Abstract Prepared in conjunction with the 2015 GSA Annual Meeting in Baltimore, Maryland, this volume contains guides to field trips in this historic region. Emanating from the Fall Line city of Baltimore, these trips reflect the diversity of geological features in the mid-Atlantic region including the Piedmont, Appalachian Mountains, and Coastal Plain, and the importance of geology on the development and construction of the Baltimore-Washington, D.C., metropolitan area. Trips to the core of the Appalachian orogen concern themselves with the tectonic and metamorphic history, early Paleozoic carbonate platform development, Devonian paleoclimate, and coal-mine fire hazards. Excursions to the Coastal Plain examine various aspects of Cenozoic stratigraphy, structure, barrier island formation, and wetland and ecosystem development. A variety of trips also explore urban geology, including building and monument stones of Baltimore and Washington, D.C., urban hydrogeology, and Civil War battlefield geology.

In New England, earthquakes pose a risk to the built environment. Emergency preparedness and mitigation planning are prudent in this region as older unreinforced masonry buildings and numerous critical facilities are common. New England state geological surveys cooperate with the Northeast States Emergency Consortium (NESEC) to improve risk communication with emergency managers. To that end, Connecticut, Maine, Massachusetts, and Vermont employed surficial geologic maps, deglaciation history, knowledge of the glacial stratigraphy, and professional judgment to reclassify surficial geologic material units into one of the five National Earthquake Hazards Reduction Program (NEHRP) site classifications (A, B, C, D, and E). These new classifications were used as a substitute for the HAZards U.S. Multi-Hazard (HAZUS-MH) site class value of “D,” which is used throughout New England as a default value. In addition, coding of surficial geologic materials for the five NEHRP site classifications was compared with classifications using the Wald methodology, a method that uses a slope analysis as a proxy for shear-wave velocity estimates. Comparisons show that coding to site classes using the Wald methodology underestimates categories A (high-velocity shear-wave materials, least relative hazard) and E (lowest-velocity shear-wave materials, greatest relative hazard) when evaluated side by side with coding done with the aid of surficial geologic maps. North of the glacial limit, derangement of drainage resulted in extensive ponding of meltwaters and the subsequent deposition of thick sequences of lacustrine mud. Inundation by the sea immediately following deglaciation in New England resulted in the deposition of spatially extensive and locally thick sequences of glacial marine mud. Surficial geologic maps better capture this circumstance when compared with the Wald topographic slope analysis. Without the use of surficial geologic maps, significant areas of New England will be incorrectly classified as being more stable than the site conditions that actually exist. By employing surficial geologic information, we project an improved accuracy for HAZUS-MH earthquake loss estimations, providing local and regional emergency managers with more accurate information for locating and prioritizing earthquake planning, preparedness, and mitigation projects to reduce future losses.

Abstract In the northeastern United States, we have been removing dams for almost as long as we have been building them, yet many communities involved in current decisions to repair, replace, or remove a dam are not aware of this. This paper highlights some of the stories that have been recorded regarding the history of decision points for dams, including the colorful history of the Billerica Dam in Massachusetts, which has been removed and rebuilt numerous times and is now under consideration for removal for at least the sixth time in its 300 yr history. By understanding that dam removal is just one of the potential dam safety decisions that needs to be analyzed over the life cycle of a dam, and that dams are man-made structures with finite life spans, we can deconstruct the notion of dam removal as a radical concept. Dam removal is just one of many dam safety options that may be discussed over the course of a dam’s history. It is most commonly implemented when a dam no longer serves any economic purpose that justifies the expense of maintaining the dam structure. In the past, dams have been removed for many of the same reasons that we remove dams today; however, the procedures currently required to remove a dam are far more complex and highly regulated. This has led to increased documentation of dam removal efforts and now allows us to compare and categorize dam removal projects, such that the lessons learned from these projects can be incorporated into a more informed decision-making process in the future.

At postglacial rebound time scales, the intraplate continental lithosphere typically behaves as an elastic solid. However, under exceptional conditions, the effective viscosity of the lower crust and lithospheric mantle may be as low as ∼10 20 Pa s, leading to ductile behavior at postglacial rebound time scales. We studied the effects of a lithospheric ductile zone on postglacial rebound–induced seismicity and deformation in eastern Canada and the northeastern United States using three types of models: (1) a reference model with no lithospheric ductile layer; (2) a model with a uniform, 25-km-thick, ductile layer embedded in the middle of the lithospheric column; and (3) a model with a dike-like vertical ductile zone, extending from mid-crust level down to the bottom of the lithosphere, along the Precambrian rift structure of the St. Lawrence Valley. Based on geothermal and rock physics data, the viscosity of the ductile zone is set to either 10 20 or 10 21 Pa s. We found that a narrow ductile zone cutting vertically through the lithosphere has larger effects than the uniformly thick horizontal ductile layer. Effects of a lithospheric weak zone on uplift rates may be large enough to be detected by global positioning system (GPS) measurements, especially for low viscosities. While the effect on fault stability is also large, the impact on the onset time of instability is small for sites within the ice margin. The impact on the onset time is more significant for sites outside the ice margin. Effects of a lithospheric weak zone are also significant on present-day horizontal velocities and strain rates and are at the limit of resolution for GPS measurements.