Fall Line

the Fall Line separates the Coastal Plain (darker green...) and the Piedmont
the Fall Line separates the Coastal Plain (darker green...) and the Piedmont
Source: National Oceanic and Atmospheric Administration (NOAA) National Geophysical Data Center

East of Interstate 95, the soil of the Coastal Plain is sandy. It is light-colored (sometimes almost yellow or even white), and flat. There are few hills, though some cliffs have been exposed where rivers have scratched out their valleys. West of the interstate, the plowed fields of the Piedmont expose red clay and the land rises in elevation towards the Blue Ridge.

I-95 is a rough guide to the location of the geologic boundary known as the Fall Line, separating the soft sediments of the Coastal Plain from the hard bedrock of the Piedmont.

Fall Line
Fall Line parallels I-95... roughly
Source: EPA Chesapeake Bay Program, Bay Atlas Interactive Map Viewer
Fall Line topography and Geology
Fall Line (white line) separates the Coastal Plain from the Piedmont
Source: US Geological Survey (USGS), A Tapestry of Time and Terrain

400 million years ago, some of what today is hard Piedmont bedrock was mud and sand, soft sediments of the Outer Continental Shelf in the Iapetus Ocean located offshore from the ancient shoreline of Virginia. Much of the modern Piedmont was further offshore, part of volcanic island chains and fragments of crust in the Iapetus Ocean.

As the tectonic plates moved, those volcanoes, crustal fragments, and soft sediments were scrunched up from the ocean bottom and pushed onto the North American continent. (You can "scrunch" icing with a fork onto a piece of cake in the same way.) The transformation of Virgnia's coastline finished when Africa/Europe bumped into the North American continent and created the Appalachian Mountains.

In the process of being scrunched, the sediments from the Iapetus Ocean floor were squeezed and baked. The former Outer Continental Shelf off the Virginia shoreline in the Iapetus Ocean were transformed and glued to the old edge of the North American continent. The offshore muds and sands were metamorphosed into the hard rock that now underlies much of Virginia between I-95 and the Blue Ridge.

In addition, as the Iapetus Ocean closed, offshore islands comparable to modern Japan and Indonesia were "docked" against the continental edge and their crust was added (accreted) to the mix. The metamorphosed sediments and crust are exposed clearly now at Great Falls, on the Potomac River just upstream from Washington, DC.

Great Falls, on the Potomac River
Great Falls, on the Potomac River

After Africa and North America collided and pushed up the Appalachian Mountains, the combined continents then split apart. The crack where they split filled with salty seawater and formed the Atlantic Ocean. When the Atlantic Ocean formed initially, the Virginia shoreline was at the eastern edge of the Piedmont (now roughly the route of I-95).

Over the last 200 million years, sediments have washed down as the mountains eroded. Those sediments have accumulated east of the Piedmont, helping to form the Coastal Plain. Virginia's land area has expanded to the east, as the North American tectonic plate has drifted to the west and the Coastal Plain has widened.

The water levels in the Atlantic Ocean have risen at times. When water rose high enough to cover the eastern edge of Virginia, marine sediments were deposited onto the Coastal Plain. Coastal Plain sediments were deposited by two processes: 1) when ocean levels were higher and 2) as freshwater rivers eroded the modern Appalachian Mountains and carried debris to the edge of the continent.

The sediments on the Coastal Plain, deposited after the continents started to split apart and the Atlantic Ocean formed, have not been baked and squeezed tight. Most sediments located east of the Piedmont are not hardened like the metamorphosed bedrock to the west, and that physical difference is why there is a Fall Line today.

Fall Line
The zone of the "Fall Line"

When today's Virginia rivers flow eastward from the Piedmont onto the Coastal Plain, they leave the Piedmont where the river's bottom is hard rock (with a thin coating of mud deposited since the last flood). When the rivers encounter the easier-to-erode Coastal Plain, the water etches into those soft sediments. The energy of the water carves a deeper channel in the softer sediments, creating waterfalls.

The edge of the Piedmont/Coastal Plain is marked by a line of waterfalls (the Fall Line) where various rivers move from harder to softer bedrock. The 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). However, the 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. On the rare occasion that Fairfax Water opens its Fall Line property to public visits, you can see the exposed rocks at the Fall Line by walking upstream from the town of Occoquan.

The Fall Line is really a zone rather than just a narrow line. The rapids and waterfalls may extend up to a mile. The zone between the Coastal Plain and the Piedmont physiographic provinces may be drawn even wider, because the actual waterfalls may be far upstream from the geologic boundary between the Piedmont and Coastal Plain bedrock.

migration of Potomac River waterfall from geologic boundary at I-66 bridge upstream to Great Falls
migration of Potomac River waterfall from geologic boundary at I-66 bridge upstream to Great Falls
Source: US Geological Survey (USGS) The National Map

For example, the bedrock in the Piedmont is the hard crystalline rock that you can see at Great Falls on the Potomac River. The eastern edge of that hard rock formation is downstream on Teddy Roosevelt Island, at the end of I-66 where it crosses the Potomac River on the Teddy Roosevelt Bridge (connecting Rosslyn with the District of Columbia at the Kennedy Center). Great Falls is 15 miles upstream, showing how the Potomac River has etched its way upstream and carved out Mather Gorge in the crystalline bedrock over the last 2 million years.1

physiographic setting of Great Falls
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

geologic setting of Great Falls
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 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:2

The Fall Line is defined as the following landmarks:
Rappahannock River: Rt. 1 Bridge;
Mattaponi River: Rt. 360 Bridge;
Pamunkey River: Rt. 360 Bridge;
Chickahominy River: Walkers Dam;
James River: 14th Street Bridge;
Occoquan River: I-95 Bridge

Fall Line, as defined by Virginia Department of Game and Inland Fisheries
Fall Line, as defined by Virginia Department of Game and Inland Fisheries
Map Source: US Geological Survey, National Map

How the Fall Line Shaped Powhatan's Area of Control

How the Fall Line Shaped Colonial Settlement in Virginia

Waterfalls of Virginia

rapids at the Fall Line of the Occoquan River, below Occoquan Reservoir (Route 123 on right)
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)

rapids at the Fall Line of the Rappahannock River in Fredericksburg (I-95 just to left)
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 James River in downtown Richmond (direction of flow indicated by arrow)
rapids at the Fall Line of the James River in downtown Richmond
(direction of flow indicated by arrow)
Source: US Geological Survey (USGS), Richmond 7.5x7.5 topographic quadrangle (Revision 1, 2013)

rapids at the Fall Line of the Appomattox River at Petersburg (west of I-95)
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)

Links

the Fall Line extends from New York to Georgia
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)

References

1. "The Origin Of The Potomac River Valley And The Carving Of Great Falls," The River and the Rocks: The Geologic Story of Great Falls and the Potomac River Gorge, US Geological Survey Bulletin 1471 - Section 4, 1970, http://www.nps.gov/history/history/online_books/grfa/ (last checked september 4, 2011)
2. "Game/Sport Fish Regulations," Virginia Department of Game and Inland Fisheries, http://www.dgif.virginia.gov/fishing/regulations/game.asp (last checked August 15, 2012)

Fall Line divides the Coastal Plain and Piedmont physiographic provinces
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
relief map of Virginia
Source: US Geological Survey Open File Report 99-11, Color Shaded Relief Map of the Conterminous United States


The Real First Families of Virginia
River and "Fall Line" Cities
Regions of Virginia - and Why Isn't There An East Virginia?
Virginia Places