58 THE CAMBRIAN AND ORDOVICIAN DEPOSITS OF MARYLAND 



The typical outcrop of the . Harpers shale extends northward from 

 Harper's Ferry into Maryland for several miles in a belt a mile or less 

 in width, following the western slope of Elk Ridge until it is terminated 

 by a fault against the Tomstown limestone, a mile south of Keedysville. 

 A second belt of outcrop is one-half mile wide and follows the western 

 slope of South Mountain across the state. The last and easternmost belt 

 occurs on the eastern side of Catoctin Mountain. 



The decay of the Harpers shale gives rise to soils of moderate value 

 when its areas of outcrop are not too deeply covered with sandstone debris 

 from the adjacent mountain sandstone ridges. As an example of the 

 latter feature, the entire area of outcrop of this shale west of South 

 Mountain in Maryland is covered with a thick deposit of such sandstone 

 boulders. So far as known the only clean exposures of the shale itself 

 are in cuts of the Western Maryland Railway in its ascent of South 

 Mountain to Pen Mar, and in certain road and stream cuttings. 



With the exception of casts of the worm burrow Scolithus lineans no 

 fossils have been found in the Harpers shale. Its age, however, is un- 

 doubtedly Lower Cambrian because it forms a part of the same series of 

 siliceous sediments as the overlying Antietam sandstone which contains 

 typical Lower Cambrian fossils. 



THE ANTIETAM SANDSTONE 



The Harpers shale forming the western slope and foot hills of South 

 Mountain is found to contain an infolded sandstone formation wherever 

 a conspicuous elevation is developed in front of the main ridge. Such 

 front ridges of South Mountain owe their origin to coarse grained white 

 to bluish-gray quartzite and sandstone about 500 feet in thickness, which 

 weathers readily to a white sand. This is the Antietam sandstone, so 

 named from the good exposures on the tributaries of Antietam Creek 

 east of Sharpsburg, Maryland. This sandstone is .the uppermost of the 

 mountain-making formations of the Blue Ridge province and is the last 

 of the siliceous deposits of Lower Cambrian age. It is composed of small 

 grains of white quartz, vorn and assorted, cemented together by a small 



