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.

Occurring only 13 months apart, the moment magnitude, M w 3.4 Germantown, Maryland (16 July 2010), and M w 5.8 Mineral, Virginia (23 August 2011), earthquakes rocked the U.S. national capital region, drawing renewed attention to the occurrence of intraplate seismicity in the Mid-Atlantic region of the eastern United States. We model the Coulomb stress transferred by these earthquakes to fault zones in the Mid-Atlantic region that were active during the Cenozoic. In most cases, the Mineral earthquake brought these preexisting Cenozoic faults further from failure. This unloading, like all changes in stress located more than 30 km from the epicenter, was very small (~1–3 mbar) and therefore unlikely to affect the occurrence of earthquakes at the regional scale. Between 30 and 15 km away, however, the maximum Coulomb failure stress change ranged one to three orders of magnitude greater (~0.05–6 bar), levels on par with the stress changes that triggered historical earthquakes in California. The Mineral earthquake generated an increase in Coulomb failure stress of ~0.5 bar 10 km from the rupture, ~5 bar 5 km from the rupture, and ~20 bar at the edge of the rupture on receiver faults oriented like the mainshock. The geographic location of the aftershock-defined Northwest fault in the epicentral region may be explained by Coulomb stress transfer from the mainshock. Elastic dislocation modeling of surface deformation caused by the Mineral earthquake indicates a maximum of ~7 cm of permanent vertical surface displacement directly above the center of the rupture.

Abstract Measuring the rate at which rivers cut into rock and determining the timing of incision are prerequisite to understanding their response to changes in climate and base level. Field mapping and measurement of cosmogenic 10 Be in 106 rock samples collected from the Great Falls area of the Potomac River show that the river has cyclically incised into rock and that the position of the knickzone, now at Great Falls, has shifted upstream over the later Pleistocene. Exposure ages increase downstream and with distance above the modern channel. The latest incision began after 37 ka, abandoning and exposing a strath terrace (the old river channel) hundreds of meters wide beginning at Great Falls and ending at Black Pond, 3 km downstream. This incision was coincident with expansion of the Laurentide ice sheet. Exposure ages of samples collected down the walls of Mather Gorge downstream of Great Falls indicate incision, at rates between 0.4 and 0.75 m/k.y., continued into the Holocene. The 10 Be data are more consistent with continued channel lowering through this 3 km reach than the steady retreat of a single knickpoint. Prior to 37 ka, the primary falls of the Potomac River were likely at Black Pond. Ongoing incision siphoned water away from these paleofalls, leaving them high and dry by 11 ka. Downstream of Black Pond, the strath terrace surface is covered with fine-grained sediment, and the few exposed bedrock outcrops are weathered and frost-shattered from periglacial processes active during the Last Glacial Maximum.