Abstract Seneca sandstone is a fine-grained arkosic sandstone of dark-red coloration used primarily during the nineteenth century in Washington, DC. Several inactive Seneca sandstone quarries are located along the Potomac River 34 km NW of Washington near Poolesville, Maryland. Seneca sandstone is from part of the Poolesville Member of the Upper Triassic Manassas Formation, which is in turn a Member of the Newark Supergroup that crops out in eastern North America. Its first major public use is associated with George Washington, the first president of the Potomac Company founded in 1785 to improve the navigability of the Potomac River, with the goal of opening transportation to the west for shipping. The subsequent Chesapeake and Ohio Canal built parallel to the river made major use of Seneca sandstone in its construction and then facilitated the stone's transport to the capital for the construction industry. The most significant building for which the stone was used is the Smithsonian Institution Building or ‘Castle’ (1847–55), the first building of the Smithsonian Institution and still its administrative centre. Many churches, school buildings and homes in the city were built wholly or partially with the stone during the ‘brown decades’ of the latter half of the nineteenth century.

Abstract This guide accompanies a walking tour of sites where masonry was employed on or near the National Mall in Washington, D.C. It begins with an overview of the geological setting of the city and development of the Mall. Each federal monument or building on the tour is briefly described, followed by information about its exterior stonework. The focus is on masonry buildings of the Smithsonian Institution, which date from 1847 with the inception of construction for the Smithsonian Castle and continue up to completion of the National Museum of the American Indian in 2004. The building stones on the tour are representative of the development of the American dimension stone industry with respect to geology, quarrying techniques, and style over more than two centuries. Details are provided for locally quarried stones used for the earliest buildings in the capital, including Aquia Creek sandstone (U.S. Capitol and Patent Office Building), Seneca Red sandstone (Smithsonian Castle), Cockeysville Marble (Washington Monument), and Piedmont bedrock (lockkeeper’s house). Following improvement in the transportation system, buildings and monuments were constructed with stones from other regions, including Shelburne Marble from Vermont, Salem Limestone from Indiana, Holston Limestone from Tennessee, Kasota stone from Minnesota, and a variety of granites from several states. Topics covered include geological origins, architectural design considerations, weathering problems, and conservation issues.