"DC Rocks, So We Need One Act of 2014" Official Rock of DCThe Potomac bluestone was added as the official rock of Washington DC in 2015 in Law 20-220, the "DC Rocks, So We Need One Act of 2014"
Sykesville Formation
Sedimentary and igneous rock layers recrystallized to become the metamorphic rock of the Piedmont. Quarries along Rock Creek, and at Little Falls, Maryland, provided Washington, DC with building stone from the Piedmont’s Sykesville Formation, also known as Potomac Bluestone (Gneiss). Foundations of the White House, Capitol, and Washington Monument, along with buildings like the Lock Keeper’s House and the Old Stone House in Georgetown were built with the Sykesville Formation.