Barrier islands are typically thin strips of sandy beaches on the ocean side, with a line of sand dunes in the middle and a salt-water marsh between the dunes and a lagoon/bay. A maritime forest may grow behind the dunes, if the island is wide enough.
Barrier islands are constantly re-shaped by storms. Overwash fans develop when waves breach the dunes, carrying sand inland and depositing it at flood tide behind the dunes - sometimes covering the maritime forest or marsh with a layer of fresh sand, but often filling some of the open water in the bay/lagoon. Barrier islands are widened by such flood-tide deposits, especially after saltmarsh cordgrass (Spartina patens) extends into the new deposit and starts growing to expand the marsh, but currents and storms can simultaneously narrow barrier islands by eroding sand from the ocean side.
If a storm surge raises the height of water in the bay, a flood in the other direction may burst through the dune line and create an ebb-tide sand bar on the ocean side of the island. Such sand bars are quickly eroded by wave action, with sand carried south on the Virginia coast by longshore currents. Southern tips of Assateague Island and Hog Island are growing today, while the northern tips are eroding away.
on Virginia's Eastern Shore, barrier islands extend from Assateague Island (with Tom's Cove on southern tip) to Fisherman's Island - and south of the Chesapeake Bay, additional islands extend to the North Carolina line at False Cape State Park
Source: National Aeronautics and Space Administration (NASA), Earth Observatory - Fall Colors in the Mid-Atlantic
The shifting landscape of barrier islands is not a recent discovery. Over a century ago, one writer commenting about the earlier settlement history of Hog Island noted:1
There are over 2,000 barrier islands worldwide, and roughly 400 barrier islands along the coastline of the Atlantic Ocean and Gulf of Mexico.3 The Delorme Atlas and Gazetteer for Virginia names about 20 major barrier islands on the Virginia coast, with a chain of sandy strips backed by marshes from Assateague to Fisherman's Island on the Atlantic Ocean side of the Eastern Shore. South of the Chesapeake Bay, the barrier islands are welded to the mainland from Cape Henry to Sandbridge, and are not actively migrating for the moment. At Back Bay, a new chain of active barrier islands starts, extending south to include Nags Head and the Outer Banks in North Carolina. At False Cape State Park, on the southeastern tip of Virginia, the now-abandoned community of Wash Woods indicates clearly how flood tides and storm surges can bring sand from the ocean side to the bay side of a barrier island.
Worldwide, there are perhaps 15,000 additional "fetch-limited" barrier islands in low energy settings, sheltered from strong wave action because the distance of open water (fetch) near the island is narrow - less than 20 miles, compared to the thousands of miles on the Atlantic Ocean side of the Eastern Shore. Fetch-limited barrier islands such as Gwynn Island and Plum Tree Island are common in the Chesapeake Bay, where they are protected from the strong swell of ocean waves by the Eastern Shore. Thin sandy beaches protect marshes from wave action, or the marshes themselves fringe the island. Dunes are rare, and the pattern of storms shapes the barrier islands in the Chesapeake Bay:4
In some cases, such as Tangier Island (balanced on the buried watershed divide of the Nanticoke and Choptank Rivers), wind blows across the Chesapeake Bay with equal fetch in two directions. The waves have created east- and west-facing barrier systems with lagoons and rapidly-changing ebb/flood tide deltas. Such islands are typically sand starved, and likely to disappear into underwater shoals once their saltmarsh anchors are drowned by rising seas.
Current barrier islands are only about 6,000 years old, but Virginia's first barrier islands formed as much as 200 million years ago after the Atlantic Ocean opened and sediments were deposited on the coastline. The source of the sand particles on modern barrier islands is ultimately the Appalachian Mountains and even Canada.
When rocks erode from the mountains, many minerals dissolved completely, but the chemical bonds of quartz - silicon dioxide, SiO2 - are highly resistant. Quartz remains a solid rock, but is crushed into small grains as rocks wash off the mountains and are carried downstream by rivers. Sand, clay, and gravel particles that survive the transport process from the mountains have been deposited at the shoreline, shifted by currents and waves, re-arranged by rivers when sea level dropped and exposed the Coastal Plain, then moved one final time as sea levels rose when the last Ice Age ended.
Erosion and currents/wave action have created many versions of the coastal shoreline, prior to the modern Delmarva Peninsula. At barrier islands and on eroding edges of rivers, the particles are still moving...
the Potomac Formation is exposed on its western end, but buried deep below the Eastern Shore
Source: US Geological Survey Engineering Geology and Design of Slopes for Cretaceous Potomac Deposits in Fairfax County, Virginia, and Vicinity (Geological Survey Bulletin 1556)
The Coastal Plain was built over the last 145 million years, starting in the Cretaceous Period when the Atlantic Ocean shoreline was roughly where I-95 is located today. Rock particles from the Appalachian Mountains to the west washed down into the ocean and settled as marine deposits.
The lowest sedimentary layer of the Coastal Plain is called the Potomac Group. It lies on top of the "basement complex" of crystalline rock that dates back to the formation of Pangea or its predecessor supercontinent, Rodinia. The Potomac Group is exposed at the surface on its western edge in Alexandria, but is at the bottom of over 6,000 feet of sediments underneath Accomack/Northampton counties on the Eastern Shore. The youngest sediments at the surface now form the barrier islands.5
aquifers beneath the Coastal Plain are located in sedimentary formations that are up to 145 million years old
Source: US Geological Survey A Surficial Hydrogeologic Framework for the Mid-Atlantic Coastal Plain (Figure 2)
Stingaree Point is growing as longshore currents carry sand south, but currents in Quinby Inlet keep a channel clear - for the moment - between Parramore Island and Hog Island
Source: US Geological Survey 7.5 minute topographic map, Quinby Inlet
heavy, dense grains of hornblend/magnetite/rutile resist movement by wind while dry grains of quartz migrate easier, forming lineaments of dark and light sand on Assateague Island
| In the last 10 million years, glaciers scraped across New York state
and deposited Long Island, and then as the ice retreated glacial meltwater
carried massive amounts of sediment downstream to the ocean. The Delaware,
Susquehanna, and Potomac rivers carried eroded sand and gravel eastward
to form the Delmarva Peninsula. At the end of the Pliocene and throughout
the Pleistocene, sea level dropped and Coastal Plain sediments were shifted
around in river deltas and outwash plains. The loose, unconsolidated sediments
that form Virginia's barrier islands were carried south by ocean currents
from the mouths of the Delaware River and even the Hudson River. Currents
along the Atlantic Ocean shoreline moved material south, blocking river/stream
exits and creating offshore sandbars.6
The sand grains in a modern dune on Virginia's Eastern Shore may once have been bedrock in Pennsylvania, New York, Quebec, or Ontario. Today, little sand washes all the way from the Appalachian Mountains to the Atlantic Ocean. Most large particles eroding off the Blue Ridge today will settle out at the Fall Line, where the rivers reach sea level. We can see cliffs at Westmoreland State Park and elsewhere on the western side of the Chesapeake Bay where erosion is still recycling Coastal Plain sediments into sandy beaches, but those are west of the Eastern Shore. Barrier islands are located on the Atlantic Ocean side, not the Chesapeake Bay side, of the Eastern Shore.
As the sea began flooding over the Continental Shelf at the end of the Ice Age 18,000 years ago, wave action eroded the "headlands" of high ground separating the many creeks that had flowed onto the Continental Shelf. The high ground (watershed divides) was composed of sediments deposited since the Cretaceous Period, reworked in previous retreats and advances of the ocean - so the eroded-and-deposited-and-eroded-and-redeposited sediment particles were mobilized one more time.
As the Continental Shelf drowned, waves and currents shifted the eroded-once-again sediments and the new bottom of the Atlantic Ocean was covered with offshore shoals. However, the shift in the shoreline, as it migrated from the Continental Slope westward to near its current location, may have been too rapid for development of barrier islands.
Once sea level rise slowed down, however, sand on the Virginia coast could have pushed by wave action into dunes, and those dunes shaped by northeasters and other storms into exposed shoals - barrier islands. The chain of barrier islands on the coast of North Carolina were in place by 1,500 BC, though hurricanes or a tsunami washed them away around 900 AD and the barrier chain at Hatteras was not reestablished until 1500 AD.7
Off Virginia's Eastern Shore, Assateague Island is a spit formed from sediments eroded from the Delmarva headland to the north, now Cape Henlopen. Erosion from the headland today does not provide sufficient quantities of sand to build barrier islands south of Assateague Island, however.
Barrier islands between Assateague and the southern tip of the Eastern Shore are a tide-dominated system, formed from sediment eroded off local headlands and accumulating on the high points of local drainage divides that have been submerged due to sea level rise. South of Assateague, the tidal channels are where the ancient streams once flowed, while the barrier islands are perched on the old ridges between those streams.8
cliff eroding to recycle sediments
Source: US Fish and Wildlife Service
"Solid earth" is more mobile than solid, when barrier islands are involved. The National Park Service summarized how sand moves on barrier islands:9
The westward migration of barrier islands on the Virginia coastline is a natural process; each barrier island is constantly being reshaped by ocean currents and storm surges. Oyster shells and tree stumps that once grew on the bay/lagoon side may be exposed and eroded from the ocean side, as the sand of a barrier island migrates. During this marine transgression, shells may be buried in the marsh in an oxygen-free reducing environment and then exposed to oxygen on the ocean side, staining the shells unusually black with deposits of iron pyrite and goethite.10
Building anything on such islands is risky, because the islands are moving. Humans can affect the deposition or erosion of sand by building seawalls or dredging channels, but we can't stop the incessant pressure of the waves as sea level rises and shifts the sand.
The natural pattern has a substantial impact on development potential, and on the cost of protecting such development. Sand moves; roads and houses are inflexible. As described by the Department of the Interior in 1979:11
In the United States, people are moving to barrier islands despite the risks. On barrier islands, population densities are three times the density of the coastal states on average.12
Major development has already occurred on barrier islands in the Town of Chincoteague and the City of Virginia Beach. In addition, the Federal government has developed Wallops Island for launching rockets. The remaining barrier islands on the Atlantic Ocean side of the Eastern Shore are protected in their natural condition by state/Federal agencies and a private conservation organization, The Nature Conservancy. The Nature Conservancy acquired its holdings in anticipation of "flipping" the real estate, reselling the islands to a Federal agency for permanent protection. When economics and politics delayed such a sale, the organization invested even more heavily to preserve the shoreline by acquiring farmland and forests on the Eastern Shore.13
The Nature Conservancy blocked potential resort development projects by obtaining ownership of key spots on the mainland that could have served as ferry ports or the start of a bridge/causeway, then committed to sustainable development of the property to provide employment in local communities and maintain the cultural heritage as well. Stopping the short-term profits associated with new resorts and industrial-scale tourism was not popular in local development or political circles. The owners of Metomkin Island refused to sell it to The Nature Conservancy at any price, seeking instead to boost the economy by selling the island to a development firm call Offshore Islands, Inc. However, the acquisition-savvy conservancy had simply created a dummy corporation, and ended up purchasing the island in disguise.14
Sandbridge, a beachfront home community built on a barrier island near Back Bay Wildlife Refuge, is at risk of washing away. A public policy issue in Virginia Beach is who should pay to replenish the sand to protect private property, an issue that is also a factor for the resort hotel strip further north in Virginia Beach. Federal subsidies for sand nourishment projects are welcomed by local officials, but taxpayer advocates question the rationale of borrowing funds at the national level to provide highly-localized benefits.15
four National Wildlife Refuges (NWR's) managed by the US Fish and Wildlife Service protect a small percentage of barrier islands on the Eastern Shore - most islands are owned by a non-government organization and two state agencies
Source: Virginia Department of Environmental Quality, Who Owns Virginia's Barrier Islands?
Inlets on barrier islands open during major storms, then silt in and close up over time. In 1933, a hurricane breached Assateague Island, carving an inlet from the ocean to Sinepuxent Bay. The U.S. Army Corps of Engineers quickly locked the inlet in place by building a jetty on the north edge, blocking sand transport that would have naturally closed up the inlet. Ocean City then developed as a fishing port and ultimately a beach resort.
cross-section of a barrier island, showing vegetation growing on lower-energy side away from wave action
Source: National Oceanic and Atmospheric Administration, Barrier Islands - Habitat of the Month
storms can push sand past the dune line
Source: US Geological Survey, The Fragile Fringe
After another storm in 1962, plans for resort-scale development on Assateague Island were dropped and the National Park Service acquired most of the land for a national seashore. That stopped most new residential and commercial construction, but the Federal government agreed to maintain the 1933 inlet. The Ocean City Inlet has been kept open by jetties and by dredging of the channel, and those artificial barriers have blocked sand from drifting southward and renewing the southern part of Assateague Island.
The normal flow of sand, now trapped by the jetty, has created a wide beach on the northern part of the island. South of the inlet, there has been extensive erosion from the barrier island, no longer resupplied with sand by longshore currents. Without natural replenishment, the barrier island just south of the inlet has shrunk and migrated westward substantially.
Sand from the longshore currents still flows to Virginia, ending up at Toms Cove near Chincoteague. That portion of Assateague Island is still growing (accreting), extending the southern end of the island and creating a sandbar "hook." Further south, the shoreline veers westward forming the Chincoteague Bight. According to the US Geological Survey, the pattern of changing ocean currents has shaped the shoreline:16
summary wind conditions at Norfolk International Airport from 1960-1990
(winds blow more often, over 27% of the time, from the north - pushing water/sand to the south)
Source: Virginia Institute of Marine Science, Chesapeake Bay Shoreline Evolution Reports - Accomack County (Table 1)
Shifting sands require constant repair/reconstruction projects to maintain a paved beach parking lot on the southern portion of Assateague Island in located Virginia. To maintain its tourism-based economy, the Town of Chincoteague has expressed its preference for perpetual Federal maintenance of the bridge from Chincoteague Island to Assateague Island and recreational facilities on Assateague, so people can drive their own vehicles to the ocean shore for fishing, swimming, and sunbathing.17
In 2011, during drafting of the Comprehensive Conservation Plan (CCP) for Chincoteague National Wildlife Refuge, the US Fish and Wildlife Service agreed to maintain vehicle access to Assateague Island for the next 15 years - but rejected proposals to build groins/jetties, or to replenish sand through long-term beach nourishment.
Fighting natural shoreline processes at the wildlife refuge is both against Federal policy and too expensive. The CCP proposal continues a long-standing Federal policy at Assateague National Seashore, articulated by a senior Department of the Interior official back in 1975:18
Any construction on a barrier island is at risk of being eroded away, or shifted inland as the beach expands. The Assateague Island lighthouse in Maryland was built in 1833 next to the ocean shoreline, but today that site is almost two miles inland.19
Along the Eastern Shore, the Virginia islands themselves are so low they are at risk of complete inundation in a northeaster or hurricane. In an 1831 storm, only the high ridges of Smith Island remained above water, providing shelter among the pine trees to cattle and sheep.20
Everything on a barrier island is subject to change. Overwash during storms is a normal process, absorbing the storm energy and redistributing the sand inland. Tree stumps from forests that once grew on the bay side of barrier islands are occasionally exposed on the ocean side, showing the migration of the beaches to the west after years of big storms.
On Hog Island, the northern part erodes when the southern part grows (accretes), but the cycle shifts after roughly 200 years. In the 1930's, residents of the Broadwater community and the rest of Hog Island moved to Willis Wharf. The former location of Broadmore is now underwater.21
hurricanes can create new inlets and spits, triggering rapid and dramatic change in barrier island configuration - but longshore drift is dominated by smaller nor 'easter storms in winter
Source: National Weather Service Hurricane Ophelia
overwash moves sand, typically from ocean-side to bay-side, so barrier islands migrate rather than drown as sea level rises
Source: National Park Service, Cape Lookout Geologic Activity
Virginia's barrier islands may be changing their shapes in three different patterns. Wallops, Assawoman, Metompkin, and Cedar islands (between Assateague and Parramore) are starved for sand replenishment and retreating westward steadily as sea level rises. Further south, Parramore, Hog, Cobb, and Wreck islands are rotating. The southern group, including Ship Shoal, Myrtle, Smith, and Fisherman's islands, are developing straighter shorelines as they move westward.
Fisherman's Island is growing rapidly, from thirty-three acres in 1738 to over 1,850 acres today. Sediments have been derived from both sides of the Delmarva Peninsula. In the 1500's, the Cape Charles paleochannel was blocked by accumulating sediments at Wise Point, as currents from the bay and Atlantic Ocean shoreface created a sand bar on the bottom that built upward until it emerged and grew over 500 years. The western (Chesapeake Bay) side of Fisherman's Island and the middle section known as "The Isaacs" formed, then merged with each other and the ancestral core of Adams Island to create the current Fisherman's Island.22
In 2003, as a result of Hurricane Isabell, the island increased in size by over 3%. A storm surge of over 4 feet (plus waves 10 feet high) pushed sand onto Fisherman's Island, especially from the landward/mainland side.23
Source: US Fish and Wildlife Service, Wetlands Mapper
sediments accumulating south of Wise Point formed Fisherman's Island
Source: US Geological Survey (USGS), Fishermans Island 7.5 x 7.5 topo (2011)
Between Assateague and Fisherman's islands, all barrier islands on Virginia's Eastern Shore are migrating towards the west. As sea level rises, the islands are "rolling over" as sand is eroded from the ocean side and redeposited on the bay side during storms. Because land subsidence is greater at Assateague Island (2mm/year) than at Fisherman's Island (1.2mm/year), the northern barrier islands are retreating westward faster than the barrier islands at the southern edge of the Eastern Shore.24
shoreline changes at Hog Island, 1852-1963
Source: US Geological Survey, Coasts in Crisis (USGS Circular 1075)
shoreline changes, 1852-1993
Source: Virginia Coast Reserve, Hog Island Chronosequence
The Corps of Engineers dredges the Chincoteague Inlet annually to maintain boat access, and dumps the spoil pile off Wallops Island to help protect that shoreline from erosion by major storms. Those sand transfers are temporary adjustments to the natural cycle; the beaches are always moving.
Similarly, "armor plating" the shoreline with jetties, breakwaters and sea walls can only slow the natural geologic process of shoreline erosion. Fixed structures to trap sand in one place cause the waves to expend their energy by moving sand further downstream (south) rather than inland (west) - assuming there is an adequate supply of sand.
Wallops Island has $800 million of rocket launching and aviation infrastructure, and a shoreline that has moved 500 feet westward since 1851. A seawall built in 1945-46, and groins built in the next 20 years, provided inadequate protection. The solution adopted by the Federal government: beach nourishment. Sand was dredged from a shoal in Federal waters between Wallops and Assateague Islands, deposited near the Wallops Island shore, then pumped and spread by bulldozers to widen the beach.
After Hurricane Sandy washed away much of the replenished beach in 2012, the Federal government committed to repeat the process.25
Ocean City (in Maryland) and Virginia Beach require constant sand replenishment ("nourishment") to maintain their current wide beaches. Those two resort communities pump sand from offshore to the beach in front of their boardwalks. That creates a wide recreational beach - until the next series of storms. Local taxes are being invested to fight the natural pattern and perpetuate a big beach as the key attraction for the local community's economy, but Federal and State funding for such projects has been shrinking since 2000.
The Nature Conservancy helped Virginia avoid the shoreline maintenance expense along the Eastern Shore. It purchased the marshes and barrier islands, choosing to perpetuate the natural setting and allow erosion/deposition to occur naturally. By minimizing the "built environment" of hotels and recreational facilities on the edge of the beach, The Nature Conservancy eliminated any need to build sea walls or pump sand.
As authorized by the Coastal Primary Sand Dune Protection Act passed by the General Assembly in 1980, special review is required by county Wetland Boards or the Virginia Marine Resources Commission before landowners can build structures that might impact the primary dune line or state-owned beaches. Virginia legislators are traditionally reluctant to limit private property rights, but shoreline development can damage adjacent properties by disrupting the normal migration of sand:26
Barrier islands are formed from unconsolidated sediments, so rainfall can quickly penetrate the sand. Wind and sun can create a very dry environment, just a few yards from the vast amount of water in the ocean. Barrier island plants must be adapted to drought-like conditions and must tolerate salt spray, since access to fresh water will be intermittent after storms. Shrubs and maritime forests on barrier islands include species such as pine trees, which have waxy coatings on the needles to conserve moisture.
In some depressions, where clay or organic particles create an impermeable layer, freshwater may accumulate. Ponds can provide fresh drinking water to animals, such as the famous herd of wild horses on Chincoteague Island. (The horses do not drink salt water...) Barrier islands also offer valuable habitat for migratory birds, such as the piping plover (which requires overwash fans for nesting habitat):27
Both recreational and market hunting were common on Virginia's barrier islands until wildlife conservation laws were passed and enforced between 1918-1940. According to one report:28
The expansion of steamboat service and railroads after the Civil War stimulated more sportsmen from East Coast cities to travel to the barrier islands to hunt. Forest And Stream published in 1876:29
President Grover Cleveland hunted ducks at the Back Bay Club (Virginia Beach) and Broadwater Club (on Hog Island)
Source: Fishing and shooting sketches, by Grover Cleveland
1. John W. A. Elliott, "Christian Guardians, 1835" in Seashore
Chronicles - Three Centuries of the Virginia Barrier Islands,
Brooks Miles Barnes and Barry R. Truit (ed.), University of Virginia Press,
1997, p. 50
Assateague Island, showing growth of Tom's Cove at southern end
Source: National Aeronautics and Space Administration (NASA), Astronaut Photography of Earth
barrier islands have a core, an overwash beach on the high-energy ocean side, wash-arounds where sand accumulates after storms, tidal marshes on the low-energy bay side, and ponds where fresh water is trapped in depressions (plus inlets that open/close due to storms)
Source: National Park Service, A hydrogeomorphic map of Assateague Island National Seashore, Maryland and Virginia (Figure 9, p.16)