CAMBRIAN AND LOWER ORDOVICIAN. 



109 



North of Little Antietam Creek there are two ridge-making sandstones above the Weverton 

 sandstone, one composing Sandy Ridge and the other Curve Mountain, and between them is 

 black-banded slate with thin ferruginous sandstones. The upper bed forming Curve Mountain 

 is undoubtedly the Antietam sandstone, and it is apparent that one of the sandstone beds in the 

 Harpers shale of Maryland increases in prominence northward, so that in Pennsylvania it 

 reaches such dimensions that it forms a distinct ridge. The Harpers formation in this area 

 therefore consists of shales and soft sandstones, with a quartzitic member near the middle 

 which is here named the Montalto quartzite, from Montalto Mountain. Northward the shale 

 gradually thins, and the sandstone continues to expand until at the northern border of the area 

 it occupies almost the whole interval of the formation, indicating a gradual change from a fine 

 mud deposit in the south to coarser siliceous sediments in the north. Similar conditions may 

 have continued into Antietam time and have affected the deposition of the Antietam sandstone. 

 The patchy occurrence of the Antietam in the Maryland area, as mapped and described by 

 Keith, may be due to its irregular deposition in that area, instead of to infolding in the Harpers 

 shale and to faulting, as previously supposed. Irregularity of Antietam sedimentation in the 

 Pennsylvania area also is indicated by the absence of ridge-making character east and southeast 

 of Montalto, where the bed is thin, disintegrated, and inconspicuous. 



In the column3,r section of the Mercersburg-Chambersburg folio these older 

 strata are given the following thicknesses by Stose : *°^ 



Shenandoah limestone. Feet. 



Antietam sandstone , 500-800 



Harpers schist and Montalto quartzite member (20-850) 2, 750 



Weverton sandstone 1, 250 



Old volcanics. 



In an article on the Cambrian and Ordovician limestones of the Appalachian 

 Valley, Stose *"'* says : 



The limestones of the Appalachian Valley, which in the South are separated into many 

 formations, have generally been treated as a unit in the North under the name Shenandoah, or 

 other local terms, such as Valley, Lancaster, Kittatirmy, and York. These rocks include all 

 the strata between the Cambrian quartzites of Georgian age and the Martinsburg ("Hudson") 

 shale of Ordovician age. 



In a paper on the sedimentary rocks of South Mountain the author briefly described the 

 formations comprising the Shenandoah group in southern Pennsylvania. Later studies of 

 these rocks in the Cumberland Valley of Pennsylvania have furnished data for a more complete 

 description of the group, including the faunal content and correlation, based on determinations 

 by E. 0. Ulrich. 



The formations comprising the Shenandoah group in southern Pennsylvania are as follows : 



Martinsburg formation . 



ft 



-a 

 o 

 •a 







ca 

 

 a 

 -d 

 OQ 



Chambersburg limestone, 100-600 feet. 



fEden. 



Utica. 



Upper Trenton. 



Lower Trenton. 



Black River. 



LowvUle. 



Upper Chazy. 



Stones River limestone, 800-1,000 feet Lower and mid- 

 dle Chazy. 

 Beekmantown limestone, 2,250-2,300 feet [includmg Stonehenge 



limestone member at the base, 500 feet] Beekmantown, 



Conococheague limestone, 1,635± feet Saratogan. 



Elbrook formation, 3,000± feet 



Wajmesboro formation, 1,250± feet 



Tomstown limestone, 1,000± feet 1 ^ 



I Georgia 

 Antietam sandstone J 



' Ordovician . 



' [Acadian. 



Cambrian. 



