MARYLAND GEOLOGICAL SURVEY 00 



original structural planes. They have been considered pre-Cambrian 

 (Algonkian and Archaean), with some infolded Cambrian-Silurian 

 quartzites, marbles and phyllites. 



It is of this eastern division of the Piedmont belt of Maryland 

 that the crystallines of Cecil county are a part. Though less than 

 one-sixth of the Maryland extension of the belt, the Cecil county 

 crystallines include a representative series of the formations of the 

 belt. 



These holocrystalline metamorphic rocks enter Maryland from the 

 southeastern corner of Pennsylvania and cross Cecil county from 

 northeast to southwest, where they pass into Harford county. Their 

 southeastern border is buried beneath the unconsolidated materials 

 of the Coastal Plain. 



Like all of eastern Maryland, Cecil county shows a striking topo- 

 graphic division into a southeastern lowly ing coastal plain district 

 and a northwestern rugged plateau region. The unconsolidated for- 

 mations: clay, sand and gravel characterize the former district, while 

 the hard rocks are confined to the plateau. Thus the southern 

 boundary of the crystalline formations of Cecil county is practically 

 coincident with the chief physiographic division of the county. 

 The Baltimore and Ohio Railroad skirts the southeastern limits of 

 the plateau and of the crystallines. The state limits toward Penn- 

 sylvania and Delaware, and the Susquehanna river furnish, the north- 

 ern, eastern and southwestern boundaries, respectively, of the trape- 

 zium which encloses at once the plateau region and the Piedmont 

 belt of Cecil county. 



The marked physiographic features of the county are in close 

 correspondence with the geologic features, and the geologic and 

 physiographic districts are coincident. 



The plateau slopes gently to the southeast. Its highest elevation, 

 in the neighborhood of Rock Springs, is about 550 feet above tide. 

 The western portion of the plateau is drained and deeply carved by 

 the Susquehanna river and two chief tributaries, the Octoraro and 

 Conowingo creeks. Basin Run, Rock Run and Happy Valley Branch 

 are the lesser tributaries. The Susquehanna, flowing transverse to 



