MARYLAND GEOLOGICAL SURVEY 111 



East of the Frederick Valley on the Piedmont Plateau, narrow, elon- 

 gated, infolded areas of limestone occur, only one of which is shown on 

 the map because of its evident relationship to the limestone of the valley 

 proper. Metamorphism has destroyed the evidence as to the age rela- 

 tions of these limestones, although it is possible that they represent an 

 eastern extension of the f ossilif erous Beekmantown strata .of the Frederick 

 Valley. 



FREDERICK VALLEY LIMESTONES 



The considerable development of Early Paleozoic limestones east of 

 Catoctin Mountain in the Frederick Valley and their economic value has 

 long been known to geologists. As these deposits occur east of the Blue 

 Ridge and are the easternmost unmetamorphosed Paleozoic strata known, 

 the determination of their exact age relations is a matter of importance 

 and interest. The Frederick Valley is bounded on the west by Catoctin 

 Mountain, composed of Lower Cambrian sandstone and shale, and on the 

 east by a range of low hills formed by pre-Cambrian schist. The length 

 of the limestone area is about 30 miles and its maximum breadth about 

 six miles. North of LeGore, Maryland, the limestones pass under cover 

 of the Newark red beds, while at the Potomac they cross into Virginia 

 where they again soon disappear under the red beds. 



This area has been studied by several geologists, but the most important 

 work upon it was that of Keyes, who published his results in 1890 in the 

 Johns Hopkins University Circular. His description of these rocks is 

 as follows : " The beds have a mean dip of about 25 degrees to the east- 

 ward. Along their western border they are covered by Triassic red sand- 

 stones (Newark formation). To the east the limestones pass gradually 

 into shales and slates, the whole forming apparently a conformable series. 

 The limestones in great part are bluish in color, compact and heavily 

 bedded ; but on approaching the shales they become more and more thinly 

 bedded and very dark blue or nearly black, owing to the bituminous 

 matter present. The latter, however, is driven off by burning, leaving a 

 pure white lime. In places this lime rock is highly siliceous on account 

 of the presence of a considerable amount of rather coarse quartz sand. 



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