CAMBRO-ORDOVICIAN LIMESTONES 699 



brian quartzites of South Mountain on the east, and the youngest 

 rocks passing beneath the Ordovician shales and sandstones of North 

 Mountain and its associated ridges on the west. The monocline is 

 not simple, however, but is modified by numerous folds and faults 

 that either repeat the strata or conceal them. 



The following detailed descriptions relate chiefly to the portion of 

 the Valley in the Mercersburg and Chambersburg 1 5-minute quad- 

 rangles, which lie just north of the Maryland state hne, but brief 

 references are made to the Carlisle quadrangle at the north end of 

 South Mountain. 



TOMSTOWN LIMESTONE 



The Tomstown limestone is not well exposed because of its near- 

 ness to South Mountain, where the surface is thickly covered by wash. 

 It is composed largely of limestone, both massive and thin bedded, 

 in part cherty, with some shale interbedded near the base. On 

 account of its relative solubility it forms a depression or valley between 

 the mountain and the irregular line of low ridges and knobs of the 

 Waynesboro formation. Its thickness, computed from the width of 

 its outcrop and the dip of its beds, is about 1,000 feet. 



The base of the formation is largely concealed but comprises some 

 hydromica shale interbedded with the limestones resting on the 

 uppermost beds of Antietam sandstone.. The top of the formation is 

 placed at the first sandstones of the Waynesboro formation. 



In the eastern or Chambersburg quadrangle the Tomstown lime- 

 stone forms a belt about one mile wide along the foot of South Moun- 

 tain, spreading out to nearly double that width in places. At Little 

 Antietam Creek it is offset by a diagonal fault and extends up the 

 Antietam Valley into the mountains several miles. 



But few fossils have been found in this formation. At Roadside, 

 in the upper limestones of the formation, excellent specimens of 

 Salter ella sp. undet., with the characteristic invaginate structure, were 

 obtained. Mr. Walcott also found in this limestone, at the foot of the 

 mountain east of Little Antietam Creek, Kutorginia n. sp. and frag- 

 ments of Olenellus. These definitely determine its age as Georgian 

 (Lower Cambrian). 



In central Virginia a formation 1,600 to 1,800 feet thick, occupying 



