160 THE CAMBRIAN AND ORDOVICIAN DEPOSITS OF MARYLAND 



by weathering. In fresh exposures these beds are hard and sometimes 

 reach several feet in thickness, but weathering brings out the intense 

 squeezing and folding to which they have been subjected. Slaty cleavage 

 usually obscures the original bedding, but occasionally a weathered sur- 

 face clearly shows the relation between cleavage and stratification. The 

 rock breaks down into small fragments not unlike shoe pegs and finally 

 weathers into soft whitish clay. With the introduction of sandy sedi- 

 ments in the Martinsburg shale, fossils again are encountered and give a 

 clew as to the age relations. At least three distinct fossiliferous zones 

 .have been discovered in these upper sandy shales. Two of these contain 

 numerous species characterizing the Eden division of the Cincinnatian, 

 whereas the third zone shows fossils of Lower Maysville age. As noted 

 before, these fossiliferous zones are encountered only in areas west of the 

 Massanutten syncline as the conditions for the preservation of organic 

 remains were not favorable during the deposition of the sandy strata in the 

 syncline itself. 



Because of the lack of good exposures of the upper Martinsburg in the 

 mountainous areas of western Maryland, it has not been possible to pre- 

 pare detailed sections showing the position of the fossiliferous beds 

 accurately. In southern Pennsylvania, however, especially in the vicinity 

 of McConnellsburg and Fort Loudon, there are several extensive exposures 

 of these beds. In the Eden portion of the Martinsburg shale 42 species 

 have been recognized. 



The interval between the top of the formation and the highest zone 

 in which Eden fossils were found is occupied by gray sandstone about 

 450 feet thick. This is locally divisible into two unequal parts, the lower 

 300 feet thick, the upper 150 feet, by a fossiliferous stratum in which the 

 brachiopod Ortliorhyncliula linneyi is a common fossil. Elsewhere in the 

 Appalachian Valley and in the Ohio Valley this Orthorhynchula bed lies 

 near the top of the Lower Maysville. This fossil zone has yielded a fauna 

 of 18 species in southern Pennsylvania, all of which are characteristic 

 Lower Maysville fossils. The overlying unfossiliferous gray sandstone 



