Geologic Field Trip – Alexandria, Virginia and Vicinity
Date: Monday, October 10, 2016, 9 AM – ?
Leader: Tony Fleming, author of the 2016 Geologic Atlas of Alexandria, VA (www.alexandriava.gov/89974)
Who: geologists, geotechnical engineers, building and planning staff, environmental professionals
Meeting Place: Ford Nature Center, 5750 Sanger Avenue, Alexandria, VA 22311
Alexandria features a strongly dissected fall line landscape with some of the most varied geology and topography in the Potomac Valley, ranging from complexly deformed crystalline bedrock of the Piedmont in the west to many steep, active hillsides developed on the eastward-thickening wedge of early Cretaceous Potomac Formation further east, along with a host of upland and lowland river terraces, debris fans, and associated hillside deposits. This trip will highlight a variety of geologic features and scientific problems of local and regional interest to geologists and geotechnical engineers working in the greater mid-Atlantic region.
Stops and topics featured on the trip include:
– Structure of the Paleozoic bedrock and the nature of the regionally significant erosional unconformity on the bedrock surface, which appears to have unusually high local relief at places in the City.
– The character, facies relations, deformation, hydrogeology, and geotechnical properties of the early Cretaceous Potomac Formation as seen at its updip end. The Potomac Formation is well exposed throughout the highlands in the western two thirds of the city and will be a major focus of the trip because of its geomorphic, hydrogeologic and geotechnical significance. We will see good examples of the informal members defined in the atlas, as well as the major aquifer system it hosts, and for which western Alexandria is part of the regional recharge area.
– Nature and evolution of the landscape from the late Tertiary through Recent. The City contains more and larger river terraces than most other parts of northern Virginia, including several widespread “upland” gravels whose ages remain poorly defined yet are of fundamental importance to our understanding of issues like seismic risk and incision history of the landscape;
– Tectonics, faults, and modern seismic hazards. Ample evidence indicates that comparatively young fault zones project into the City from both the north (DC/Rock Creek fault system) and the south (Stafford fault system), but defining and mapping their locations with a reasonable degree of precision in this heavily urbanized area is a real challenge. We will visit some of the localities where faults have been documented or are strongly suspected, and revisit subsurface data presented in the atlas for some of these “suspect” places.
Alexandria contains all of these features within a compact area, which will greatly reduce travel time between stops and allow for maximum time in the field. In addition to illustrating the strata and features depicted in the city atlas, the intent of the trip is to stimulate lively discussion among this group of regional experts and professionals. I look forward to your participation.
Please contact the field trip leader (firstname.lastname@example.org) if you plan to attend or have any questions, and feel free to forward this announcement to colleagues who may be interested in the outing.