[Note: In the Web version of this bulletin, instead of the standard Cambrian map symbol (a "C" with a dash through it), a capital "C" preceded by a dash is used (-Clc, Loudoun Formation).]
The geology of the Harpers Ferry quadrangle was mapped in 1989 and 1990 under a cooperative agreement between the U.S. Geological Survey (USGS) and the Maryland Geological Survey. In Virginia, the mapping was part of a cooperative agreement between the USGS and the Loudoun County Department of Environmental Resources (Southworth, 1991a; Burton and others, 1992b; Southworth and others, in press).
The area is underlain by rocks of the northeast-plunging Blue Ridge-South Mountain anticlinorium, a late Paleozoic (Mississippian?) Alleghanian structure (fig. 1). Middle Proterozoic paragneisses and granitoids intruded by Late Proterozoic metadiabase and metarhyolite dikes and Jurassic diabase dikes underlie broad valleys in the areas and form the core of the anticlinorium. Late Proterozoic metasedimentary and metavolcanic rocks and Lower Cambrian metasedimentary rocks underlie the high ground of Blue Ridge-Elk Ridge and Short Hill-South Mountain where topographic relief ranges from 100 to 450 m (300 to 1,476 ft). The Short Hill fault transects the quadrangle and causes a repetition of the west limb of the anticlinorium. Alluvium is found along the Potomac and Shenandoah Rivers and all tributaries. Colluvium is abundant on the flanks of Blue Ridge-Elk Ridge and Short Hill-South Mountain.
The map area (pl. 1) includes the Harpers Ferry National Historical Park, and this report commemorates its 50th anniversary. A segment of the Appalachian Trail and the Chesapeake and Ohio Canal National Historical Park are also included on plate 1. Abundant trails on the mountains and along the Shenandoah and Potomac Rivers provide access to excellent exposures of bedrock.
Rocks of Late Proterozoic and Early Cambrian age in this quadrangle are heterogeneous, and some units pinch and swell abruptly. Postdepositional erosion (Jonas and Stose, 1939; Stose and Stose, 1949) and deposition on a paleotopographic surface (Nickelsen, 1956) have formed unconformities. Unconformities occurring in rocks of Late Proterozoic and Early Cambrian age reflect changes from the Iapetan rift to a passive continental margin (drift). Fluvial clastic sediments of the Late Proterozoic Swift Run Formation lie unconformably on Middle Proterozoic gneiss and mark the beginning of rifting of Laurentia, which formed the Iapetus Ocean (Rankin, 1976). Metadiabase and metarhyolite dikes and extrusive metabasalt of the overlying Catoctin Formation were emplaced during the main phase of continental rifting. Conglomerate of the Late Proterozoic and Lower Cambrian Loudoun Formation and conglomeratic quartzite of the Lower Cambrian Weverton Formation were deposited in the rift-to-drift transition. Tabular crossbedded quartzite of the Owens Creek Member (upper part) of the Weverton Formation comprises shoreface deposits of initial drift facies (Simpson and Eriksson, 1989). The drift regime was fully established during deposition of clastic rocks of the Lower Cambrian Harpers and Antietam Formations. Rocks of the Lower Cambrian Tomstown Formation were the first carbonate rocks deposited during initial phases of the evolving passive continental margin.
The area was first mapped by Keith (1894) at a scale of 1:125,000, and this map (pl. 1, scale 1:24,000) commemorates the centennial anniversary of his Harpers Ferry folio. Subsequent mapping was by Jonas and Stose (1939) and Cloos (1941) at a scale of 1:62,500, Nickelsen (1956) at a scale of 1:25,000, and Dean and others (1990) and Howard (1991) at a scale of 1:24,000.
Keith (1894) established the basic stratigraphic nomenclature for the stratified cover rocks that overlie the basement rocks, and the type localities of his Catoctin Schist, Weverton Sandstone, and Harpers Shale are within the map area (fig. 1A). Jonas and Stose (1939) and Stose and Stose (1946) subdivided Keith's (1894) Loudoun Formation into the Swift Run Tuff, Catoctin Metabasalt, and Loudoun Formation. The formational names of the Swift Run and Catoctin were revised by King (1950) and Bloomer and Bloomer (1947), respectively. Nickelsen (1956) considered the Swift Run and Catoctin Formations to be of Precambrian age; his Loudoun Formation, Weverton Quartzite, Harpers Formation, and Antietam Quartzite constituted the Chilhowee Group of Early Cambrian age. Recently, Brezinski (1992) described and named a tripartite subdivision for the Weverton Formation and subdivided the Tomstown Formation (Stose, 1906) into four members, (fig. 1B).
U.S. Geological Survey, U.S. Department of the Interior
This page is https://pubs.usgs.gov/bulletin/b2123/intro.html
Contact: Scott Southworth
Last modified 08.29.00
This page is https://pubs.usgs.gov/bulletin/b2123/intro.html
Contact: Scott Southworth
Last modified 08.29.00