the Fall Line zone (black line) separates the Coastal Plain of eastern Virginia (yellow) from the hard bedrock of the Piedmont
Source: US Geological Survey, Virginia Geologic Map Data
The edge of the Piedmont/Coastal Plain, where various rivers cross from hard bedrock to soft sediments, is marked by a zone of rapids and waterfalls called the Fall Line where Virginia rivers drop 50-100' in elevation. The Fall Line is closer to a zone of turbulent flow; it is not single, fixed line with a 50-100' tall waterfall at the end.
The eastern edge of the Fall Line is where rivers reach sea level. The western edge of the Fall Line can be marked as the spot where the gradient of a river gets steeper, as the channel from the Blue Ridge begins to cut through the bedrock at a faster rate.
In Richmond, the zone is seven miles wide. It stretches from 100' in elevation at Bosher's Dam downstream to Mayo Island, where the water reaches sea level where 14th Street Bridge crosses the James River. Half of the elevation loss is in the last half-mile, next to Richmond's downtown. The urban rapids are popular with whitewater enthusiasts.1 /p>
the Fall Line zone in Richmond ends at Mayo Island
Source: ESRI, ArcGIS Online
John Smith was the first European to report on this natural feature. In April, 1607, Captain Christopher Newport and John Smith led an expedition upstream from the site just chosen for Jamestown, until rapids at the current location of Richmond blocked further exploration by ship:2
Captain Newport's exploration up the James River in April, 1607 ended with the discovery of rapids near what is now called Mayo's Island in downtown Richmond (direction of flow indicated by arrow)
Source: US Geological Survey (USGS), Richmond 7.5x7.5 topographic quadrangle (Revision 1, 2013)
The Fall Line, which has been part of Virginia's landscape since the formation of the Atlantic Ocean 200 million years ago, is a geologic feature that has had great impact on the cultural geography of Virginia. That physical pattern of rapids/waterfalls blocked ships from sailing further upstream, limiting water-based transportation of the European colonists. The natural geologic barrier to shipping delayed European settlement of the Piedmont and shaped the location of major Virginia's cities, including Alexandria, Fredericksburg, Richmond, and Petersburg.
Great Falls, on the Potomac River, reveals the hard metamorphic rock of the Piedmont - and how rivers in flood stage carve channels
Today, Interstate 95 is a rough guide to the location of the geologic boundary that separates the soft sediments of the Coastal Plain physiographic province from the hard bedrock of the Piedmont physiographic province. East of Interstate 95, the soil of the Coastal Plain is sandy. The Coastal Plain soils are light-colored, sometimes almost yellow or even white. Topography is flat, but there are a few hills in the Coastal Plain because erosion has been uneven on the sedimentary formations. Some cliffs have been exposed where rivers have scratched out their valleys.
Fall Line (red line) separates the Coastal Plain from the Piedmont, essentially the route of modern I-95 in Virginia
Source: US Geological Survey (USGS), ">A Tapestry of Time and Terrain
West of Interstate 95 and Interstate 85 in the Piedmont, various plowed fields and construction sites expose reddish clay soils. Topographically, the land rises gradually in elevation towards the Blue Ridge. Virginia rivers flow eastward from the Blue Ridge and across the Piedmont to the Fall Line, then on to the Chesapeake Bay or to Albemarle/Pamlico Sound in North Carolina. Water from just one major Piedmont stream, the Ararat River in Patrick and Carroll counties, flows south though North and South Carolina and merges into the Yadkin and Pee Dee rivers before entering the Atlantic Ocean.
The Fall Line was created naturally. It reflects the different geologic histories of the Piedmont and Coastal Plain.
as Africa and North America collided, the sediments in the Iapetus Ocean were compressed and metamorphosed to become part of the bedrock in the Piedmont physiographic province
Source: Ron Blakely, "Paleogeography and Geologic Evolution of North America," Late Mississippian (325 million years ago)
Piedmont Geology - West of the Fall Line
The bedrock directly to the west of the Fall Line, the Piedmont physiographic province, was created between 250-450 million years ago in the tectonic collisions that formed Pangea. Various volcanoes, crustal fragments, and soft sediments were scrunched up from the ocean bottom and pushed onto the North American continent in a series of mountain-building events known as the Taconic, Neo-Acadian, and Alleghenian orogenies.
In those orogenies, chunks of silica-rich crust in island arcs similar to Japan or Indonesia, eroded sediments that accumulated next to the island arcs on the ocean floor, and basalt ocean crust were added to Virginia. Virginia got wider as it accreted new crust at its Iapetus Ocean coastline. In addition, the mud, sand, and soft sediments on the Iapetus Ocean's Continental Shelf were compressed, metamorphosed, and pushed onto Virginia with each orogeny.
The terranes that were squeezed onto the edge of Virginia when Pangea formed are now exposed on the surface in the Piedmont physiographic region.
Those terranes were metamorphosed about 250 million years ago, as Africa bumped into the North American continent and the Iapetus Ocean was terminated with extreme prejudice. Once again, Continental Shelf sediments were squeezed onto Virginia.
As the continental plates pressed together, the soft sands, shales, and muds - equivalent to what is on the bottom of today's Chesapeake Bay and Atlantic Ocean off the coast at Virginia Beach - were buried, baked, and metamorphosed. They became the hard crystalline rock now located between I-95/I-85 and the Blue Ridge. The collision formed the bedrock of today's Piedmont, the largest physiographic province in Virginia today.
the Fall Line extends from New York to Georgia
Source: US Geological Survey (USGS), Hydrogeologic Framework Of The Virginia Coastal Plain (Professional Paper 1404-C)
About 250 million years ago, as the Iapetus Ocean disappeared and Africa merged with North America, Virginia ended up in the middle of the supercontinent known as Pangea. A tall mountain range was located where I-95 is today. It had been pushed up by the tectonic plate collisions, just as the Himalaya mountain range has been uplifted over the last 50 million years by the collision of India with Asia. The granitic roots of those mountains extended down as much as the mountains towered up above the surface, perhaps 20,000 feet.
The Fall Line developed much, much later, but its beginning was the formation of that hard Piedmont basement rock and the roots of those ancient mountains. The old continental shelf of the Iapetus Ocean and the terranes accreted onto Virginia during the Taconic and Acadian orogenies were metamorphosed, while the igneous mountain roots cooled to form additional hard bedrock. The basement consists of the rocks formed during the Grenville orogeny one billion years ago, the younger metamorphosed rocks formed during the Taconic, Neo-Acadian, and Alleghenian orogenies, and the igneous chunks remaining from old mountain roots and the terranes that were accreted onto the continent.
What goes up must come down. After Pangea split up, the 20,000 feet high mountains eroded away over 200 million years to expose the Piedmont's bedrock.
The metamorphic and igneous formations of the Piedmont do not stop at modern I-95/I-85. That hard crystalline bedrock is buried below the soft sediments of the Coastal Plain, and the "basement complex" continues all the way from the Blue Ridge to the edge of the Continental Shelf. Wells drilled through the coating of sediments east of the Fall Line encounter the crystalline bedrock at ever-greater depths to the east. Underneath the Eastern Shore, the basement complex is about 10,000' deep.
The last eastern exposure on the surface of the Piedmont bedrock is zone of waterfalls and rapids known as the Fall Line. When Gov. Alexander Spotswood led an expedition west across the Piedmont and across the Blue Ridge to the Shenandoah River in 1716, his party discovered that the sandy soils common near Williamsburg were rare west of the Fall Line.
The trip through the Piedmont crossed soils with hard stones which had formed as the crystalline bedrock decayed into saprolite. According to legend, Spotswood later gave mementos in the form of horseshoes to his fellow travelers and they became known as "Knights of the Golden Horseshoes," because on the trip:3
how bedrock dips underneath the Coastal Plain, between Fall Line and Atlantic Ocean
Source: US Geological Survey, Professional Paper 1404-C: Hydrogeologic Framework of the Virginia Coastal Plain (Figure 5)
Coastal Plain Geology - East of the Fall Line
East of the Fall Line, the geology story is simpler. The ground exposed at the surface, the material of concern to farms and builders, consists of sediments that are younger and softer than the Piedmont bedrock.
Much of those sediments in the Coastal Plain eroded off of the ancient Appalachian Mountains that were pushed up in the final Alleghenian orogeny. For the last 300 million years, rain and wind has etched away those mountains, carrying sand and mud westwards towards Illinois and eastward towards the Atlantic Ocean. Road cuts through West Virginia expose the layering of the oldest layers of sediments, even some dating back to erosion from the mountains that were lifted up by the Taconic and Acadian orogenies.
On the eastern side of the old Appalachians, the oldest sedimentary layers from the Taconic and Acadian orogenies disappeared when Pangea broke up and Africa moved away. The sedimentary layers still present in eastern Virginia have been deposited for the last 150 million years, and document the final stages as the ancient Appalachians once located along I-95 have eroded to a flat plain. Only their igneous roots remain, exposed in granite quarries and outcrops at Occoquan, Goochland County, Richmond, and Petersburg.
In addition to sediments deposited by mountain erosion, marine sediments were deposited in eastern Virginia when sea level was high enough. Today's Coastal Plain includes sediments washed off the ancient Appalachians over the last 150 million years, plus deposits from the Atlantic Ocean when it covered the land up to (and occasionally beyond) the current location of I-95/I-85.
the Fall Line separates the Coastal Plain (darker green...) and the Piedmont
Source: National Oceanic and Atmospheric Administration (NOAA), National Geophysical Data Center
Two processes occurred after the last orogeny in Virginia 300 million years ago to create the Coastal Plain: 1) freshwater rivers eroded the modern Appalachian Mountains and carried debris to the edge of the continent and 2) limestone and other minerals were deposited on top of submerged Virginia land when ocean levels were higher.
Neither process altered the sediments to convert them into hard crystalline rock. The sediments on the Coastal Plain have not been baked and squeezed tight; there has been no metamorphism comparable to the older bedrock exposed in the Piedmont now. That physical difference between the hard Piedmont bedrock and the soft Coastal Plain sediments is why there is a Fall Line today.
the Fall Line on the Rappahannock River led to the establishment of Fredericksburg/Falmouth and the location of Route 1
Source: US Geological Survey (USGS), Fredericksburg 7.5x7.5 topographic quadrangle (2013)
Early colonists moving west beyond the Fall Line quickly discovered locations where river currents regularly scoured the bottom of streams flowing eastward across the Piedmont. Those rivers have a thin coating of recent sediments; mud, sand and stones deposited on the river bottom since the last major flood make crossing the rivers difficult in most locations. Colonists located the fords and later built their roads to take advantage of the places where hard bedrock was exposed underwater in the Piedmont. It was far easier to cross those rivers at a ford where bedrock was exposed, compared to dragging a wagon through mud...
Such fords are very rare east of the Fall Line, except on small streams. River bottoms are soft Coastal Plain sediments east of the Fall Line; no underwater bedrock offers a hard bottom for an easy river crossing.
a Digital Elevation Model (DEM) shows how the Rappahannock River carved a broader valley in the soft Coastal Plain sediments, east of where I-95 crosses the river (red circle)
Source: US Geological Survey (USGS), National Map - DEM Viewer
In Tidewater, colonists avoided building a road network that crossed rivers. Instead, farmers carried tobacco and other agricultural products to wharves and docks built on the edge of the rivers. Shipping mostly by water minimized the need to get wagons across a river. When necessary, colonists such as George Mason III (father of the man who built Gunston Hall and was principal author of the Virginia Declaration of Rights) created ferry crossings to get across Coastal Plain rivers.
East of the Fall Line, the Piedmont bedrock is buried beneath young, unconsolidated sediments that hardly qualify to be labeled "rock." Rapids occur throughout the Piedmont physiographic province, from the Blue Ridge east to the Fall Line. The gradient at which a river channel drops can steepen briefly at knickpoints where rivers cross from harder to softer bedrock, reflecting ancient zones of different metamorphism or faults in the terranes. There are no such knickpoints on the Coastal Plain, and there are no rapids downstream of the Fall Line.
rapids occur in the middle of the Piedmont on the South Anna River northwest of Richmond, but there are no rapids on Virginia rivers downstream of the Fall Line
Source: ESRI, ArcGIS Online
When a river flowing east towards the Chesapeake Bay reaches the eastern edge of the Piedmont and encounters the easier-to-erode sediments on the Coastal Plain, the flowing water washes away those soft sediments. At the Fall Line, the energy of the river carves a deeper channel in the softer Coastal Plain sediments than in the Piedmont bedrock. The difference in the depth of the channel results in rapids and waterfalls; the water flowing off the Piedmont falls to sea level at the Fall Line.
Such rapids and waterfalls are most obvious at Great Falls on the Potomac River, on the Rappahannock River at Fredericksburg (look westward from the I-95 bridge), and on the James River near downtown Richmond (look westward from any bridge between I-95 to the Huguenot Bridge).
physiographic setting of Great Falls
Source: US Geological Survey (USGS) Bulletin 1471, The River and the Rocks: The Geologic Story of Great Falls and the Potomac River Gorge
The Fall Line waterfall on the Occoquan River near Lorton has been "dried out" by the construction of a dam, and only a trickle of water flows over the Occoquan Reservoir dam in the summer months. Fairfax Water kept the area closed to public use after acquiring the dam and reservoir complex in the 1960's, but starting in 2015 the utility agreed to open its Fall Line property upstream from the town of Occoquan to public visits. People will no longer need special permission (or have to sneak past the gates...) to see the exposed rocks at the Fall Line.
rapids at the Fall Line of the Occoquan River, below Occoquan Reservoir (Route 123 on right)
Source: US Geological Survey (USGS), Occoquan 7.5x7.5 topographic quadrangle (Revision 1, 2013)
The Fall Line zone between the Coastal Plain and the Piedmont physiographic provinces is wide, because the actual waterfalls may be miles upstream from the geologic boundary between the Piedmont and Coastal Plain bedrock. As the rivers etch their way through the sediments at the contact between Piedmont-Coastal Plain, the harder crystalline bedrock is exposed. The waterfalls are migrating upstream, towards the west, as the sedimentary coating is eroded. At a far slower rate, the waterfalls are disappearing on the eastern edge of the Fall Zone as the bedrock is etched away to sea level.
the last rapids on the North Anna River are just upstream from North Anna Battlefield Park, almost two miles west of I-95
Source: ESRI, ArcGIS Online
Today the hard crystalline rock exposed at Great Falls on the Potomac River is the bedrock in the Piedmont, while the eastern edge of that hard bedrock formation is downstream on Teddy Roosevelt Island. The contact between Coastal Plains sediments and exposed Piedmont is underneath the Teddy Roosevelt Bridge at the eastern end of I-66, where it crosses the Potomac River to connect Rosslyn with the District of Columbia at the Kennedy Center. The whitewater rapids of Great Falls are about 14 miles upstream of the geologic contact.
The Potomac River has etched its way 14 miles upstream over the last 2 million years. In the process, it has carved out Mather Gorge in the crystalline bedrock. The gradient of most Virginia rivers changes when the rivers are about 100' in elevation above sea level. Great Falls is close to the 140' contour line, because the large Potomac River has cut deeper into the bedrock. In comparison, the slow-flowing Mattaponi and Pamunkey rivers change their gradient at only the 50' elevation contour.4
Teddy Roosevelt Island marks the geologic boundary between the hard crystalline bedrock of the Piedmont and the soft sediments of the Coastal Plain
Source: US Geological Survey (USGS), Washington West 7.5x7.5 topographic quadrangle (2014)
500-million-year-old (Cambrian Period) crystalline bedrock outcrops are buried beneath younger Coastal Plain sediments and artificial fill east of Teddy Roosevelt Island and Rock Creek Park
Source: US Geological Survey (USGS), Geologic map of the Washington West quadrangle, District of Columbia, Montgomery and Prince George's Counties, Maryland, and Arlington and Fairfax Counties, Virginia (1994)
Over time, the erosive power of the Potomac River will move the waterfall further upstream, past Riverbend Regional Park in Fairfax County. Great Falls will move far enough upstream to get past the Piedmont bedrock, and to etch away at the softer Triassic Basin sediments near Seneca Creek.
Because the resistance of the bedrock will be less in the Triassic Basin, Great Falls will change into a series of gentler rapids over the sandstone of the Triassic Basin. It took 2 million years to etch a channel 14 miles, an average of one mile each 150,000 years. There are about eight more miles of Piedmont bedrock before reaching the Triassic Basin. Using very simplistic math, the remaining lifespan of Great Falls is about 1-1.5 million years.
migration of Potomac River waterfall from geologic boundary at I-66 bridge upstream to Great Falls (blue), and on to Seneca Creek (red) in next million or so years
Source: ESRI, ArcGIS Online
geologic setting of Great Falls
Source: US Geological Survey (USGS) Bulletin 1471,
The River and the Rocks: The Geologic Story of Great Falls and the Potomac River Gorge
The Fall Line may be interpreted as a 14-mile wide zone on the Potomac River, but the Virginia Department of Game and Inland Fisheries (DGIF) requires a specific location for the Fall Line to facilitate enforcement of different regulations for freshwater vs. anadromous/saltwater fish.
According to that state agency, the Fall Line zigs eastward from Richmond to Walkers Dam on the Chickahominy River. The Route 360 bridges over the Pamunkey and Mattaponi rivers are used to define the Fall Line, before zigging back to Route 1/Interstate 95 at Fredericksburg/Occoquan:5
Fall Line, as defined by Virginia Department of Game and Inland Fisheries
Map Source: US Geological Survey, National Map
rapids at the Fall Line of the Rappahannock River in Fredericksburg (I-95 just to left)
Source: US Geological Survey (USGS), Frdericksburg 7.5x7.5 topographic quadrangle (Revision 1, 2013)
rapids at the Fall Line of the Appomattox River at Petersburg (west of I-95)
Source: US Geological Survey (USGS), Petersburg 7.5x7.5 topographic quadrangle (Revision 1, 2013)
a dam on Aquia Creek at the Fall Line forms Smith Lake in Stafford County
Source: National Oceanic and Atmospheric Administration (NOAA), Sea Level Rise Viewer
no hydropower facilities in Virginia are located on the Coastal Plain - all rely upon the topographic relief at the Fall Line, or further west
Source: Energy Information Administration, State Profile and Energy Estimates - Virginia
Fall Line divides the Coastal Plain and Piedmont physiographic provinces
Source: US Geological Survey Design, Revisions, and Considerations for Continued Use of a Ground-Water-Flow Model of the Coastal Plain Aquifer System in Virginia, Water Resources Investigations Report 98-4085
relief map of Virginia
Source: US Geological Survey (USGS), Open File Report 99-11, Color Shaded Relief Map of the Conterminous United States