Local DC-MD-VA Geology
Washington is centrally located in the middle of the North American Plate
✦ Screen shot of Earth showing crustal elevation, tectonic plates, and plate boundaries. Note that Washington sits near the locked (tectonically inactive) continent-ocean margin that marks the western rift margin for the opening of the Atlantic Ocean basin approximately 200 million years ago. (Original image from Tasa Graphic Arts, Inc.)
Washington sits at the edge of a long-ago assembled supercontinent
✦ Depiction of Archean through Neoproterozoic basement features of Precambrian North America (Laurentia). Significant terranes, orogenic belts, basins, rifts, and structural features are highlighted by individual colors. Original figure and parts of caption from Whitmeyer, S.J. and Karlstrom, K. (2007) Tectonic model for the Proterozoic growth of North America. http://doi.org/10.1130/GES00055.1, used with permission.
Physiographic provinces of Maryland and northern Virginia
✦ Map of the physiographic provinces of Maryland and Northern Virginia. From Southworth, S., Brezinski, D.K., Orndorff, R.K., Chirico, P.G., and Lagueux, K (2001) Digital Geologic Map and Database of the Chesapeake and Ohio Canal Hational Historical Park, District of Columbia,Virginia, Maryland, and West Virginia, USGS OFR 01-188.
✦ Simplified geologic cross section across the southern region (in northern Virginia) of the physiographic provinces map above. From northwest to southeast: west- vergent folded and faulted rocks of the Valley and Ridge (surf green), basement gneisses (stippled blue) and cover rocks (dark green) of the Blue Ridge, suspect terranes of the Piedmont (tan), Triassic basins (stippled green), and the Coastal Plain onlap in the eastern Piedmont (thin yellow band at the surface). Major structural features indicated: LCD—Lower Carbonate duplex; NMT—Little North Mountain thrust; MS—Massanutten synclinorium; BRF—Blue Ridge fault sys- tem; SM—Brookneal/Shores Mountain Run zone; SZ—Spotsylvania zone; HZ—Hylas zone; T—Taylorsville basin. Original figure and slightly modified caption from Whitmeyer, S. J., Bailey, C. M., & Spears, D. B. (2015). A billion years of deformation in the central Appalachians: Orogenic processes and products. In Tripping from the Fall Line: Field Excursions for the GSA Annual Meeting, Baltimore, 2015 (Vol. 40, pp. 11–33). Geological Society of America. http://doi.org/10.1130/2015.0040(02), used with permission.
State of geologic mapping at the 1:24,000 and greater scale
around the Washington, DC area
See the index maps from the National Geologic Map Database to actively scroll and get geologic topographic basemaps for anyplace in the US.
Basemaps for the DC area can be found by clicking on the map above or here. *Note: This basemap series currently runs in Adobe Flashplayer which only runs on MacOs. It will in run on iOs (Apple iPhones and iPads). The USGS says that future updates will move off Flashplayer.
Geologic/Geomorphic setting of Washington DC and the Capital Beltway
The boundary between the Coastal Plain in the southeast of the map area (yellow) and the metamorphosed crystalline basement of the Piedmont Province in the northwest (blue and purple) cut across the the DC area and the beltway. The boundary between these provinces known as the "fall line" is a fundamental change in rock type, and often topography and drainage. Washington, like many other cities along the east coast of the US, was designed to be at the fall line because it marked the upriver end of the navigable and tidal Potomac River. EPL is located just to the west of the black square labelled Rock Creek Park near the center of the map at the red star.
Original figure from Means (2010) Roadside Geology of Maryland, Delaware and Washington, DC. Mountain Press Publishing Company, Missoula Montana, p. 177. —Figure was drawn from the work of Cleaves, Edwards, and Glaser, 1968; Kunk and others, 2004; Southworth and others, 2001; Cloos and others, 1964; and Southworth and others, 2006.
Geologic map of Washington, DC area from the USGS
✦ Geologic Map of the District of Columbia and the proximal Potomac River. From Scott Southworth and Danielle Denenny (2006) Geologic Map of the National Parks in the National Capital Region, Washington, D.C., Virginia, Maryland, and West Virginia USGS OFR 2005-1331.
✦ Legend for geologic map above. From USGS OFR 2005-1331.
The local geology around Carnegie's Earth and Planets Laboratory
Geologic Sketch map for the local campus area
The EPL campus is located just west of the Rock Creek Shear Zone (the region around the Rock Creek Fault in the map at left). Here Laurel Formation (light purple) is in fault contact with the sheared Kensington tonalite. The Laurel Formation is a Cambrian age sedimentary 'melange' (sedimentary rock that is composed of block and fragments of various sizes) that is part of the Potomac Terrane. According to Southworth and Denenny (2005) the rock is a matrix of quartz and feldspar with elongate cobbles and bodies of other sandstone and schist. Some Laurel Formation localities show evidence of incipient partial melting.
The Laurel Formation has been intruded by graniodioritic magma known as the Kensington Tonalite in the middle Ordovician (463±8 Ma; Aleinikoff et al., 2002). In the Rock Creek Shear Zone this rock has been severely metamorphosed to become a mylonite with augen, porphyoblasts of microcline, and local garnet. (Southworth and Denenny; 2005)
Coastal plain deposits are evident in the eastern and southern parts of the map area.
This original figure and its annotations are from Means (2010) Roadside Geology of Maryland, Delaware and Washington, DC. Mountain Press Publishing Company, Missoula Montana, p. 190 which was modified from Southworth and others, 2006.
A more detailed USGS Geologic Quadrangle Map at the 1:24,000 scale