almost entirely of sandy slates with a few sandstone 

 layers developed in its upper portion. These shales are of a dull bluish- 

 gray color when freshly exposed, but they weather to a light greenish- 

 gray. Northward in Maryland the sandstone layers increase in thickness 

 until, in the region of Pen Mar, and especially at Montalto Mountain in 

 southern Pennsylvania, a massive quartzite 750 feet in thickness is 

 developed in the middle portion of the schist. This is the Montalto 

 quartzite member mapped by Stose in the Chambersburg (Pennsylvania) 

 quadrangle, but it is hardly of sufficient importance in Maryland to be 

 distinguished as a separate unit. This Montalto member is only 20 feet 

 thick just north of the Maryland line, but it thickens to 850 feet going 

 northward a distance of 20 miles in Pennsylvania. 



As no complete section of the Harpers shale is exposed in Maryland 

 or even in its other areas of outcrop, its thickness is difficult to determine. 

 Moreover one or often both sides of its areas of outcrop are cut off from 

 adjoining formations by faults. At Harper's Ferry, the type area of 

 outcrop, the thickness has been estimated by Keith as 1200 feet. . In 

 southern Pennsylvania northeast of Waynesboro the thickness is increased 

 to 2750 feet, due in part to the development of the Montalto quartzite 

 member. 



