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 The cliffs along the Potomac River at Stratford Hall display extensive exposures of Miocene marine strata that belong successively to the Calvert, Choptank, St. Marys, and Eastover Formations. Within the lower part of this sequence, in the Calvert and Choptank Formations, there is well-developed cyclic stratigraphy. Above the Miocene units lies the marginal marine to deltaic Pleistocene Bacons Castle Formation, which is the highest and youngest formation exposed in the cliffs. The goals of this field trip guide are to (1) show the Miocene formations exposed in the cliffs and discuss the paleoenvironments within which they formed, (2) demonstrate the cyclicity in the Miocene marine formations and discuss its origin, (3) compare and contrast the section exposed at the Stratford and Nomini Cliffs with the classic Miocene Calvert Cliffs sequence exposed to the northeast in Calvert County, Maryland, and the Miocene sequence recovered in the Haynesville cores to the southeast in Richmond County, Virginia, (4) discuss and explain why a detailed correlation among these three places has been so difficult to attain, and (5) show typical lithologies of the Bacons Castle Formation and discuss the paleoenvironments in which they formed.

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 The Salisbury embayment is a broad tectonic downwarp that is filled by generally seaward-thickening, wedge-shaped deposits of the central Atlantic Coastal Plain. Our two-day field trip will take us to the western side of this embayment from the Fall Zone in Washington, D.C., to some of the bluffs along Aquia Creek and the Potomac River in Virginia, and then to the Calvert Cliffs on the western shore of the Chesapeake Bay. We will see fluvial-deltaic Cretaceous deposits of the Potomac Formation. We will then focus on Cenozoic marine deposits. Transgressive and highstand deposits are stacked upon each other with unconformities separating them; rarely are regressive or lowstand deposits preserved. The Paleocene and Eocene shallow shelf deposits consist of glauconitic, silty sands that contain varying amounts of marine shells. The Miocene shallow shelf deposits consist of diatomaceous silts and silty and shelly sands. The lithology, thickness, dip, preservation, and distribution of the succession of coastal plain sediments that were deposited in our field-trip area are, to a great extent, structurally controlled. Surficial and subsurface mapping using numerous continuous cores, auger holes, water-well data, and seismic surveys has documented some folds and numerous high-angle reverse and normal faults that offset Cretaceous and Cenozoic deposits. Many of these structures are rooted in early Mesozoic and/or Paleozoic NE-trending regional tectonic fault systems that underlie the Atlantic Coastal Plain. On Day 1, we will focus on two fault systems (stops 1-2; Stafford fault system and the Skinkers Neck-Brandywine fault system and their constituent fault zones and faults). We will then see (stops 3-5) a few of the remaining exposures of largely unlithified marine Paleocene and Eocene strata along the Virginia side of the Potomac River including the Paleocene-Eocene Thermal Maximum boundary clay. These exposures are capped by fluvial-estuarine Pleistocene terrace deposits. On Day 2, we will see (stops 6-9) the classic Miocene section along the ~25 miles (~40 km) of Calvert Cliffs in Maryland, including a possible fault and structural warping. Cores from nearby test holes will also be shown to supplement outcrops.

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.