MARYLAND GEOLOGICAL SURVEY 161 



bringing Martinsburg shale deposition to a close is equivalent on strati- 

 graphic grounds to the Oswego sandstone of New York, and like it, is 

 apparently either of continental origin or its formation is connected with 

 some phase of sea withdrawal. It is fairly well agreed upon by geologists 

 that the Juniata shales or Eed Medina immediately following the Oswego 

 sandstone, is a continental expression of the marine Richmond group of 

 the Ohio Valley. Indeed the two formations have actually been traced 

 into each other. The underlying fossiliferous Upper Maysville (Mc- 

 Millan) formation of the Ohio Valley, which occurs between the Richmond 

 above and the Orthorhynchula bed at or near the top of the Lower Maysville 

 (Fairview) below, is thus almost certainly the equivalent of the un- 

 fossiliferous Oswego sandstone which has the same boundary planes. 

 This correlation is further indicated by the fact that in both instances the 

 Lower Maysville strata pass into the Upper Maysville without any clear 

 evidence of a stratigraphic break. 



TOPOGRAPHIC FEATURES AND AREAL DISTRIBUTION. The largest area 

 of Martinsburg shale in Maryland forms a low plateau averaging two 

 and one-half miles in width. This crosses the state in a belt trending 

 southwest-northeast through the central part of the Appalachian Valley. 

 These shales resist weathering much more effectively than the subjacent 

 limestones, the result being the low, yet topographically conspicuous 

 plateau already mentioned. Although it is much dissected by Conoco- 

 cheague Creek and its tributaries, the upland part of this plateau has an 

 altitude of about 580 feet at the Mason-Dixon line, but descends to about 

 540 feet at the Potomac River. These upland areas are remnants of an 

 old peneplain that is still well preserved in the vicinity of Harrisburg, 

 Pennsylvania, and after which it has been named. The excellent and 

 almost continuous exposures of the Massanutten syncline belt of Martins- 

 burg shale along the Western Maryland Railway between Williamsport 

 and Pinesburg Station have been mentioned before. 



In the limestone valley west of the Massanutten syncline a narrow strip 

 of this shale has been brought down to the surface by faulting. The only 



