6. the Incision History of a Passive Margin River, the Potomac Near Great Falls
6. The Incision History of a Passive Margin River, the Potomac Near Great Falls By Paul Bierman,1 E-an Zen,2 Milan Pavich,3 and Luke Reusser1 Introduction was done at the Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, Ca., in This field trip focuses on the emerging significance of collaboration with Robert Finkel. complex geomorphic processes that have operated in a pas- sive margin setting in the mid-Atlantic region (fig. 1A). The application of cosmogenic exposure dating to understanding the responses of rivers to Quaternary tectonic, eustatic, and Regional Framework: The Complexity of climatic variations is providing unprecedented information Passive Margin Settings about landscape histories. This trip presents new information about the response of the Potomac River to regional variabili- Passive margins, the trailing edges of continental plates, ty in sea level and climate over the late Pleistocene and are geomorphically complex. Despite the absence of active Holocene. We present a regional framework, field observa- tectonics, passive margins exhibit features such as great tions of the morphology of the Potomac River gorge below escarpments (Matmon and others, 2002; Bank and others, Great Falls (fig. 1B), and data on the age relations of strath 2001), river gorges, and marine terraces (Flint, 1940; Cooke, terraces associated with the Potomac River gorge. 1952). New analytical techniques, such as fission track ther- mochronology (Naeser and others, 2001) and cosmogenic iso- tope exposure dating (Bierman and others, 2002) are provid- Acknowledgments ing insights into the processes that control passive margin evolution. Passive margins exhibit both stable and active fea- We want to recognize the exceptional contributions of tures.
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