ABSTRACT During the Pleistocene, the Laurentian Ice Sheet extended southward into northwestern Pennsylvania. This field trip identifies a number of periglacial features from the Appalachian Plateaus and Ridge and Valley provinces that formed near the Pleistocene ice sheet front. Evidence of Pleistocene periglacial climate in this area includes glacial lake deposits in the Monongahela River valley near Morgantown, West Virginia, and Sphagnum peatlands, rock cities, and patterned ground in plateau areas surrounding the Upper Youghiogheny River basin in Garrett County, Maryland, and the Laurel Highlands of Somerset County, Pennsylvania, USA. In the high-lying basins of the Allegheny Mountains, Pleistocene peatlands still harbor species characteristic of more northerly latitudes due to local frost pocket conditions. Pleistocene fauna preserved in a cave deposit in Allegany County, Maryland, record a diverse mammalian assemblage indicative of taiga forest habitat in the Ridge and Valley province.

ABSTRACT This field guide presents a one-day excursion in Prince George’s County, Maryland, USA, and documents the transition across the Cretaceous-Paleogene boundary by examining sediments from the upper Maastrichtian of the Severn Formation to the Paleocene sediments of the Brightseat and Aquia formations. Emphasis is placed on understanding how differences in depositional character and lithostratigraphy are related to changes in both microfossil and macrofossil assemblages. Particular attention is given to the difficulty in distinguishing Upper Cretaceous sediments from lower Paleocene sediments in the field, a problem that has traditionally led to misrepresentation of the distribution and thickness of these units and their correlation on a regional scale. Regarding the Late Cretaceous geology, the guide presents information on the lithology and microfossil biostratigraphy of the Severn Formation, which consists predominantly of silty quartz sand, with less than 5% clay. These sediments are placed in calcareous nannofossil Zone CC25a, suggesting an early late Maastrichtian age. Low abundances of planktic foraminifera combined with sedimentological evidence suggest deposition most likely occurred in a middle neritic environment. Macrofossils in the outcrops along the field trip consist primarily of fragmented bivalve mollusk and cephalopod shell material. A hiatus of ~5 m.y. separates the Cretaceous sediments from the overlying Paleocene deposits. As for the Paleocene geology, the guide presents information on the Brightseat and Aquia formations. The Brightseat represents early Danian age deposition and consists of clayey, silty sand at the base that grades upward into a silty sand. Glauconite is present at <5% throughout the formation in outcrop. Sediments of the Brightseat Formation are placed in calcareous nannofossil Zone NP3. Macrofossils are limited to small bivalve fragments that are scattered throughout. A hiatus representing ~3 m.y. separates the Brightseat from the overlying Aquia Formation, which is Selandian to Thanetian in age and consists of a glauconite-rich (~10%–20%), silty sand with common to abundant macrofossils, including both fragmented and complete gastropods and bivalves.

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 By virtue of its use in iconic monuments and historic buildings in the USA, Cockeysville marble, a dolomitic to calcitic lower Paleozoic (Cambrian/Ordovician) marble quarried in Baltimore County and adjacent areas in Maryland, is proposed as a potential Global Heritage Stone Resource. The most important use of this stone was for the Washington Monument in Washington, DC whose construction began in 1848; the second most important use was for the 108 columns of the United States Capitol's wings, completed in 1868. It was also used for two of the oldest major marble monuments in the USA, Baltimore's Battle Monument (dedicated in 1827) and Washington Monument (completed in 1829), as well as Baltimore's City Hall, Buffalo's Adkins Art Museum, Detroit's Fisher Building and parts of St Patrick's Cathedral in New York City. During the nineteenth century white Cockeysville was most desired, but a colourful variety, Mar Villa marble, was also used in the first decades of the twentieth century. Cockeysville marble is no longer quarried for dimension stone. All Cockeysville used outdoors has weathered to a lesser or great extent, but early testing indicating that the dolomitic marble would be more durable has proved to be true.

ABSTRACT The Upper Ordovician Juniata Formation, Central Appalachian Basin, USA, is a thick succession of cyclically bedded arenites, wackes, and mudrocks. Sedimentary facies of the formation in West Virginia, Virginia, and Maryland indicate cyclic peritidal deposition along the northern shoreline of the basin. The subsurface Juniata Formation has been drilled throughout the basin, and long, continuous well logs are available for analysis of the cyclic deposition. A 2400-ft-long (731.52-m-long) gamma-ray (GR) log from the Preston 119 well, northern West Virginia, provides a proxy of terrigenous siliciclastic fluxes originating from the Taconic highlands, from the early Ashgillian to the Ordovician–Silurian transition. Strong cycling in the GR log shows evidence for Milankovitch-forced sea-level oscillations, hypothesized to have been produced by dynamic Late Ordovician glaciation in polar (southern) Gondwana. Juniata cycle frequencies are different from those of Quaternary Milankovitch cycles, with significantly higher obliquity and precession index frequencies, consistent with a 21.5 h Ordovician day and an Earth-Moon distance that was 95% of present day. These results support John Dennison’s long-held view that Milankovitch forcing of sedimentation took place in the early Paleozoic Appalachian Basin by action of remotely generated glacio-eustatic oscillations powered by glaciation on southern Gondwana, and that this sedimentary record has tracked “Earth’s movement through space.”

ABSTRACT This trip explores three different occurrences of a diamictite-bearing unit in the transition between Upper Devonian redbeds of the Hampshire Formation (alluvial and fluvial deposits) and Mississippian sandstones and mudstones of the Price/Pocono Formations (deltaic deposits). Palynology indicates that all the diamictites examined are in the LE and LN miospore biozones, and are therefore of Late Devonian, but not latest Devonian, age. Their occurrence in these biozones indicates correlation with the Cleveland Member of the Ohio Shale, Oswayo Member of the Price Formation, and Finzel tongue of the Rockwell Formation in the central Appalachian Basin and with a large dropstone (the Robinson boulder) in the Cleveland Member of the Ohio Shale in northeastern Kentucky. Although several lines of evidence already support a glaciogenic origin for the diamictites, the coeval occurrence of the dropstone in open-marine strata provides even more convincing evidence of a glacial origin. The diamictites are all coeval and occur as parts of a shallow-marine incursion that ended Hampshire/Catskill alluvial-plain accumulation in most areas; however, at least locally, alluvial redbed accumulation continued after diamictite deposition ended. The diamictites are parts of nearshore, marginal-marine strata that accumulated during the Cleveland-Oswayo-Finzel transgression, which is related to global eustasy and to foreland deformational loading during the late Acadian orogeny. Detrital zircon data from clasts in a diamictite at Stop 3 (Bismarck, West Virginia) indicate likely Inner Piedmont, Ordovician plutonic sources and suggest major Acadian uplift of Inner Piedmont sources during convergence of the exotic Carolina terrane with the New York and Virginia promontories. Hence, the Acadian orogeny not only generated high mountain source areas capable of supporting glaciation in a subtropical setting, but also through deformational foreland loading, abetted regional subsidence and the incursion of shallow seas that allowed mountain glaciers access to the open sea.

ABSTRACT The Baltimore terrane, the Baltimore Mafic Complex (BMC), and the Potomac terrane are telescoped tectonostratigraphic packages of metasedimentary and meta-igneous rocks that record the geologic history of eastern Maryland from 1.2 Ga to 300 Ma. These terranes provide insight into the understanding of the rifting of Rodinia and the initial amalgamation of eastern Laurentia. The oldest of these rocks are exposed as gneiss domes in the Baltimore terrane, with gneissic Grenvillian crust overlain by a metasedimentary cover succession believed to have been deposited during Rodinian rifting and the formation of the Iapetus ocean. These rocks are interpreted to be analogous to the Blue Ridge sequence in western Maryland. Late Cambrian ultramafites and amphibolites of the BMC discordantly overlie the Baltimore terrane to the east and north, and may represent ophiolitic oceanic crust obducted over eastern Laurentia continental rocks as an island-arc collisional event during the Taconian orogeny. To the west, a thick assemblage of schist, graywacke, metadiamictite, and ultramafic bodies comprises the Potomac terrane, a polygenetic mélange that may have formed in an accretionary wedge during Taconian subduction and collision with the Laurentian continental margin. The Pleasant Grove fault zone marks the Taconian suture of these accreted terranes to Laurentian rocks of the central Maryland Piedmont, and preserves evidence of dextral transpression during the Alleghenian orogeny in the Late Pennsylvanian.

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.