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 field guide presents a one-day trip across the northern Virginia Blue Ridge geologic province and highlights published geologic mapping of Mesoproterozoic rocks that constitute the core of the Blue Ridge anticlinorium and Neoproterozoic cover-sequence rocks on the fold limbs. The guide presents zircon SHRIMP (sensitive high-resolution ion microprobe) U-Pb crystallization ages of granitoid rocks and discusses the tectonic and petrologic evolution of basement rocks during the Mesoproterozoic. U-Pb data show more of a continuum for Blue Ridge Mesoproterozoic magmatic events, from ca. 1.18–1.05 Ga, than previous U-Pb TIMS (thermal ionization mass spectrometry)-based models that had three distinct episodes of plutonic intrusion. All of the younger dated rocks are found west of the N-S–elongate batholith of the Neoproterozoic Robertson River Igneous Suite, suggesting that the batholith occupies a fundamental Mesoproterozoic crustal boundary that was likely a fault. Narrow belts of paragneiss may represent remnants of pre-intrusive country rock, but some were deposited close to 1 Ga according to detrital zircon U-Pb ages. For late Neoproterozoic geology, the guide focuses on lithologies and structures associated with early rifting of the Rodinia supercontinent, including small rift basins preserved on the eastern limb of the anticlinorium. These basins have locally thickened packages of clastic metasedimentary rocks that strike into and truncate abruptly against Mesoproterozoic basement along apparent steep normal faults. Both basement and cover were intruded by NE-SE–striking and steeply dipping, few-m-wide diabase dikes that were feeders to late Neoproterozoic Catoctin Formation metabasalt that overlies the rift sediments. The relatively weak dikes facilitated the deformation that led to the formation of the Blue Ridge anticlinorium during the middle to late Paleozoic as the vertical dikes were transposed and rotated during formation of the penetrative cleavage.

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.

Abstract This field trip highlights the current understanding of the tectonic assemblage of the rocks of the Central Appalachians, which include the Coastal Plain, Piedmont, and Blue Ridge provinces. The age and origin of the rocks, the timing of regional deformation and metamorphism, and the significance of the major faults, provide the framework of the tectonic history which includes the Mesoproterozoic Grenvillian, Ordovician Taconian, Devonian to Mississippian Neoacadian, and Mississippian to Permian Alleghanian orogenies.