SEDIMENTARY ROCKS OF SOUTH MOUNTAIN 21 1 



The Knox is limited above by homogeneous, fine-grained, dove- 

 colored, pure limestone, extensively quarried throughout the Valley. 

 It contains a few leperditia, gasteropods, and brachiopods of Stones 

 River age, and since the rock is lithologically the same as the Stones 

 River of Tennessee, and apparently occupies the same interval, the 

 name "Stones River Hmestone" is apphed here. 



Overlying the Stones River are darker and more crystaUine 

 hmestones, somewhat cherty at the base and interbedded in the 

 upper portion with argillaceous Hmestone. Fossils are not numerous 

 in the lower cherty beds, but the upper portion contains a large 

 and interesting fauna, referred by Ulrich to the Chazy and Black 

 River. The formation is well exhibited along the edge of the shale 

 belt west of Chambersburg, and is therefore named the "Cham- 

 bersburg hmestone." The thickness of this and the Stones River 

 formation was not determined by the writer. Sections recently 

 made by Ulrich near Greencastle, 8 miles west of Waynesboro, 

 show 400 -H feet of Stones River limestone and 1,000 -f feet of Cham- 

 bersburg limestone. 



The Martinsburg group. — The calcareous strata are followed 

 by a series of shales and soft sandstones previously called the " Mar- 

 tinsburg shale." At the base are a few feet of dark calcareous 

 shale and thin beds of carbonaceous hmestone, transition beds, con- 

 taining a fauna regarded as Trenton in age. These are followed by 

 dark to gray platy shale, with Leptobulus insignia, Triarthus becki, 

 and other Utica forms, including numerous graptoHtes, and it is there- 

 fore named the "Utica shale. " It is generally intricately folded and 

 crinkled, but the thickness is estimated to be 1,000 feet. Above 

 it are greenish to buff, soft sandstone which is named Eden because 

 it contains a fauna referred by Ulrich to the Eden, and is regarded 

 by him as stratigraphically its equivalent. It has a thickness of 

 about 500 feet. These shales and sandstones form the tablelands 

 rising out of the hmestone lowland west of Chambersburg. 



STRUCTURE 



All the rocks of the region have been folded and highly com- 

 pressed, and nearly all have acquired a secondary structure. In 

 the mountains the rocks have a marked schistosity, most completely 



