72 THE CAMBRIAN AXD ORDOVICIAX DEPOSITS OF MARYLAND 



THE ELBROOK FORMATION 



Overlying the purple shales of the Waynesboro formation in the normal 

 section is a thick series of light-blue and gray shaly limestone and 

 calcareous shales which, in Maryland, are seldom exposed in natural 

 outcrops. These strata were not recognized as a distinct formation until 

 1910, when Stose 1 named them the Elbrook formation from the village 

 on the Western Maryland Railway in southern Pennsylvania. 



LITHOLOGIC CHARACTER AND THICKNESS. The shaly limestone and 

 calcareous shale making up the major portion of the Elbrook formation 

 weathers very rapidly into shale fragments, so that usually there are few 

 natural outcrops. In stream valleys and artificial exposures the following 

 general succession has been determined. At the very base of the formation 

 are beds of rather pure dark blue massive limestones not over 100 feet 

 thick which have afforded the only fossils found. Succeeding this and 

 constituting approximately the lower third of the formation is 1000 or 

 more feet of minutely laminated shaly limestone and calcareous yellow 

 to green and some reddish shale which weathers into calcareous shaly 

 plates. The middle of the formation is marked by siliceous limestones 

 and massive beds of dolomite which form a slight elevation in the gen- 

 erally low area of outcrop of the formation. The upper half of the 

 formation is composed of light colored calcareous shale and impure 

 laminated limestones which, like the lower part, weather shaly. How- 

 ever, it is slightly more siliceous than the lower third and weathers into 

 more irregular often cubical sandy red to brownish fragments. It is 

 followed by the limestone conglomerates and sandy oolite marking the 

 base of the succeeding Conococheague limestone. The total thickness of 

 the Elbrook as determined in both northern and southern Maryland is 

 about 3000 feet. 



AREAL DISTRIBUTION. Notwithstanding its great thickness the 

 Elbrook formation occupies less area in Maryland than almost any other 

 of the Cambrian or Ordovician formations. It appears at the surface in a 

 narrow northeast-southwest band crossing the state in the .eastern part 



1 Folio 170, U. S. Geol. Surv. 



