24 THE CAMBRIAN AND ORDOVICIAN DEPOSITS OF MARYLAND 



Eidges on the west and the Great Valley on the east, the latter known as 

 the Hagerstown Valley in Maryland, the Cumberland Valley in Pennsyl- 

 vania, and the Shenandoah Valley in Virginia. The Great Valley is a 

 broad lowland with an elevation averaging between 500 and 600 feet and 

 gradually increasing in height to the northward. It extends from New 

 York state to Alabama and in Maryland lies between North Mountain 

 on the west and the Blue Eidge on the east. The Blue Eidge district con- 

 sists of the Blue Eidge and Catoctin mountains which unite immediately 

 north of the Maryland-Pennsylvania boundary to form the greater high- 

 land known as South Mountain. 



The Piedmont Plateau borders the Blue Eidge district on the east and 

 comprises the hill country of ancient rocks lying between the Blue Eidge 

 on the west and the Coastal Plain district on the east the latter district 

 sloping gradually to the southeast and becoming submerged beneath the 

 Atlantic. The Piedmont Plateau is divided into an Eastern Division and 

 a Western Division by the upland known as Parr's Eidge which forms the 

 low divide at an average elevation of between 800 and 900 feet of the 

 streams flowing directly into Chesapeake Bay and those flowing into 

 Potomac Eiver. The Western Division in Maryland corresponds rather 

 closely to what is known as the Frederick Valley. 



THE GEOLOGY 



The Cambrian and Ordovician formations in Maryland are confined 

 to the eastern division of the Appalachian Eegion, previously described 

 as the Great Valley and Blue Eidge, and to the western division of the 

 Piedmont Plateau Eegion. 



The Cambrian formations consist of shales, limestones, and sandstones 

 of sedimentary origin which have been subjected to much metamorphism 

 and marked structural disturbances since they were deposited. They 

 cover considerable areas in Washington and Frederick counties. The 

 Ordovician formations are found in association with the Cambrian in the 

 Great Valley and Blue Eidge regions and also in the Frederick Valley. 

 The Ordovician sediments have been much folded and faulted, but they 

 are, on the whole, less metamorphosed than those of Cambrian age. 



