130 THE CAMBRIAN AND ORDOVICIAN DEPOSITS OF MARYLAND 



In Maryland, as in southern Pennsylvania, the fossiliferous, thin- 

 bedded limestone with argillaceous partings, formerly regarded as the 

 uppermost division of the Shenandoah group, is now called the Chambers- 

 burg limestone, so named by Stose from outcrops in the vicinity of 

 Chambersburg, Pennsylvania. This limestone is subject to great varia- 

 tions not only in thickness, but in the character of the rocks from place to 

 place along the strike and across it. The rather broad expanse in southern 

 Pennsylvania exposing Cambrian and Ordovician strata exhibits many 

 outcrops of Chambersburg limestone. This is particularly true in the 

 Chambersburg and Mercersburg quadrangles in Pennsylvania where good 

 collections of fossils have been made and numerous sections of the strata 

 were studied. This broad area of outcrop is divided by a wide synclinal 

 belt of Martinsburg shale that continues southward through Maryland 

 into Virginia and lies in a continuation of the Massanutten syncline. 

 As the towns of Chambersburg and Mercersburg are located respectively 

 in the eastern and western belts, it has been found convenient to term 

 these two belts of outcrop the Chambersburg and Mercersburg troughs. 

 The very different composition and thickness of this formation in the 

 two troughs has been used by Ulrich in his Eevision of the Paleozoic 

 Systems as a striking illustration of the instability of the continental 

 floor during Middle Ordovician time. A resume of these differences is 

 presented under the discussion of the sections. 



LITHOLOGIC CHARACTER. The Chambersburg limestone as a whole 

 is characterized by thin-bedded, fossiliferous, dark -blue, argillaceous 

 limestone with clayey partings. Many of its layers upon weathering have 

 a tendency to break up into rounded cobblestone-like fragments. This 

 " cobbly " nature of the weathered outcrop is so noticeable and character- 

 istic of the formation that it may be safely employed in the discrimination 

 of this limestone. In natural or artificial cuts exposing the weathered 

 and unweathered zones of the limestone a great abundance of cobblestones 

 at the surface may always be noted. Eoads which happen to pass along the 

 strike of the cobbly beds where these are highly tilted often clearly show 

 the arrangement of the cobblestones in definite thin-bedded layers. 



