MARYLAND GEOLOGICAL SURVEY 63 



uppermost beds of the formation, on the contrary, are very commonly 

 exposed in Maryland because there are so many small areas of the over- 

 lying Waynesboro formation yet remaining in the valley to mark the top 

 of the Tomstown. These upper strata, while still retaining pinkish to 

 pearl-colored marble beds, also comprise massive dark blue magnesian 

 limestones. Most of the limestones of the Tomstown, and especially the 

 marbles, exhibit some lamination with the result that upon weathering the 

 rock is easily split into thin slate-like fragments. This lamination is 

 usually quite regular, but in southern Maryland there is a body of Toms- 

 town limestone where the rock is so irregularly laminated that it weathers 

 into a mottled effect. Shearing of this laminated limestone is frequent, 

 especially in the marbles. Such strata give the characteristic fracture due 

 to the combination of lamination and shearing. 



The shape of the characteristic shale fragments resulting from the 

 weathering of this limestone is due to this same combination of lamination 

 and shearing, so that, while many of the pieces are broken, others are 

 undulated or twisted. The shearing planes are marked on the residual 

 shale-like fragments by thin films of silky, sericite-like material. Some- 

 what similar shale fragments result from the weathering of the Elbrook 

 formation. When the Tomstown and Elbrook limestones are brought in 

 contact by f aulting-out of the intervening Waynesboro formation, careful 

 discrimination is necessary to identify the formations correctly. In 

 doubtful cases it is necessary to search for an outcrop of the rock furnish- 

 ing the shale residue. When this has been found it should be easy to 

 distinguish the sheared marbles of the Tomstown from the dull laminated 

 clayey limestone of the Elbrook. 



RESIDUAL PRODUCTS. One of the characteristic residual products of 

 the Tomstown is black banded chert in small blocky pieces left in the soil 

 upon the weathering of the upper beds of the formation. This chert is 

 almost chalcedonic in nature and the black bands passing through it give 

 it somewhat the aspect of agate. Chert of this particular nature is not 

 found again until the middle division of the Stones Eiver limestone, and 

 as there is little danger of confusing these two widely separated forma- 



