Coastal Plain

the Coastal Plain is the land between the Fall Line and the shoreline
the Coastal Plain is the land between the Fall Line and the shoreline
Source: Virginia Division of Mineral Resources Bulletin 82, Post-Miocene Stratigraphy and Morphology, Southeastern Virginia (Figure 7)

The Coastal Plain extends eastward from the Fall Line to the shoreline of the Atlantic Ocean. Sediments deposited since the Cretaceous Period cover crystalline bedrock. The basement bedrock is one billion years old, formed during the Grenville Event when the supercontinent of Rodinia was created.

The surface of the Coastal Plain is unconsolidated sediments. Modern construction projects require no blasting; bulldozers and shovels can move the compacted sediments with little difficulty.

Native Americans living east of the Fall Line made stone tools from cobbles in the streambeds, since there were no rock outcrops or suitable sites for quarries. The earliest Virginians also traveled and traded to acquire better materials from west of the Fall Line, including hard metamorphic rock from the Blue Ridge and the Uwharrie Mountains now in North Carolina.

Early colonists found no stone to use for house foundations, and throughout the 1600's most strutures were made of readily available wood. The gentry displayed their wealth in the 1700's by using enslaved labor to make bricks for mansions such as Westover, and importing stone. The pavers installed in 1735 on the floor of Christ Church in Middlesex County were brought from southern England, carved out of the Purbeck Limestone Group.1

limestone pavers on the floor of Christ Church were imported from England, since there were no rock outcrops on the Coastal Plain of Virginia
limestone pavers on the floor of Christ Church were imported from England, since there were no rock outcrops on the Coastal Plain of Virginia
Source: ESRI, ArcGIS Online

The sand, clay, and silt were deposited when sea level was higher. Erosion from the Blue Ridge brought sand and clay particles downstream, and at sea level they settled to the bottom of the ocean. Marine deposit also included silica-rich and calcium-rich shells from phytoplankton and zooplankton. Remnants of ocean reefs, sharks teeth, and whale fossils are scattered within the Coastal Plain.

Since North Anerica and Africa split apart, sea levels have changed dynamically as tectonic plates and the climate shifted. When a bolide (comet or meteor) struck what is now the Chesapeake Bay 35 million years ago, there was no bay. The shoreline was west of Richmond, and marine sediments were accumulating on what is now part of the Piedmont as well as the current Coastal Plain.

Many of the sediments which accumulated during high stands of the Atlantic Ocean later eroded away when the sea level dropped. Sediments deposited when shorelines reached furthest west have washed away, exposing the metamorphic bedrock and soils of the Piedmont physiographic province. The edge of the Coastal Plain is where sediments still remain, itting on top of the same sort of metamorphic rocks exposed further west to the edge of the Blue Ridge. The boundary between the Coastal Plain and the Piedmont is marked roughtly north of Petersburg by I-95, and I-85 south to Raleigh.

Geological studies completed before construction of the Surry Nuclear Power Plant revealed that all deposits there during the Miocene Epoch have washed away, along with the last deposits with settled on the ocean floor in the older Miocene and the first deposits of the more-recent Pleistocene epochs. After assessing how the Miocene deposits had been compressed by the overburden, geologicst calculated that 150-200 feet of sediments once at that site eroded away before newer sediments accumulated there during the Pleistocene.2

During the Ice Age, sea levels dropped by roughly 400 feet and Virginia's Coastal Plain extended an additional 40 miles to the east. About 18,000 years ago, sea levels began to rise again, and flood that land. What is now the Outer Continental Shelf is comparable to today's Coastal Plain, with a thin layer of recent sediments on top.

The remaining Coastal Plain, between the Piedmont and the modern shoreline, is a flat surface with a very gentle dip from the Fall Line to the ocean. The flatness is interrupted by two topographical features, cliffs and scarps.

There are steep cliffs on major river valleys. Smaller streams have carved their own valleys into the sediments.

At Westmoreland State Park, the Potomac River has etched a channel with cliffs that expose sharks teeth, whale vertebrae, crocodile bones, and other fossils buried in the Miocene-age sedimentary formations. Beaches directly beneath eroding cliffs there and at Stratford Hall Plantation are closed to fossil collecting because of safety concerns. A teenager died in 1975 after a section of the Stratford Cliffs collapsed and the landside crushed him.

there are cliffs on the Coastal Plain, including Westmoreland State Park
there are cliffs on the Coastal Plain, including Westmoreland State Park
Source: Department of Conservation and Recreation, Westmoreland State Park Master Plan Executive Summary (2017 Update)

On the Rappahannock River, the light-colored diatomaceous earth of Fones Cliffs is exposed for four miles on the river's north bank. In 2015, a developer got authorization from Richmond County to build the "Rappahannock Cliffs" golf course and luxury resort. Near the edge of the 75' tall cliff, 22 houses would have spectacular views to the west.

The Chesapeake Bay Foundation and other conservation groups opposed the project, noting the environmental sensitivity of the area. After the developer cleared 13 acres of forested land next to the cliffs for a golf course, but failed to obtain the required permits of install the appropriate stormwater controls, a section of the cliffs collapsed. In response, the state of Virginia sued to block the development from adding excessive sediment to the Rappahannock River. The normal process of erosion had been accelerated by the clearing of the forest above the cliff.

Another developer with adjacent property planned to build 200 housing units in 10 condominiums. In 2018, he abandoned his plans and agreed to sell the property to a conservation group and ultimately have it transferred to the US Fish and Wildlife Service's Rappahannock River Valley National Wildlife Refuge.3

clearing the forest at Fones Cliffs created a surge of erosion and cliff collapse
clearing the forest at Fones Cliffs created a surge of erosion and cliff collapse
Source: Friends of the Rappahannock, Fones Cliffs

Scarps were created by series of sea level changes. The Atlantic Ocean has flooded over and retreated from the Coastal Plain since the Cretaceous Period, when Pangea split up. During different periods of steady sea level, the waves at the temporary shoreline etched into sediments that had been deposited previously and built barrier islands, dunes, and beaches. When sea level dropped later, a ridge of sand was left inland of the new shoreline.

A 1973 report from the Virginia Division of Mineral Resources noted that the remaining Coastal Plain:3

is known for its "stair-step" appearance... consisting of wide, gently eastward-sloping plains separated by linear, steeper, eastwardfacing scarps. Earlier workers called these features, respectively, benches or terraces and scarps or escarpments. The plains are successively lower and less dissected from west to east; thus it was inferred that the higher western plains are older. The plains slope eastward at less than 2 feet per mile, whereas the scarps have slopes of as much as 50 to 450 feet per mile through short distances.

the topography of the Coastal Plain is marked by almost-flat terraces separated by steeper scarps, plus stream channels eroded into the sediments
the topography of the Coastal Plain is marked by almost-flat terraces separated by steeper scarps, plus stream channels eroded into the sediments
Source: Virginia Division of Mineral Resources Bulletin 82, Post-Miocene Stratigraphy and Morphology, Southeastern Virginia (Figure 6)

Links

wedges of sediment cover the tilting Coastal Plain bedrock known as the Basement Complex
wedges of sediment cover the tilting Coastal Plain bedrock known as the Basement Complex
Source: US Geological Survey, Hydrogeologic Framework of the Virginia Coastal Plain (Figure 5)

References

1. Marcus M. Key, Jr., Robert J. Teagle, Treleven Haysom. "Provenance of the Stone Pavers in Christ Church, Lancaster Co., Virginia," Quarterly Bulletin of the Archeological Society of Virginia, Volume 65, Number 1 (2010), https://scholar.dickinson.edu/cgi/viewcontent.cgi?article=1298&context=faculty_publications (last checked February 12, 2019)
2. "Surry Power Station Units 1 and 2 Application for Subsequent License Renewal, Appendix E," Dominion Power submission to Nuclear Regulatory Commission, October 2018, page E-3-57, https://www.nrc.gov/docs/ML1829/ML18291A834.pdf (last checked February 12, 2019)
3. "Exploring Stratford's cliffs," Free Lance-Star, November 23, 2001, https://www.fredericksburg.com/local/exploring-stratford-s-cliffs/article_2dcaf577-c647-5988-b62b-1aba2bf9f19e.html; Plan," Rappahannock Cliffs, http://rappahannockcliffs.com/html/plan.html; "Developer shelves condo plan for Fones Cliffs, will sell to conservation group," Free Lance-Star, October 3, 2018, https://www.fredericksburg.com/news/va_md_dc/developer-shelves-condo-plan-for-fones-cliffs-will-sell-to/article_2c64bab9-ea47-5cd2-a00f-f50e6042e826.html; "Fones Cliffs," Chesapeake Bay Foundation, https://www.cbf.org/about-cbf/locations/virginia/issues/fones-cliffs.html; "Developerís environmental violations at historic Richmond County cliffs referred to attorney general," Virginia Mercury, October 1, 2018, https://www.virginiamercury.com/2018/10/01/developers-environmental-violations-at-historic-richmond-county-cliffs-referred-to-attorney-general/ (last checked February 14, 2019)
4. Robert Q. Oaks Jr., Nicholas K. Coch, "Post-Miocene Stratigraphy and Morphology, Southeastern Virginia," Virginia Division of Mineral Resources Bulletin 82, 1973, p.8, https://www.dmme.virginia.gov/commercedocs/BUL_82.pdf (last checked January 21, 2019)

the Coastal Plain (light tan) is located on the eastern edge of the North American continent
the Coastal Plain (light tan) is located on the eastern edge of the North American continent
Source: US Geological Survey, North America Tapestry of Time and Terrain


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