The Fall Line, which has been part of Virginia's landscape since the formation of the Atlantic Ocean, is a geologic feature that limited water-based transportation of the European colonists who began arriving in 1607. The Fall Line is a set of rapids and waterfalls that blocked ships from sailing further upstream. The natural barrier to shipping shaped the location of major Virginia's cities, including Alexandria, Fredericksburg, Richmond, and Petersburg.
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. 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.
Fall Line parallels I-95... roughly
Source: EPA Chesapeake Bay Program, Bay Atlas Interactive Map Viewer
Fall Line (white line) separates the Coastal Plain from the Piedmont
Source: US Geological Survey (USGS), A Tapestry of Time and Terrain
The bedrock directly to the west of the Fall Line, the Piedmont physiographic province, was created 200-450 million years ago through a series of mountain-building events known as the Taconic, Acadian, and Alleghenian orogenies. In the series of orogenies, chunks of silica-rich crust, island arcs similar to Japan or Indonesia, and ultimately the continent of Africa smashed into Virginia's coastline.
About 250 million years ago, Africa/Europe bumped into the North American continent and created the Appalachian Mountains. The Iapetus Ocean disappeared, Virginia was located in the middle of Pangea, and there was no Fall Line - but the geological pattern had been created for its future development.
In the 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. As the tectonic plates moved, the mud, sand, soft sediments of the Iapetus Ocean's Outer Continental Shelf was converted (metamorphosed) into rock hard rock. Today, that metamorphic rock (together with chunks of the crust and island arcs squeezed onto the edge of Virginia when Pangea formed) is exposed on the surface of the Piedmont physiographic region between the Blue Ridge and I-95.
Great Falls, on the Potomac River, reveals the hard metamorphic rock of the Piedmont - and how rivers in flood stage carve channels
East of I-95, the hard rock of the Piedmont continues, extending eastward all the way to the edge of the Outer Continental Shelf - but the bedrock is buried underneath the soft sediments of the Coastal Plain. Those sediments have been deposited over the last 200 million years, as the mountains pushed up in the final Alleghenian orogeny have been eroded by rain and wind. The sand and mud washed off the eastern edge of the mountains have been carried eastward by various rivers, and dumped at the shoreline of the Atlantic Ocean to form the Coastal Plain. In addition, when sea level was high enough to cover the eastern edge of Virginia, marine sediments were deposited on top of the bedrock.
Today's Coastal Plain includes both sediments washed off the mountains and deposits from the Atlantic Ocean, created by two processes: 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. In contrast to the metamorphic bedrock exposed in the Piedmont, the sediments on the Coastal Plain have not been baked and squeezed tight. That physical difference is why there is a Fall Line today.
When today's Virginia rivers flow eastward across the Piedmont, river bottoms are hard rock with a thin coating of mud, sand and stones deposited since the last flood. When the rivers encounter the easier-to-erode sediments on the Coastal Plain, the flowing 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.
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
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