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 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
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.
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.
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.
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