Prepared in conjunction with the GSA Southeastern and Northeastern Sections Joint Meeting in Reston, Virginia, the four field trips in this guide explore various locations in Virginia, Maryland, and West Virginia. The physiographic provinces include the Piedmont, the Blue Ridge, the Valley and Ridge, and the Allegheny Plateau of the Appalachian Basin. The sites exhibit a wide range of igneous, metamorphic, and sedimentary rocks, as well as rocks with a wide range of geologic ages from the Mesoproterozoic to the Paleozoic. One of the trips is to a well-known cave system in West Virginia. We hope that this guidebook provides new motivation for geologists to examine rocks in situ and to discuss ideas with colleagues in the field.

Abstract The mid-Atlantic region and Chesapeake Bay watershed have been influenced by fluctuations in climate and sea level since the Cretaceous, and human alteration of the landscape began ~12,000 years ago, with greatest impacts since colonial times. Efforts to devise sustainable management strategies that maximize ecosystem services are integrating data from a range of scientific disciplines to understand how ecosystems and habitats respond to different climatic and environmental stressors. Palynology has played an important role in improving understanding of the impact of changing climate, sea level, and land use on local and regional vegetation. Additionally, palynological analyses have provided biostratigraphic control for surficial mapping efforts and documented agricultural activities of both Native American populations and European colonists. This field trip focuses on sites where palynological analyses have supported efforts to understand the impacts of changing climate and land use on the Chesapeake Bay ecosystem.

Abstract Urbanization is a major process now shaping the environment. This field trip looks at the hydrogeology of the general Washington, D.C., area and focuses on the city’s lost springs. Until 150 years ago, springs and shallow dug wells were the main source of drinking water for residents of Washington, D.C. Celebrating the nation’s bicentennial, Garnett P. Williams of the U.S. Geological Survey examined changes in water supply and water courses since 1776. He examined old newspaper files to determine the location of the city’s springs. This field trip visits sites of some of these springs (few of which are now flowing), discusses the hydrologic impacts of urbanization and the general geological setting, and finishes with the Baltimore Long Term Ecological Research site at Dead Run and its findings. The field trip visits some familiar locations in the Washington, D.C., area, and gives insights into their often hidden hydrologic past and present.

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.

The moment magnitude (M w ) 5.8 Mineral, Virginia, earthquake of 23 August 2011, was centered ~130 km south-southwest of Washington, D.C. (USA), and caused minor damage across Virginia and the Washington metropolitan area. The Washington Monument sustained masonry damage; a post-earthquake survey of the monument performed for the National Park Service identified cracking and spalling of the pyramidion (the topmost piece of the obelisk), and cracking, spalling, and lesser damage over the entire length of the monument shaft. A seismic vulnerability assessment of the monument was then performed to evaluate the potential for damage to the monument from future earthquakes. No ground-motion recordings of the Mineral earthquake were available for the monument site; therefore, deterministic and probabilistic seismic hazard analyses were performed to develop site-specific response spectra representing the Mineral earthquake and the maximum considered earthquake (MCE). These spectra were initially developed for a firm rock site condition, and each event was represented by a suite of seven three-component time histories. The rock motions were then modified through site-response analyses to develop time histories and response spectra representing ground motions in the clayey and gravelly soils that support the base of the monument. The results of the site-response analysis show significant amplification at short to intermediate response periods; this amplification is also observed in recordings of the 2011 Mineral earthquake obtained from another site in the general vicinity of the monument. The ground shaking conditions, along with expected foundation load-deflection behavior, were used in detailed structural modeling of the monument to help understand the structure’s response and damage during the 2011 Mineral earthquake and to predict expected performance during future MCE-level ground shaking. The analyses indicate that the period range of the pyramidion’s modes of vibration corresponds closely with the characteristic period range of the monument’s subsurface profile, and is reflected by the peak response of the site-specific spectra analyzed for the Mineral scenario earthquake. This similarity caused amplification of motions experienced by the monument and increased damage to the pyramidion. The nonlinear analysis for the MCE ground motions indicates damage to the pyramidion similar to that from Mineral earthquake effects, but larger lateral displacements of the top of the shaft due largely to the greater soil-bearing stresses associated with a greater first-mode response. The monument was found to generally meet accepted seismic safety criteria without need for strengthening.

This paper investigates the potential causes of the damage to the Washington Monument sustained from the 2011 Mineral, Virginia (USA), earthquake through time-history dynamic analysis. Ambient vibration field test data were obtained and utilized to calibrate a finite element model of the structure and its foundation. The impact of the foundation modeling and the uncertainties associated with the material properties of the stone and iron, in the absence of in situ material testing, were investigated through several parametric studies, in which the material property values are permuted at three (upper, average, and lower) levels to bound the predicted dynamic characteristics of the structure. Because ground-motion data recorded in the Washington, D.C., area during the earthquake are scarce, the ground motion at the Washington Monument site was simulated using an angular transformation of the recorded ground motions in Reston, Virginia, deconvoluted to the bedrock level and upward propagation of the rotated motions to the ground surface based on soil profiles in Reston and the Washington Monument site provided by the U.S. Geological Survey. The finite element model of the Washington Monument shaft subjected to these bidirectional earthquake records showed high acceleration amplification at the observation level, as well as tensile stress concentration at the ~107 m level. These observations correlate with the damage observed in the pyramidion section and upper levels of the Washington Monument shaft following the 2011 Virginia earthquake.

Characterizing geologic features associated with major earthquakes provides insights into mechanisms contributing to fault slip and assists evaluation of seismic hazard. We use high-resolution airborne geophysical data combined with ground sample measurements to image subsurface geologic features associated with the 2011 moment magnitude (M w ) 5.8 central Virginia (USA) intraplate earthquake and its aftershocks. Geologic mapping and magnetic data analyses suggest that the earthquake occurred near a complex juncture of geologic contacts. These contacts also intersect a >60-km-long linear gravity gradient. Distal aftershocks occurred in tight, ~1-km-wide clusters near other obliquely oriented contacts that intersect gravity gradients, in contrast to more linearly distributed seismicity observed at other seismic zones. These data and corresponding models suggest that local density contrasts (manifested as gravity gradients) modified the nearby stress regime in a manner favoring failure. However, along those gradients seismic activity is localized near structural complexities, suggesting a significant contribution from variations in associated rock characteristics such as rheological weakness and/or rock permeability, which may be enhanced in those areas. Regional magnetic data show a broader bend in geologic structures within the Central Virginia seismic zone, suggesting that seismic activity may also be enhanced in other nearby areas with locally increased rheological weaknesses and/or rock permeability. In contrast, away from the M w 5.8 epicenter, geophysical lineaments are nearly continuous for tens of kilometers, especially toward the northeast. Continuity of associated geologic structures probably contributed to efficient propagation of seismic energy in that direction, consistent with moderate to high levels of damage from Louisa County to Washington, D.C., and neighboring communities.

The Stafford fault system, located in the mid-Atlantic coastal plain of the eastern United States, provides the most complete record of fault movement during the past ~120 m.y. across the Virginia, Washington, District of Columbia (D.C.), and Maryland region, including displacement of Pleistocene terrace gravels. The Stafford fault system is close to and aligned with the Piedmont Spotsylvania and Long Branch fault zones. The dominant southwest-northeast trend of strong shaking from the 23 August 2011, moment magnitude M w 5.8 Mineral, Virginia, earthquake is consistent with the connectivity of these faults, as seismic energy appears to have traveled along the documented and proposed extensions of the Stafford fault system into the Washington, D.C., area. Some other faults documented in the nearby coastal plain are clearly rooted in crystalline basement faults, especially along terrane boundaries. These coastal plain faults are commonly assumed to have undergone relatively uniform movement through time, with average slip rates from 0.3 to 1.5 m/m.y. However, there were higher rates during the Paleocene–early Eocene and the Pliocene (4.4–27.4 m/m.y), suggesting that slip occurred primarily during large earthquakes. Further investigation of the Stafford fault system is needed to understand potential earthquake hazards for the Virginia, Maryland, and Washington, D.C., area. The combined Stafford fault system and aligned Piedmont faults are ~180 km long, so if the combined fault system ruptured in a single event, it would result in a significantly larger magnitude earthquake than the Mineral earthquake. Many structures most strongly affected during the Mineral earthquake are along or near the Stafford fault system and its proposed northeastward extension.

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.

Abstract Washington, D.C., is the first and largest planned city in the United States. The city lies along the Fall Line at the boundary between the Atlantic Coastal Plain and the Piedmont Plateau and at the head of navigation on the estuary of the Potomac River. This site combines the engineering complexities of two vastly different geologic terranes with the other complications introduced by the terraces and channels of a major river-estuary system. The western part of the city and most of the suburbs to the west and north are on the Piedmont Plateau, an upland underlain by complexly deformed metasedimen-tary and metaigneous rocks of late Precambrian or early Paleozoic age. These crystalline rocks are mantled by soil, saprolite, and weathered rock to depths of as much as 50 m, which adds both to their geologic inscrutability and to the problems of excavation and design of structures. The Atlantic Coastal Plain is underlain by unmetamorphosed and little deformed fluvial and marine strata of Cretaceous through Miocene age. These deposits form a prism that thickens southeastward from a wedge edge at the Fall Line to as much as 450 m in the southeastern part of the metropolitan area. Unconformities, facies changes, and variations in physical properties with age and depth of burial add spice to the life of the engineering geologist dealing with these strata. Terrace deposits ranging in age from Miocene(?) to Holocene bevel across the contact between the Coastal Plain deposits and the crystalline rocks of the Piedmont. The oldest deposits underlie a broad, deeply dissected upland that stands at an elevation of 80 to 90 m southeast of the Fall Line; isolated outliers cap hills and interfluves at elevations of as much as 150 m northwest of the Fall Line. Lower and younger terraces flank the major drainages and occur at various levels down to the modern flood plains. Much of the central city is built on low terraces of Sangamon or Wisconsin age. These younger terraces locally fill and conceal deep bedrock channels cut by the ancestral Potomac during low stands of sea level during the Pleistocene. The terrace deposits show conspicuous differences in degree of weathering and soil development, depending on their age and physiographic position. Estuarine and marsh deposits flank the tidal reaches of the Potomac and Anacostia Rivers, and considerable parts of the central city are built on artificial fill over these deposits. Considerable experience in underground excavation has been gained in the last decade during construction of METRO, a regional rapid transit rail system. Tunneling techniques have been developed for both crystalline rocks and Coastal Plain deposits, but cut and cover methods are generally used in the young materials, which are generally weakest. Foundation and slope stability problems are widespread in some geologic units in the metropolitan area and are locally serious. They affect structures ranging from single family dwellings to the Washington Monument.