Rivers and Watersheds - The Geology of Virginia

The Potomac-Shenandoah System

The Shenandoah River: North Fork | South Fork | The Potomac River

The South Branch of the Potomac River heads in the Valley & Ridge province of northwestern Highland County along the eastern edge of the Allegheny Front. The South Branch flows to the northeast into West Virginia and has carved a linear valley underlain by Devonian shales and limestones. The North Branch of the Potomac originates in the Appalachian Plateau province of western Maryland and West Virginia. It is the Shenandoah River system that drains much of the northwestern part of Virginia and has two major forks, the North and South. The Shenandoah River has a storied human history, but also flows over a geologically diverse and interesting landscape.

The North River, tributary to the South Fork of the Shenandoah River.
The North River, Augusta County
(scale 1:100,000)

The South Fork of the Shenandoah

The South Fork has three major tributaries, the North River, the Middle River, and the Back Creek/South River system. The North River arises amongst 1200 meter-high (4000') ridges in the Allegheny Mountains to the northwest of Staunton and emerges from the mountains onto carbonate rocks of the Great Valley near Natural Chimneys. Here the river gradient decreases dramatically and the North has deposited a broad belt of alluvial sediment. The Middle River heads in the western part of the Great Valley to the southwest of Staunton. For most of its length the Middle River flows over carbonates and shales, forming a low gradient stream with many large meander loops. The Back Creek/South River system heads along the western flank of the Blue Ridge in Augusta County. Back Creek drains ridges underlain by resistant quartz sandstones of the Antietam Formation and Catoctin metabasalts. North of Waynesboro, the South River is a northeast flowing stream with a moderate gradient flowing over Cambrian carbonate rocks.

William & Mary geologists Jon Relyea and Ryan McKinney examine the contact between sand and gravel terrace deposits and underlying Paleozoic saprolitic carbonate rocks along the South Fork of the Shenandoah River, Rockingham County, Virginia.
Contact between sand and gravel terrace
deposits and underlying Paleozoic saprolitic
carbonate rocks along the South Fork

At Port Republic the South Fork is borne, here the river is 50-70 meters wide with a gradient of approximately 1 meter per kilometer. Terraces are well developed along this reach of the South Fork. Each terrace represents an ancient surface cut by the river and filled with a veneer (1-5 meters thick) of sand and gravel. Over time the South Fork has downcut abandoning former surfaces and leaving elevated terraces behind. Overlapping alluvial fans extend westward from the Blue Ridge into the valley of the South Fork. These alluvial fans are currently being incised by erosion. Construction of these alluvial fans may have occurred during colder periods in the Pleistocene when more sediment was transported from the Blue Ridge highlands.

Near Elkton, the South Fork enters a valley bounded on the east by the Blue Ridge Mountains and on the west by the Massanutten Mountain massif. Massanutten Mountain rises to over 1000 meters in elevation (3300 feet) and is held up by resistant Silurian quartz sandstones of the Massanutten Formation. The geologic structure of Massanutten is that of a synclinorium (a regional scale syncline with subordinate anticlines and synclines). Although Massanutten Mountain is a dramatic topographic expression of the synclinorium, the geologic structure can be traced for over 200 kilometers from southern Augusta County into Jefferson County, West Virginia.

The South Fork of the Shenandoah River.
The South Fork at Bixler Bridge.
Masanutten Mountain is the feature
in the background.

In the Page Valley, the South Fork is quite sinuous and forms a number of dramatic meanders that swing the river across the valley from the Blue Ridge to Massanutten. Rapids and riffles in the South Fork are of two basic types: 1) bedrock ledges and 2) cobble to boulder rock gardens . Caverns and sinkholes are common in the Cambro-Ordovician carbonate rocks of the Page Valley. Sand and gravel deposited by an ancient South Fork occur as dissected terrace remnants up to 50 meters above the modern river. Current research is attempting to date the age of these terraces and thereby estimate the downcutting and erosion rates in the Shenandoah River basin.

North of Luray, the Page Valley is only a few kilometers wide and the South Fork meanders across nearly the entire valley. Along Gooney Creek, a few kilometers to the southwest of Front Royal Proterozoic basement rocks of the Blue Ridge structurally overlie and are in fault contact with Ordovician carbonates. Rocks of the Blue Ridge were transported many kilometers to the northwest over lower Paleozoic sedimentary rocks of the Valley & Ridge during the late Paleozoic (~300 Ma) Alleghanian phase of Appalachian mountain building and plate collision.

The North Fork of the Shenandoah

Structural features of the Shenandoah Valley.  Click for more information.

The North Fork of the Shenandoah River heads in the mountainous terrain of the Ridge & Valley subprovince of northwestern Rockingham County. At Cootes Store the North Fork crosses into the Great Valley subprovince. Here the river crosses the North Mountain fault zone a major central Appalachian structure that brings Cambro-Ordovician carbonates and shales over middle Paleozoic clastic rocks that can be traced for over 300 kilometers from southern Pennsylvania to southwest of Lexington, Virginia. The North Fork flows east-northeast across the carbonate rocks of the Great Valley, until it reaches the northwestern flank of Massanutten Mountain. Between Edinburg and Strasburg the North Fork flows to the northeast, but follows an extremely sinuous course. The meanders along this reach are entrenched into bedrock; straight reaches of the river trend northwest/southeast and appear to be controlled by northwest/southeast striking cross fractures in the bedrock . Northeast of Strasburg, the topographic prominence of Massanutten Mountain yields to the relatively low ground in the Great Valley and the North Fork turns towards the east, joining the South Fork at Front Royal.

The Shenandoah River meanders northeast from Front Royal along the western edge of the Blue Ridge. The average gradient in this reach is typically less than 1 meter per kilometer. As the Shenandoah approaches Harpers Ferry its gradient increases to 3-5 meters per kilometer as it cuts obliquely across the weakly metamorphosed sandstones and mudstones of the Chilhowee Group. Most of the rapids and riffles in this stretch of the river occur as the water drops over bedding and foliation planes.

The Potomac River

At Harpers Ferry the Shenandoah meets the Potomac River and collectively they have carved a dramatic water gap through the Blue Ridge. For many kilometers to the northeast and southwest of Harpers Ferry, the Blue Ridge forms a prominent, unbroken ridge rising 300 meters (~1000 feet) above the Great Valley. Resistant metamorphosed quartz sandstones (quartzites) of the Weverton Formation underlie this ridge. Thomas Jefferson, in his Notes on the State of Virginia, stated that “the passage of the Potowmac through the Blue Ridge is perhaps one of the most stupendous scenes in nature” and “…this scene is worth a voyage across the Atlantic.”

Four kilometers to the east of Harpers Ferry the Potomac cuts another water gap through South Mountain. This ridge is also underlain by resistant quartz sandstones of the Weverton Formation that have been repeated by movement along the Short Hills fault. Recent research indicates that the Short Hills fault is an early Paleozoic normal fault that developed during Iapetan rifting and was reactivated as a reverse fault during the Paleozoic. East of South Mountain the Potomac flows over Mesoproterozoic basement rocks in the core of the Blue Ridge anticlinorium. Near Catoctin Mountain the Potomac crosses a major unconformity between the Mesoproterozoic granitic basement rocks and the 570 Ma Catoctin Formation metabasalts. These resistant metabasalts crop out on the eastern limb of the Blue Ridge anticlinorium.

One kilometer east of Catoctin Mountain, the Potomac crosses the western border fault of the Mesozoic Culpeper basin. On the eastern side of this normal fault rocks of the Blue Ridge were dropped at least 2 kilometers, as slip on this fault occurred sediment was deposited on the hanging wall forming the Culpeper basin. The Potomac flows with a modest gradient over Mesozoic sedimentary rocks for over 20 kilometers. Near the Loudoun/Fairfax County line the Potomac crosses the west-dipping unconformity onto the Paleozoic to Neoproterozoic metasedimentary rocks of the western Piedmont.

At Great Falls, the Potomac starts its rapid descent to sea level. Great Falls is a gorge carved into metasedimentary rocks of the Mather Gorge Formation, these schists and gneissic metagraywackes were originally deposited as marine sediment. During Appalachian mountain building these rocks were buried to depths of ~15 km and heated to temperature approaching 600 C. The Potomac drops 30 meters in a few kilometers at Great Falls. The drop of the Potomac River at the Fall Zone is the most dramatic of any eastern river. The Mather Gorge has formed by downcutting of the Potomac into resistant rocks and the upstream retreat of the falls over time.

The Potomac is a tidal river by the time it reaches Washington. Here the river dramatically broadens and is flanked by low marshes in many places. The river flows south-southwest from Washington. Bluffs along its western bank are composed of Cretaceous sedimentary units. These rocks are poorly cemented sandstones and siltstones that preserve some of the oldest angiosperm (flowering plants) fossils . The river turns southeast at King George County and continues to widen towards the Chesapeake. The low flats that border the river are underlain by Quaternary sediments deposited by an ancient Potomac River during higher stands of sea level. The prominent bluffs on the south bank of the Potomac in Westmoreland County are scarps in the process of forming. These slopes are composed of 4 to 10 Ma sediment of the Chesapeake Group. Abundant fossils including sharks teeth, whales, and a diverse array of bivalves are clear evidence that Chesapeake Group sediments were deposited in a shallow marine setting when much of the Virginia Coastal Plain was flooded. South of Piney Point, Maryland the Potomac merges with the Chesapeake Bay.

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