Reports
The geology of Cecil County, Maryland
1990, Higgins, M.W. and Conant, L.B.
Bulletin 37
Abstract
In the eastern Maryland Piedmont, southeast of the extensive terrane of pelitic and psammitic rocks that have traditionally been assigned to the Wissahickon Formation, is a terrane of layered gneisses, massive but fine-grained granofels, and layered, massive, and pillowed amphibolites collectively called the James Run Formation. Interspersed with and locally grading into the James Run Rocks are coarser grained granitic plutons. The plutons and the James Run Rocks are believed to represent the roots, products, and debris of ancient volcanoes. There also is a large outcrop belt of ultramafic and mafic rocks belonging to the Baltimore Complex, called Baltimore Gabbro by earlier workers. The relations among the metasedimentary rocks, the James Run Formation and its associated plutons, and the Baltimore Complex are important to our understanding of the history of the central Appalachians. The James Run metavolcanic rocks and associated plutons are perhaps best exposed in Cecil County, the northeasternmost county in Maryland (fig. 1; pl. 1), where a small part of the sequence has long been called "the volcanic complex of Cecil County." Thick and diverse sections of the metasedimentary rocks also occur, as well as the most intact section known of the Baltimore Complex.
This paper gives descriptions of the rocks exposed in Cecil County, interpretations of the origins of these rocks, and interpretations of the geologic, structural, and tectonic history of the area. The terminology used herein is that of the U.S. Geological Survey and does not always conform to Maryland Geological Survey practice.
