MARYLAND GEOLOGICAL SURVEY 131 



The cobbly effect is caused by the clay partings in the limestone cross- 

 ing it at a high angle so that weathering breaks up the solid limestone 

 into rounded fragments. Also concentration of the lime in weathering 

 and the relative facility of its loss by the more clayey parts has much to 

 do with the formation of the cobbles, as suggested by Ulrich. Fresh 

 exposures, as in quarries, indicate these partings very obscurely. The 

 limestone then appears massive and solid and exhibits regular bedding 

 planes five or six inches apart. The change from the fresh, massive 

 limestone to the cobblestones is well shown in railroad cuttings where the 

 surface outcrop capping the fresh cutting is preserved. 



fWsburg Mason-.Dlxon Grn<jt-l Ch>ersbur$ 



FlG. 17. DIAGRAMMATIC SECTION OF CHAMBEBSBURG LIMESTONE FROM CHAMBERS- 

 BURG, PA., TO STRASBTJRG, W. VA. 



Although dark blue nodular limestone weathering cobbly constitutes 

 the greater or at least the most conspicuous part of the Chambersburg 

 formation in Maryland, other types of rock are widely developed and 

 often exposed in the continuation of the outcrop belts to the north and 

 south. These strata consist of interbedded thin calcareous shales and 

 shaly limestones, thin-bedded, dark gray limestone, and bluish grano- 

 crystalline limestone. All these strata form a composite unit that is 

 decidedly different from any of the underlying limestone formations. 



In Maryland the thickness of the Chambersburg limestone does not 

 exceed 300 feet; in southern Pennsylvania it reaches a maximum of 800 

 feet, although at the type exposure around Chambersburg, Pennsylvania, 

 it is about 500 feet in thickness. This great variation is expressed in the 



