100 YEARS OF WISSAHICKON — WHAT'S IN A NAME?

The Wissahickon Formation has been, and continues to be, one of the more controversial units of the Central Appalachian Piedmont. When Florence Bascom compared the Cecil County, Maryland mica-schists to the Wissahickon Creek mica-gneisses in 1902, she extended the Wissahickon from the Philadelphia area southwest into the Maryland Piedmont. She then correlated the Wissahickon mica-schist and gneiss with the Berkshire schist of New England and the Hudson schist of New York, while Maryland researchers first used the name Wissahickon Formation and extended the unit southwest through Maryland and into Virginia. With this extension, the Wissahickon became a massive formation traversing three states and correlated with other metasedimentary rocks in New York and New England. In 1932, Bascom mapped the Wissahickon Formation into Delaware. Since that time, researchers have excised parts of the Wissahickon Formation throughout the Central Appalachian Piedmont. In Maryland and Pennsylvania, the Octoraro Schist and Peters Creek Schist were created from parts of the Wissahickon. The Wissahickon was raised to the “group” level in Maryland, assigning the mica schists and gneisses to the Loch Raven Schist and Oella Formation. The name Wissahickon was abandoned in Virginia by Pavlides. This left an isolated mass of mica schists and gneisses mapped as Wissahickon in northeast Maryland, Delaware and southeastern Pennsylvania. This package of metasediments, which is physically separated by the Wilmington Complex and the Rosemont Fault from the type locality in Philadelphia, is referred to as “Glenarm Wissahickon” for its relationship to the Glenarm Group (Cockeysville Marble and Setters Formation). Newer work has provided further support for isolating this metasedimentary package and renaming it for clarity. Current endeavors include radiogenic ages from detrital zircons, geochemistry and monazite ages for a direct comparison between the Wissahickon sensu stricto in the Philadelphia area and the “Glenarm” Wissahickon to bring closure to 100 years of wandering Wissahickon creating stability for future geologic work in the Central Appalachian Piedmont.