CAMBRO-ORDOVICIAN LIMESTONES 701 



ELBROOK FORMATION 



The Elbrook formation is the thick series of gray to light-blue 

 shaly limestone and calcareous shale that overlies the purple Waynes- 

 boro formation. 



The formation is decidedly shaly, most of the included limestones 

 being minutely laminated and weather readily into calcareous shaly 

 plates. When unweathered the limestone appears massive and 

 homogeneous and in places is quarried. 



Near the middle of the formation are massive beds of dolomite and 

 very siliceous or quartzitic limestone that weathers to porous slabby 

 sandstone and frequently forms knobs and ridges. The formation is 

 limited above by limestone conglomerates containing rounded vit- 

 reous quartz grains and others containing tabular fragments of lime- 

 stone, which characterize the base of the overlying formation. 



The thickness of the Elbrook, determined on the two limbs of 

 the unsymmetrical syncline west of Quincy, is about 3,000 feet. 



The Elbrook formation crosses the Chambersburg quadrangle in 

 a belt about 2 miles wide, with numerous projections and re-entrants 

 due to intricate folding. It has also been traced across the Carlisle 

 quadrangle where the character of the limestones is the same, but 

 shales increase in prominence. 



The only fossils found in this formation were fragments of trilobites 

 obtained from rather pure limestone near the base northeast of 

 Waynesboro. The age of the rocks could not be determined from 

 these fragments, but they suggest Acadian. 



The limestone was once extensively quarried for ballast for the 

 Western Maryland Railroad at Elbrook, in the Chambersburg quad- 

 rangle, from which the name of the formation is taken. 



CONOCOCHEAGUE LIMESTONE 



The Conococheague limestone is characterized by beds containing 

 thin sandy laminae and quartz grains that weather into hard shale frag- 

 ments and thin slabby sandstones which generally give rise to rocky 

 hills and rugged topography. 



The base of the formation is usually easily determined because it is 

 marked by siliceous beds and conglomerates that produce a ridge. 

 The conglomerates are of two kinds; one is composed of rounded 



