MARYLAND GEOLOGICAL SURVEY 129 



crowded with Tetradium syringoporoides, while Leperditia fabulites and 

 several species of undetermined shells occur in great number in other beds* 



THE CHAMBERSBURG LIMESTONE 



Most students of Appalachian stratigraphy have recognized a so-called 

 transition zone of argillaceous limestone between the massive limestones 

 of the Shenadoah group and the overlying great shale formation, the 

 Martinsburg shale. These argillaceous limestones are thin bedded and 

 quite fossiliferous in comparison with the underlying and overlying strata. 

 They were classed as the closing phase of the Shenandoah group and their 

 contained fossils established the Ordovician age of the upper part of that 

 group. As their lithologic character is clearly enough intermediate 

 between that of the underlying pure and magnesian limestones and the 

 typical shale group above, it was natural to believe that they were really 

 transition beds. However, detailed comparison of the faunas of these 

 argillaceous beds in different parts of the Appalachian Valley in recent 

 years has shown that these strata are either not transition beds at all or 

 that the supposed transition occurred at widely different times in dif- 

 ferent parts of the valley. The fossils showed that as a rule shale deposi- 

 tion set in much earlier in the eastern bands of the Appalachian Valley 

 than in the western. Also that these dates varied considerably even in 

 north and south directions. The " transition " beds when closely studied 

 proved to represent formations of totally different ages in various parts 

 of the valley and that stratigraphic breaks, sometimes of considerable 

 extent, commonly separate them from both the underlying and overlying 

 strata. For example, in the eastern bands of the valley of east Tennessee 

 this argillaceous limestone phase of deposition is the Lenoir limestone of 

 middle Chazyan age, while at the western side of the valley the limy beds 

 just under the first shale are late Trenton in age. Again, in southern 

 Pennsylvania the Chambersburg limestone of Black Eiver age directly 

 or immediately precedes the Ordovician shale group in the eastern or 

 Cumberland Valley, whereas in central Pennsylvania the shale is under- 

 lain by uppermost Trenton. The basal part of the shale itself varies 

 correspondingly in age from upper Chazyan to Utica. 



