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VIMS Books and Book Chapters Virginia Institute of Marine Science

2016

Atlantic Coast and Inner Shelf

David E. Krantz

Carl H. Hobbs III Virginia Institute of Marine Science

Geoffrey L. Wikel

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Recommended Citation Krantz, David E.; Hobbs, Carl H. III; and Wikel, Geoffrey L., "Atlantic Coast and Inner Shelf" (2016). VIMS Books and Book Chapters. 117. https://scholarworks.wm.edu/vimsbooks/117

This Book Chapter is brought to you for free and open access by the Virginia Institute of Marine Science at W&M ScholarWorks. It has been accepted for inclusion in VIMS Books and Book Chapters by an authorized administrator of W&M ScholarWorks. For more information, please contact [email protected]. ATLANTIC COAST AND INNER SHELF

David E. Krantz Department of Environmental Sciences, University of Toledo, Toledo, OH 43606, [email protected] Carl H. Hobbs, III Virginia Institute of Marine Science, College of William & Mary, Gloucester Point, VA 23062, [email protected]

Geoffrey L. Wikel Office of Environmental Programs, Bureau of Ocean Energy Management, Herndon, VA 20170, [email protected]

INTRODUCTION alternating deep channels and shallow shoals between the Virginia Capes–Cape Charles on the southern tip of the Eastern The continental margin of Virginia, and of North America Shore and Cape Henry on the northern edge of Virginia Beach. more broadly, is the physical transition from the high elevation The Virginia coast shares its evolutionary history with of the continent to the low of the ocean basin. This transition adjacent areas, and must be viewed in a larger, more regional, was created as rifting pulled apart the ancient supercontinent context. The Virginia coastal zone can be divided by geologic Pangaea to create the Atlantic Ocean basin. Tectonic forces character into three broad geographic areas: the Atlantic coast fractured and stretched the bedrock to create a stair-step ramp of the Delmarva Peninsula, the mouth of Chesapeake Bay, and that subsequently would be mantled with sediment built up by the mainland shore of southeastern Virginia continuing into erosion and transport off the continent. northeastern North Carolina (Figure 1). Each of these main The Coastal Plain and Continental Shelf of Virginia sections can be subdivided further into compartments defined are contiguous and discrete physiographic provinces of the primarily by the effects of wave and tide energy. A fundamental continental margin delimited by the present elevation of sea characteristic of the Virginia coastal zone is that it is composed level. On geologic time scales of thousands to millions of entirely of unconsolidated sediments, such as sand and silt, years, the coastal zone—the boundary between the coastal plain with no exposures of bedrock or hard, consolidated sediments. and shelf—is dynamic and migrates hundreds of kilometers Consequently, sedimentary processes—erosion, transport, and landward and seaward. Today, the Atlantic shore of Virginia lies deposition—are active on timescales of minutes to millennia, just past halfway across the margin: about 150 km (93 mi) from and are constantly reshaping the coast. the edge of the Piedmont at the Fall Zone, and about 100 km (62 This chapter presents the physiography, geological mi) from the seaward edge of the shelf (Figure 1). The modern processes, and evolutionary history of the Atlantic Coast of coastal zone occupies nearly the same position as during several Virginia. The coastal zone, as described above, encompasses the previous interglacial highstands of sea level that have recurred at outer Coastal Plain and inner Continental Shelf, and the Atlantic approximately 100,000-year (abbreviated 100 ky, for “kilo year”) Ocean shorelines of the Eastern Shore and southeastern Virginia. intervals since the middle Pleistocene (about the last 750 ky). This region inherits a geologic foundation that spans millions of The Virginia coastal zone encompasses the outer Coastal years, but has been shaped most directly by changes in sea level Plain and the barrier islands and mainland shoreline fronting and associated processes operating over the past approximately the Atlantic Ocean, as well as the back-barrier lagoons or 125,000 years. coastal bays, the inner shelf, and the mouth of Chesapeake Bay (Figure 1). A broader definition includes the lower reaches of PHYSIOGRAPHY AND PROCESSES OF THE the estuarine tributaries to Chesapeake Bay, such as the James, VIRGINIA ATLANTIC COAST York, Rappahannock, and Potomac Rivers. Within the coastal zone, the emergent and submerged Along an eastward transect from the front of the Blue Ridge landforms are the result of a geologic heritage modified by across central Virginia, the rolling hills of the Piedmont, with modern processes driven by the energy of waves and tides. High shallowly exposed bedrock, give way to the broad, low-relief wave energy from the Atlantic Ocean shapes the mainland shore Coastal Plain near Richmond and Fredericksburg. The western of southeastern Virginia and the long, narrow barrier islands of boundary of the Coastal Plain is the Fall Zone (Figure 1), where the central Delmarva (Delaware–Maryland–Virginia) Peninsula. the gradients of the rivers flowing seaward drop dramatically Wave and tide energy together create the segmented barrier as the river profile reaches sea level. At the Fall Zone, the islands of the Virginia Eastern Shore (the southern extension of sediments of the Coastal Plain lap onto the crystalline bedrock of the Delmarva Peninsula) that are separated by tidal inlets and the Piedmont. The wedge of sediment that underlies the Coastal accompanying tidal deltas. Strong flood and ebb currents move Plain has been deposited in fluvial, deltaic, marginal marine, and water and sand through the mouth of Chesapeake Bay, creating marine environments since the early Cretaceous (about the past 341 The Geology of Virginia

39° DELAWARE BAY T I N N E A O N L O M Z P D L ENINSULA E L A OUTHERN P I L S T P A ARYLAND S M F ELMARVA Delaware A D

O Potomac R. Maryland C Northern C HESAPEAKE Delmarva Wilmington Coast Canyon

Rappahannock R. F Baltimore L 38° B Canyon E

HORE H AY S S 50 m E P L 100 m Accomac O A Canyon L ASTERN T E S N 250 m 500 m L E York R. Southern A N IRGINIA T Delmarva I V Coast T N Washington E N Canyon N

20 m O I LAIN

T

P C ONE

James River N Z

O

C IEDMONT A L L Norfolk

F Chesapeake 37° P Canyon

O A S T A L Bay Mouth C Nottoway R. SOUTHEASTERN VIRGINIA ATLANTIC Virginia OCEAN Meherrin R. North Carolina Southeastern Virginia & Northern North Carolina 0 50 miles Roanoke R. Coast

0 50 km ALBEMARLE SOUND 36°N 78° 77° 76° 75° 74° 73°W

Figure 1. Regional setting of the Atlantic coast of Virginia, showing the Piedmont, Coastal Plain and Continental Shelf physiographic provinces. The Virginia coastal zone may be divided by geologic character into three broad geographic areas: the Atlantic coast of the Delmarva Peninsula, the mouth of the Chesapeake Bay, and the mainland shore of southeastern Virginia continuing into northeastern North Carolina.

145 million years). This wedge thickens to almost 2 km (1.2 mi) of predominantly continental-shelf sediments during highstands at the Atlantic coast (Meng and Harsh, 1988). from the late Cretaceous through most of the Cenozoic (Powars et al., 2016 [this volume]). Fluvial and deltaic sediments were Building the Outer Coastal Plain deposited on what is now the Piedmont. The last major fully- marine flooding event occurred in the late Pliocene (about 3 My The Virginia Coastal Plain has evolved through three major ago) when a series of transgressions lapped onto the Piedmont episodes of construction, each with its own combination of and deposited the Yorktown Formation across the width of the dominant physical factors controlling deposition of sediment. present Coastal Plain (Ward and Blackwelder, 1980). During the first period, from initial rifting in the Triassic A significant increase in global ice volume near 2.6 My through the Jurassic (roughly from 220-145 My) (LeTourneau ago dropped sea level worldwide and initiated the final phase and Olsen, 2003; Smoot, 2016 [this volume]), what would of evolution of the margin (Krantz, 1991). Since that time, become the Atlantic margin was dominated by regional uplift global climate has oscillated between glacial cold conditions that created highlands and extensional rift basins. This setting with low sea level and interglacial warm conditions with high generated huge volumes of sediment delivered by river systems sea level. The mainland Coastal Plain of Virginia built up on into the developing ocean basin. During the Jurassic, tectonic top of the Yorktown Formation as a foundation during the spreading switched to the mid-ocean ridge system in the ever- highstands of the latest Pliocene and Pleistocene (Oaks et al., widening Atlantic Ocean, and crustal cooling and subsidence 1974; Mixon et al., 1989; Ramsey, 1992; Ator et al., 2005). The progressively lowered the edge of the continent. maximum transgression of each highstand carved a high-energy By the early Cretaceous, the continental margin had dropped ocean shoreline, expressed geomorphically as a scarp, with significantly in elevation and sea level was high globally (Owens lagoonal and estuarine environments landward, and shoreface and Gohn, 1985). The second major episode in the evolution of the and shelf environments seaward (Oaks and Coch, 1973; Oaks Coastal Plain was dominated by marine flooding and deposition et al., 1974). Build-up of the Coastal Plain during highstands 342 Atlantic Coast and Inner Shelf alternated with deep incision of the major regional rivers during Formation, which developed as a braided-river outwash plain lowstands. The broader valleys thus created would be partially of coarse sediments (sand and gravel, with some cobbles and filled by deposition of estuarine sediments during the succeeding boulders) during the early Pleistocene (Ramsey and Schenck, highstand. 1990; Ramsey, 1992; Pazzaglia, 1993). Similar sheets of coarse The Coastal Plain of central Virginia has a series of stair- clastics from the ancestral Potomac River built out the peninsula steps that are broad coast-parallel or river-parallel plains, or that is now Southern Maryland, between the modern Potomac terraces, each bounded on the landward side by a scarp. The and Patuxent Rivers (McCartan et al., 1990). Age control for the river-parallel scarps represent the laterally eroding banks of the major depositional events is generally poor because these deposits estuaries that occupied the river valleys during highstands. The have few fossils for biostratigraphy or materials for radiometric coast-parallel scarps represent the previous maximum landward or other geochemical dating (Groot and Jordan, 1999). position of erosional ocean shorelines, and the terraces would Since the last major depositional event in the early have been the floor of the inner shelf stretching seaward. In Pleistocene, the high-energy Atlantic shoreline has cut into the some cases, the terraces were re-occupied by later highstands fluvial-deltaic deposits of central Delmarva during highstands, of sea level that left barrier islands stranded during regression and the Delaware and Susquehanna Rivers have incised deep as paleo-shorelines. The entire system of scarps and terraces valleys during lowstands to carve out the basins for Delaware was created through the Pliocene and Pleistocene by a series of and Chesapeake Bays, respectively. On the ocean side of the successively lower highstands of relative sea level that cut into peninsula, sand eroded by wave attack was transported south and planed off existing sediments. The older scarp and terrace and southwest by the regional longshore transport (Figure 2). sets were preserved because of the slow but continuous uplift of Deposition downdrift produced the series of large-scale spits that the Piedmont and Coastal Plain. prograded into the basin created by the Eocene Chesapeake Bay The Pliocene–Pleistocene history of the Delmarva Peninsula Impact Crater (Poag, 1997; Powars et al., 2016 [this volume]) differs somewhat from that of central Virginia because of the and formed the Virginia Eastern Shore (Colman and Mixon, prominent role of the Delaware and Susquehanna Rivers that 1988; Hobbs, 2004). drained the southern margin of the continental ice sheets. The core of the Delmarva Peninsula was built up by the delivery of Major Rivers in the Region large volumes of sediment, mostly from the Delaware River system, to deposit the Pensauken Formation in the late Miocene Regionally, the major rivers that drain the Appalachian and the Beaverdam Sand in the late Pliocene (and possibly highlands, the Delaware, Susquehanna, Potomac, James, into the early Pleistocene) (Owens and Denny, 1979; Olsson and Roanoke Rivers, have remained in essentially the same et al., 1988; Ramsey, 1992; Pazzaglia, 1993). The last major positions for at least the past 10 My (Pazzaglia, 1993). Fluvial depositional event onto Delmarva was that of the Columbia sands and gravels from these rivers cap the eroded hills of the

Table 1. Mean and spring tidal ranges at selected stations along the Virginia and Delmarva ocean coast. Mean Spring Mean Spring Site ID Tide Station range (m) range (m) range (ft) range (ft) LE RB DELAWARE IR LE Lewes 1.25 1.43 4.1 4.7 FI RB Rehoboth Beach 1.19 1.43 3.9 4.7 DE MD OC IR Indian River Inlet 0.77 0.90 2.5 2.9 AI N MARYLAND FI Fenwick Island Light 1.13 1.37 3.7 4.5

MD OC Ocean City (fishing pier) 1.02 1.22 3.4 4.0 VA AI Assateague Island 1.04 1.25 3.4 4.1 WI VIRGINIA EASTERN SHORE

WA WI Wallops Island 1.10 1.34 3.6 4.4 WA Wachapreague Inlet 1.19 1.43 3.9 4.7 GMI GMI Great Machipongo Inlet 1.19 1.43 3.9 4.7 SI Smith Island 1.07 1.28 3.5 4.2 SOUTHEASTERN VIRGINIA SI CB Chesapeake Bay Bridge Tunnel 0.79 0.88 2.6 2.9 CB CH CH Cape Henry 0.94 1.07 3.1 3.5 RI RI Rudee Inlet 1.01 1.13 3.3 3.7 SB SB Sandbridge 1.04 1.16 3.4 3.8

343 The Geology of Virginia

SE Virginia

Chesapeake Bay Mouth Virginia CHESAPEAKE Mixed-energy Barriers River barrier island BAY Potomac compartment

Chincoteague Bight Chincoteague Inlet Chincoteague Bay

Assateague DELMARVA Island P ENINSULA

Wave-dominated Ocean City Inlet barrier island compartment

Fenwick Bethany Island Headland nodal zone Indian River Inlet

Rehoboth Headland ATLANTIC Eroding headland & O CEAN cuspate foreland Cape compartments Henlopen

Delaware Bay Mouth

Figure 2. Oblique view from the north of the Atlantic coast of Delmarva, with the Chesapeake Bay mouth and southeastern Virginia in the distance. The coast is subdivided into four coastal compartments defined primarily by the effects of wave and tide energy (Oertel and Kraft, 1994). Arrows indicate direction of net longshore transport. Image generated from Google Earth; Landsat image from April 2013.

Piedmont and rim the western edge of the Coastal Plain along Coastal Compartments the Fall Zone (Mixon et al., 1989). As introduced above, the regional rivers play major roles in landscape evolution through From the record of ancient shoreline features preserved the alternation between processes operating during sea-level on the Coastal Plain, the coastal processes operating today are highstands and those during lowstands. The major rivers have fundamentally the same as those operating through the previous episodically delivered large volumes of coarse sediment that several million years of the geologic past. As the Atlantic Ocean built up the Coastal Plain and shelf, and excavated valleys that cut into and reworked the sediments of the Delmarva Peninsula trend southeast across the continental margin toward the offshore and southeastern Virginia, longshore transport carried sand submarine canyons. The channels and broader valleys incised downdrift to build barrier islands, capes, and prograding spits. during lowstands by the rivers and their tributary drainage Erosion of two primary sources has supplied sand to the littoral networks provided accommodation space (basins) that would be system: erosion of headlands intersecting the shoreline, and a flooded by rising sea level to form estuaries during highstands. broader planation and excavation of the entire inner shelf down The intermediate rivers on the Virginia mainland, the to water depths of about 12 m (40 ft) by the combined action of Rappahannock and York Rivers, drain the Piedmont and make storm waves and currents (Kraft, 1971). short excursions into the front of the Blue Ridge. Geologic The modern Mid-Atlantic coastal system may be subdivided evidence suggests that these smaller rivers also have been into four large sections, each having similar sequences of in approximately the same positions at least since the earliest geomorphic zones and each bounded by the mouths of major Pleistocene (about the last 2 My) (Newell, 1985; Ramsey, 1992). Both of these rivers presently are flooded as tidal estuaries, with estuaries (Fisher, 1967). These individual sections comprise the head of tides close to the Fall Zone. However, their most the coastlines of Long Island, New Jersey, Delmarva, and significant contribution to the evolution of the Virginia coast was Virginia-North Carolina, bounded respectively by the mouths carving the valleys that would later provide outlet courses for the of Long Island Sound, Hudson River Estuary, Delaware Bay, combined Susquehanna-Potomac drainage as it was displaced and Chesapeake Bay. The southern boundary of the Virginia– southward through the Pleistocene (Colman and Mixon, 1988; North Carolina section is Cape Lookout, where the coastline Colman et al., 1990; Oertel and Foyle, 1995; Hobbs, 2004). turns abruptly west. Swift (1969), Belknap and Kraft (1985), and McBride (1999) used this framework to compare the coasts of New Jersey and the Delmarva Peninsula, and to explain 344 Atlantic Coast and Inner Shelf the compartments in terms of geologic setting and sediment- of the Chesapeake Bay mouth has created the cuspate foreland of transport processes. Cape Henry and the sand-rich shoreline that extends past Little In their summary of the coastal geology of New Jersey and Creek and Ocean View. Willoughby Spit is the relatively small Delmarva, Oertel and Kraft (1994) refined Fisher’s (1967) model terminal end of this littoral system prograding into the mouth such that each of the large sections has four segments (Figure of the James River estuary. Based on maps from the Colonial 2): (1) cuspate spit; (2) eroding headland; (3) barrier spits and Period, Willoughby Spit itself appears to have formed sometime long linear barrier islands (wave-dominated barrier islands); and between 1799–1807 (Melchoir, 1970), although prior to 1800 (4) short barrier islands with tidal inlets (mixed-energy barrier Willoughby Bay was shallow with numerous sand shoals. (Refer islands). Oertel and Overman (2004) added the emergent barrier forward to Figure 9 for locations of these features.) island section created by convergence of two sediment-transport pathways at the northern margin of the Chesapeake Bay mouth. Sedimentary Processes in the Coastal Zone This coastal segmentation scheme was used by Gutierrez et al. (2007) to better predict the variable response of barrier islands Sediment transport processes of the inner shelf are both to sea-level rise in the Mid-Atlantic region. complex and extremely dynamic. Moderate storms that impact Unlike in the Georgia Bight (Hayes, 1980, 1994), the the Virginia coast several times each year typically mobilize geomorphic transitions along the Virginia coast do not appear to sediments on the seafloor to a storm wave base of 40–50 m be dominated by changes in the tidal range. The entire Atlantic (130–165 ft). The major storms that occur a few times each coast of Delmarva and southeastern Virginia is microtidal, with century move sediment on the entire width of the shelf, to water mean tidal ranges between 0.94–1.2 m (3.1–3.9 ft), and spring deeper than 100 m (330 ft), and can agitate the bottom to depths tidal ranges of 1.1–1.4 m (3.6–4.6 ft) (Table 1). In contrast, greater than 200 m (660 ft) (Wright, 1995). Besides the obvious within the Georgia Bight spring tidal ranges increase from 1.4 m turbulent action of large waves, storms also generate strong (4.6 ft) at Cape Fear, North Carolina to 2.8 m (9.2 ft) at the apex longshore currents flowing parallel to the shoreline in a zone of the Bight south of Savannah, Georgia. Here the segmentation that may be several kilometers wide. Additionally, the pressure of the coast into mixed-energy (“drumstick”) barrier islands gradient of seawater piled up against the shoreline during storms coincides spatially with the transition to mesotidal range (>2 m generates rip currents, and strong downwelling and offshore [6.6 ft]). flow, commonly called “undertow.” All of this energetic water Along the entire Mid-Atlantic coast, net longshore transport motion lifts sediment off the seafloor, holds it in suspension, and of sand in the shoreface—the inner shelf affected by moderate moves it down the flow path. waves, typically to a water depth of about 20 m (65 ft)—is to To provide a framework for understanding the dominant the south or southwest. The net southerly transport is set up by processes, Swift et al. (2003) and Parsons et al. (2003) seasonal variations in the regional weather systems (Belknap synthesized previous studies to formulate the following “rules and Kraft, 1985). Gentle breezes from the south and southeast for bedload sediment dispersal on the Virginia Coast.” predominate during the summer, but stronger winds blow from • At time scales of 50 years or greater, the entire shoreface the west and northwest in winter. Coastal storms of the late in transgressive sectors of the Middle Atlantic Bight fall through early spring are the dominant force driving sand (MAB) is undergoing landward retreat. transport. Storm winds blowing from the northeast quadrant • Net along-shore sediment flux in the surf zone (0–3 produce extremely strong south-flowing longshore currents that m [0–10 ft] water depth) of the MAB is southward move more sand in a few days than the cumulative transport to southwestward and parallel to isobaths, with the throughout the rest of the year. Wave transformation and exception of local reversals south of inlets and major refraction over shallow nearshore shoals and tidal deltas result estuaries. in local reversals in longshore transport. • Net across-shelf transport averaged over decades, The regional trend of southerly shore-parallel sediment to include major storms, has a component directed transport is altered by the mouths of the major estuaries, where onshore, reflecting episodic storm washover of the the gradient of wave energy goes from high along the Atlantic barrier islands. coast to low within the estuary. Additionally, shore-perpendicular • Sand size on the beaches of all MAB coastal tidal currents are focused and strengthened by constriction compartments fines downdrift to the south. at the bay mouths. As a result, net sediment transport is from • Sand on the Delmarva inner shelf (shallower than the ocean into the estuary (Meade, 1969, 1972; Hobbs et al., 20 m [66 ft]) becomes progressively finer along the 1992). Bathymetric effects, reversing tidal flow, and interactions transport path to the southwest, from medium sand at between waves and tides result in sediment convergence along Chincoteague Shoals to fine sand at Smith Island. the northern flank of the bay mouth, producing large shoal • Sediment transport on the middle and lower shoreface systems such as Middle Ground and Nine-Foot Shoals at the (water depths of 5–20 m [16–66 ft]) is directed mouth of Chesapeake Bay (Figure 3) (Ludwick, 1970, 1975; southward and offshore, and is oblique to isobaths. Swift, 1975; Oertel and Overman, 2004). • The intensity of storm-driven, across-shelf sediment Net transport of sand into the bay mouth also produces a flux diminishes seaward as a function of increasing reversal of longshore transport along the ocean shoreline south of water depth and decreasing intensity of wave orbital the bay. For Chesapeake Bay, this produces northward flow from motion during peak storm flows. a nodal zone north of False Cape (Figure 3) (Everts et al., 1983). • Sediment transport over storm-generated shoreface This sand moving north and then west along the southern flank sand ridges in the MAB is typically south and offshore, 345 The Geology of Virginia

moving obliquely across the crests. ft) below present sea level (Kraft, 1971). • The troughs between the shoreface ridges undergo Sand in the littoral transport system in these northern erosion, as do the up-current flanks of the ridges, while sections moves north, counter to the regional transport the crests and down-current flanks aggrade, as the direction. Longshore transport diverges in a nodal zone along ridges migrate slowly downdrift. the Delaware–Maryland coast (Figure 2). The actual boundary between northward and southward flow varies between years as Atlantic Coast of the Delmarva Peninsula net transport balances the effects of winter northeaster storms and summer southeasterly breezes. Consequently, the point of Fisher’s (1967) model of coastal compartments, as modified transport divergence migrates between South Bethany and the by Oertel and Kraft (1994), characterizes the geomorphology of easternmost point on the Delmarva coast near the Delaware– the Atlantic coast of the Delmarva Peninsula in terms of supply Maryland state line at Fenwick Island (Dalrymple and Mann, of sand to the shoreline by erosion and redistribution by transport 1985). The northward flow of sand along the Delaware coast is processes. This review of the coastal geomorphology follows the driven both by the north–northwest orientation of the shoreline coastal compartment model and relates the geomorphic changes (rotated west from the northeast orientation of the Maryland along the coast to setting and processes. coast) and the wave-energy gradient into Delaware Bay. The flow of sand in the littoral system has created the baymouth barrier Assateague Island and North or barrier spit that closed two incised valleys to form Rehoboth From Cape Henlopen, Delaware, to the southern tip of and Indian River Bays, and to build the cuspate foreland of Assateague Island in Virginia (Northern Delmarva Coast, Cape Henlopen, where the shore abruptly turns westward into Figure 1), the Delmarva coast is essentially one long, continuous, Delaware Bay (Kraft et al., 1978). narrow barrier island, with short headlands at Rehoboth Beach From the Delaware–Maryland state line, regional longshore and Bethany Beach, Delaware (Figure 2). This 110–km (68–mi) transport is to the south all the way to Cape Charles at the stretch of the shoreline is interrupted by only two inlets, both mouth of Chesapeake Bay. Local reversals of transport occur of which are maintained: Indian River, Delaware, and Ocean immediately downdrift of tidal inlets. The typical scale of City, Maryland. The northern 30 km (18.5 mi) of this coastline downdrift effects by an inlet is about 6 km (3.7 mi) (Fenster and comprise the eroding headland and cuspate foreland elements Dolan, 1996) and varies with the size and stability of the inlet of the Delmarva coastal compartment (Oertel and Kraft, 1994). and associated ebb-tidal delta. Erosion of the shoreface and the two Pleistocene headland sections Fenwick and Assateague Islands extend south 73 km provides sand to the shore. Shoreface ravinement is the primary (45 mi) from the Delaware–Maryland border to the southern tip erosional process, as scour by storm waves and longshore currents of Assateague Island in Virginia (Figure 2). The resort towns of bevels Holocene back-barrier deposits exposed in the shoreface Fenwick, Delaware, and Ocean City, Maryland, occupy Fenwick and excavates shallow Pleistocene sediments to 11–13 m (36–43 Island, which is heavily developed and often hosts upwards of

York River CBIC Outer Rim James River CHESAPEAKE BAY

Shore Eastern

Mockhorn Is. Cape Charles Hog Island Virginia astern outhe S Cape Henry

False Cape

ATLANTIC OCEAN

Figure 3. Perspective view of the topography of southeastern Virginia, the lower Delmarva Peninsula, the Chesapeake Bay mouth region, and adjacent portions of the continental shelf. The rim of the Chesapeake Bay Impact Crater is expressed geomorphically by the arcuate escarpment starting at Cape Henry and continuing clockwise past the peninsulas separating the James, York, and Rappahannock Rivers.

346 Atlantic Coast and Inner Shelf

300,000 visitors on a summer weekend. In contrast, Assateague Estimates of sand supplied to the spit complex of Toms Island is minimally developed, with extensive pristine areas and Cove Hook and Chincoteague Inlet range from 165,000 m3 restricted access to much of the island. The Maryland section of (215,000 yd3) per year to 1,100,000 m3 (1,440,000 yd3) per year Assateague Island has been owned and managed by the National (Finkelstein, 1983; Headland et al., 1987). The closure of the Park Service since 1965 as Assateague Island National Seashore, historic Green Run and Pope Island inlets and reworking of their with a Maryland State Park as an inholding. The Virginia section ebb-tidal deltas in the late 1800s may have contributed to the is managed principally by the U.S. Fish and Wildlife Service as spit’s rapid growth in the 20th century (Halsey, 1978; Demarest Chincoteague National Wildlife Refuge. and Leatherman, 1985; McBride, 1999). Because of this littoral Assateague Island is bounded to the north by Ocean City transport, Chincoteague Inlet has one of the longest histories Inlet, which was cut by a hurricane in 1933 and stabilized with of dredging to maintain a navigable channel of any seaport jetties soon thereafter by the U.S. Army Corps of Engineers. The entrance along the U.S. East Coast. The quantity of material jetties and, more important, the flood- and ebb-tidal deltas of the dredged from the inlet during eight projects between 1995 and inlet have trapped a significant volume of sand that otherwise 2006 varied from 53,500 to 94,000 m3 (70,000 and 122,900 would have been transported to Assateague Island (Kraus, yd3), with a total volume removed of 473,000 m3 (620,000 yd3) 2000). As a consequence, the northern 5-km (3-mi) section of (Morang et al., 2006). Assateague has migrated landward a full island width, more than The nearshore shelf surface off Assateague Island is floored 500 m (1,600 ft), and experiences nearly complete overwash by a discontinuous sheet of medium to fine sand, molded into during major storms. The depletion of the island is so severe that shore-oblique ridges and swales that are spaced 2–4 km (1.2–2.5 the Park Service took the unusual, for it, action of placing sand mi) apart and extend kilometers to tens of kilometers (Swift et to stem the retreat (U.S. National Park Service, 2007; Schupp et al., 2003). Most of these sand ridges are oriented southwest– al., 2007). The arc of erosion created by the inlet extends nearly northeast with a maximum relief of 5–10 m (16–33 ft). Shoreface 16 km (10 mi) south on Assateague Island (Galgano, 1998). ridges are the most significant source of coarse sediment Central Assateague Island, south to Green Run Bay near the on the inner shelf (Wells, 1994; Conkwright and Gast, 1995; Maryland–Virginia border, is a typical wave-dominated barrier Toscano and York, 1992), and the largest number and highest island, between 500 m and 1 km (0.3 to 0.6 mi) wide, with density of ridges along the East Coast occur off Assateague broad overwash flats extending into the back-barrier lagoon of Island (McBride and Moslow, 1991). However, the sand ridges Chincoteague and Sinepuxent Bays. Higher-elevation sections disappear abruptly in the comparatively featureless basin south of the island are dissected by the low-lying traces of previous of Fishing Point (Figure 4). tidal inlets (Krantz, 2010), some of which were open for a few A prominent erosional surface generally underlies the decades during the past 150 years (McBride, 1999). One of actively transported sands of the shoreface ridges (Toscano the more prominent inlets is Green Run Inlet, which was open et al., 1989; Toscano and York, 1992). Seismic data show an through much of the early to mid-1800s, but shallowed, filled, inclined, planar to hyperbolic surface with a fairly uniform and finally closed between 1880 and 1900. geometry, except for outcropping platforms where sediment is From Green Run Bay south, the modern Assateague being exhumed from fine-grained, back-barrier facies. The sand Island is offset about 1 km (0.6 mi) seaward of Pope Island ridges are post-transgressive expressions of modern shoreface and Chincoteague Island, which have been interpreted as late processes, in particular a response to wave action and alongshelf Holocene precursors to the modern barrier island (Halsey, 1978, currents generated by storms (Swift and Field, 1981; Toscano 1979). Assateague Bay Inlet cut through Assateague Island and York, 1992; Snedden and Dalrymple, 1998). These energetic seaward of the north end of Chincoteague Island during the events mobilize sand in the shoreface, where waves may Colonial period (1600s and 1700s) (Amrhein, 1986). By about transport it across the barrier island to create overwash fans, or 1800, this inlet was closed by the first of a set of spits extending downwelling currents may sweep it off the upper shoreface into south and wrapping around the eastern side of Chincoteague deeper water (Parsons et al., 2003). One prevailing model for the Island (Goettle, 1978). The original Assateague Lighthouse was initial formation of these sand shoals is that the ridges develop constructed in 1833 on the highest-elevation and most prominent as waves rework the sands of an ebb-tidal delta after an inlet of these spits; at the time, this was the ocean shoreline. Today closes (McBride and Moslow, 1991). the second lighthouse, built on the same site in 1867, is 3 km (1.8 mi) inland from the beach. Chincoteague Bight The southern end of Assateague Island is the terminus of One of the major geomorphic transitions along the Virginia the regional longshore sediment transport system moving sand coast occurs immediately south of Chincoteague Inlet, where south 75 km (46 mi) from the Bethany headland (Oertel and Wallops Island is offset nearly 6 km (3.7 mi) west of Assateague Kraft, 1994). The informally named Toms Cove Hook is the spit Island (Figures 2 and 4). The next three islands south— complex of south Assateague, with Fishing Point as the distal tip Assawoman, Metompkin, and Cedar Islands—are offset as (Figure 4). This spit extended south nearly 6 km (3.7 mi) from a much as 10 km (6 mi) west of a line drawn between Assateague cuspate foreland that was the end of the island in the mid-1800s and Parramore Islands, to form an arcuate embayment of the (Field and Duane, 1976). Historical charts show initial extension coast known as Chincoteague Bight (Oertel et al., 2008). The of the spit to the south–southwest, but by 1890 the leading edge transition at Chincoteague Inlet marks the boundary between of the spit turned westward. Continued progradation to the west- the wave-dominated islands to the north and the mixed-energy, northwest during the 1900s partially enclosed a relatively deeper “drumstick” islands with large ebb-tidal deltas to the south basin to form Toms Cove. (Finkelstein, 1983; McBride and Moslow, 1991; Oertel and 347 The Geology of Virginia

75°45' 75°30' 75°15' 38°00' CB CB Chincoteague Bay CI CI Chincoteague Island AI WQS AI Assateague Island TC WQS Winter Quarter Shoal BB CS WI CIn WI Wallops Island

AwI CIn Chincoteague Inlet MI TC Toms Cove 37°45' us CS Chincoteague Shoal PoB BB Blackfish Bank

MIn AwI Assawoman Island CeI MI Metompkin Island WIn PB us unnamed shoal MIn Metompkin Inlet PoB Porpoise Banks PI 37°30' CeI Cedar Island QIn WIn Wachapreague Inlet PB Parramore Banks HI PI Parramore Island GMIn QIn Quinby Inlet

HI Hog Island GMIn Great Machipongo Inlet 37°15'

20 km

Figure 4. Coastal zone and shaded-relief bathymetry of the inner shelf for the Delmarva coast from southern Assateague Island to Hog Island, showing the landward offset of barrier islands forming Chincoteague Bight.

Kraft, 1994) (Figures 2 and 5). The morphologic change also starvation and rapid retreat (2–9 m/yr, or 6.5–30 ft/yr) of the coincides with a transition to finer sand to the south (Swift, 35 km (22 mi) arc from Wallops Island to Cedar Island (Rice 1975). and Leatherman, 1983; Kraus and Galgano, 2001). The southern Wallops Island, the northernmost barrier in Chincoteague section of Wallops Island, which is the most stable island of the Bight, has the classic bulbous updrift end and narrow downdrift group, retreated about 400 m from 1857 to 1994 for an average spit of a mixed-energy island (Hayes, 1979, 1980). Wave rate of 2.9 m/yr (9.5 ft/yr) (Morang et al., 2006). Locally refraction around Chincoteague Shoals, Fishing Point, and the near Assawoman Inlet, between Wallops and Assawoman ebb-tidal delta of Chincoteague Inlet drives the bypassing of Islands, the rate increases to 3.7 to 5.5 m/yr (12 to 18 ft/yr), sand around the outer edge of the delta. The re-attachment of and Assawoman Island itself retreated about 5 m/year (16.5 ft/ swash bars migrating around the delta, and a local reversal in yr) between 1911 and 1994 (Morang et al., 2006). The present longshore transport to the north result in accretion to the northern surfaces of Assawoman and Metompkin Islands are extremely end of Wallops Island. However, the process with the greater low and are essentially an amalgamation of thin overwash fans effect on the coastline is the capture of a huge volume of sand by migrating across the back-barrier marsh. It is difficult to define the flood-tidal delta landward of the inlet. The 12-km by 6-km the shoreline of either island on aerial photographs. Because (7.4-mi by 3.7-mi) section of the back-barrier lagoon behind of sediment starvation and rapid transgression, at times these Chincoteague and Wallops Islands is filled by this extensive barrier islands essentially cease to exist. Gutierrez et al. (2007) tidal-delta and marsh complex. interpret that the islands may be at a threshold such that with The interruption of regional longshore transport by further sea-level rise they might segment or disintegrate. trapping of sand in Chincoteague Shoals, Toms Cove Hook, The inner shelf of Chincoteague Bight also is anomalous and Chincoteague Inlet is a primary factor contributing to the along the Delmarva coast. The shoreface ridges and offshore 348 Atlantic Coast and Inner Shelf shoals prominent off Assateague Island end abruptly southeast of The tidal inlets and coast-perpendicular tidal flow dominate the Toms Cove Hook (Figure 4). In contrast, the inner shelf directly sand-transport dynamics. The inlets and associated ebb-tidal off Metompkin and Assawoman Islands is smooth, featureless, deltas trap and hold sand from the upper shoreface. Even so, and relatively deep. Along the southern margin of the Bight, the sand in the middle to lower shoreface (roughly 10 to 20 m, or sand shoals of Parramore Banks create some bathymetric relief. 33 to 65 ft, water depth) continues to move south toward the Otherwise, within the Bight the only shoals are the thin stringers mouth of Chesapeake Bay (Swift et al., 2003). Overall net sand of Porpoise Banks nearly 15 km (9 mi) offshore in water 16–18 transport in the shoreface is slowed dramatically relative to that m (52–59 ft) deep, and an unusual, unnamed, set of nearshore in the wave-dominated compartment to the north. shoals 2 km (1.2 mi) off Metompkin Island (Figure 4). Wave refraction around the outer edge of the ebb-tidal deltas, In a study of the inner shelf adjacent to Cedar Island, combined with bypassing via the migration of shoals around immediately north of Wachapreague Inlet, Wright and Trembanis the seaward apron of the deltas, produces a zone downdrift of (2003) and Trembanis (2004) observed that the shoreface is the inlets in which longshore transport reverses (flowing to the deficient in potentially mobile sand. Mapping the seafloor north). As a result, additional sand is added to the updrift end of with high- resolution side-scan sonar revealed considerable the island, and most of the Virginia barrier islands have a bulbous variability and showed areas of exposed, relict, Holocene marsh updrift end that is relatively wide with multiple beach ridges. In peat and filled back-barrier channels. Localized deposits of contrast, the downdrift end typically is a narrow, low, prograding shell and a thin, mobile, sheet of sand overlying the relict back- spit that is unstable and frequently washed over during storms. barrier deposits create sharp transitions between bottom types. Most of the Virginia barriers also exhibit downdrift offsets The bottom roughness in these areas may vary by as much as (Hayes et al., 1970; Hayes, 1979, 1980); the southern island of an order of magnitude, which substantially affects the dynamics two framing an inlet will be as much as 1 km (0.6 mi) seaward of currents interacting with the seafloor (Masden et al., 1993; of the northern island. Because of the differences in width and Wright, 1993). stability of the island sections, they tend to rotate in response to transgression rather than undergoing parallel retreat (Rice and The Virginia Barrier Islands Leatherman, 1983). The maximum distances of inlet influence From Parramore Island to Cape Charles at the southern tip extend to 6.8 km (4 mi) updrift and 5.4 km (3.3 mi) downdrift of the Delmarva Peninsula, the barriers are the short, stubby of inlets along the Virginia barrier islands (Fenster and Dolan, islands typical of a mixed-energy coast (Figure 5). They sit 1996). 6–8 km (3.7–5 miles) to as much as 12 km (7.4 miles) away The Virginia barrier islands may be categorized into three from the mainland. The back-barrier is a complex of tributary geomorphic groups based on the response to transgression (Rice tidal channels, shallow subtidal to intertidal mudflats, extensive et al., 1976; Dolan et al., 1979; Leatherman et al., 1982; Rice tidal marshes, and open bays. The larger inlets, such as Great and Leatherman, 1983): 1. a northern group that extends from Machipongo and Sand Shoals Inlets, have deeply scoured Assateague Island to Cedar Island (within Chincoteague Bight) channels 20 to nearly 25 m (65 to 82 ft) deep, 5 km (3 mi) long, is characterized by parallel beach retreat; 2. a middle group and 500 m (more than 0.25 mi) wide. The ebb-tidal deltas extend including Parramore, Hog, Rogue, Cobb, and Wreck Islands is seaward 5 to 6 km (3 to 3.7 mi), and as much as 8 to 9 km (5 to 5.5 characterized by rotational instability; and 3. a southern group mi) alongshore, with a pronounced downdrift (southward) skew. of Ship Shoal, Myrtle, and Smith Islands experiences non-

ATLANTIC OCEAN N Ship Shoal Myrtle Hog Cobb Wreck Parramore Smith Cedar Metompkin

Fishermans Mockhorn CHARLES CAPE CHESAPEAKE BAY MOUTH

VIRGINIA EASTERN SHORE

10 km CHESAPEAKE BAY

Figure 5. Mosaic of the Virginia Eastern Shore from photos taken from the International Space Station (NASA images ISS004-E-11554 and 11555). The southern Delmarva section of the coast includes three smaller segments: the arc of severe erosion from Chincoteague Inlet to Wachapreague Inlet, the central section of Parramore, Hog and Cobb Islands, and the arcuate reach of Wreck through Smith Islands which follows the inner rim of the Chesapeake Bay Impact Crater.

349 The Geology of Virginia parallel beach retreat. The variations in geomorphology are a have opened and closed on decadal time scales. Several primary response to differing sediment supply, underlying Pleistocene physical factors influence the geographic distribution and modes topography, and vertical tectonic movements. It is worth noting of response of the tidal inlets (McBride, 1999). These are the that this categorization preceded the discovery of the Eocene balance between tidal range and wave energy, which is the most Chesapeake Bay Impact Crater. The Inner (or “Peak”) Ring important factor, wave direction, tidal prism, storm magnitude of the crater neatly separates the middle and southern group and recurrence, and antecedent topography. of islands, and the southern group essentially rests atop the As an example of the morphology of a single island, Inner Ring. The Outer Ring of the crater is slightly south of Parramore Island has three distinct zones that respond differently Wachapreague Inlet, which separates the middle and northern to transgression (Figure 7) (McBride and Vidal, 2001). The groups (Poag, 1997; Powars, 2011). It is likely that differential northernmost zone is influenced by Wachapreague Inlet and its compaction and local tectonics associated with the crater account ebb-tidal delta, and has retreated since 1980. The second zone is for the “tectonics” cited by Leatherman et al. (1982). The filled the north-central segment of the island, which is characterized Eastville and Exmore paleochannels cut by the Susquehanna- by truncation of tree-lined beach ridges (Figure 8). Although Potomac River system (Colman et al., 1990) also underlie the historically fairly stable, the central zone has experienced transitions between island groups. accelerating retreat since 1962 (4.6 m/yr from 1959/62–1980 The tidal inlets are dynamic through time as documented increasing to 9.0 m/yr from 1998–2000). The retreat rates of the in historic maps of the Delmarva coast (Figure 6). The barrier northern and central sections increased further between 1997 and islands exhibit two primary modes of inlet evolution (McBride, 2004 to 12 m/yr and 10 m/yr, respectively, based on LIDAR data 1999). For the wave-dominated barriers of Assateague and (Wikel, 2005). The third zone is the southern half of the island, Fenwick Islands, inlets primarily open by breaching during major which is a low, narrow spit dominated by overwash processes. storms. The inlets then rapidly migrate laterally to the south in The rapid retreat of the southern section—7.8 m/yr from 1871 response to net longshore transport, and close after a few years to 1959/62, 6.4 m/yr from 1959/62 to 1980, 12.8 m/yr from to a few decades (i.e., these inlets are short-lived). For the tide- 1980 to 1998, and 15.6 m/yr from 1998 to 2000—contributes dominated barriers from Wallops Island south, the larger inlets to the clockwise rotation of the island. McBride and Vidal tend not to migrate (i.e., they oscillate in place), and remain open (2001) attributed the accelerating erosion rate to four factors: much longer. However, McBride (1999) also describes a suite 1.extension of the arc of erosion south of Assateague Island; 2. of smaller, ephemeral inlets in the Virginia barrier islands that impact of the 1962 Ash Wednesday storm; 3. morphodynamics

Cedar Is. Wachapreague Inlet Assateague Is. Wachapreague Green Run 1790 - 1882 (?) Upshur Neck MD closed 1877 & 1880 VA Pope Is. 1850 - 1890 Chincoteague Is.

Chincoteague 1849 - present Wallops Is. Italian Assawoman 1851 - present Ridge Assawoman Is. Gargathy 1851 - 1980 Metompkin Is. breaches various dates Metompkin 1852 - present new ~1990 - 2005 CHESAPEAKE BAY Cedar Is. Parramore Island Wachapreague 1852 - present

Parramore Is. breaches various dates Quinby 1852 - present Hog Is. Great Machipongo 1853 - present Cobb Is. Sand Shoal 1870 - present [ Bone Island pre-1877 Quinby Inlet Wreck Is. New 1852 - present Wine pre-1877 Ship Shoal Is. } [ Little & Ship Shoal 1852 - present Myrtle Is. Mudhole various dates Bungalow 1942 - 1981 Smith Is. breaches various dates ATLANTIC OCEAN Hog Smith Island Inlet 1849 - present Is. 5 km Fishermans Is. [Fishermans Is. Inlet 1849 - present

Figure 6. Location of present and historic inlets in the Virginia Figure 7. Parramore Island, Virginia, with three geomorphic barrier island chain. Initial dates refer to dates of the first chart zones that respond differently to transgression, as discussed in on which inlet was named. Names of barrier islands are in italics. text. Image generated from Google Earth; USDA Farm Service Modified from McBride (1999). photographs from May 2006.

350 Atlantic Coast and Inner Shelf

contrast, more protected areas with limited fetch, such as around Mockhorn Island in the southern part of the lagoon system (shown on Figure 5), sustained slow marsh growth through the mid-1990s (Erwin et al., 2004). The observational evidence from the behind the Virginia barriers suggests a fundamental shift from net marsh expansion 30 years ago (Finkelstein and Ferland, 1987) to net marsh loss since 2000 (Erwin et al., 2006). During that period the global rate of sea-level rise more than doubled (IPCC, 2013). In the Wachapreague area between 2000 and 2004, the local rate of sea-level rise of 3.9 mm/yr exceeded by factors of 3 to 5 times the rates of accretion of 1.4 mm/yr for the mid marsh and 0.7 mm/yr for the high marsh (Erwin et al., 2006). However, at the same time marsh ponds filled at a substantial rate, 11.5 mm/ yr, well above that of the relative sea-level rise. In response, Spartina alterniflora (salt-marsh cord grass) moved laterally into pond edges and onto tidal flats (Erwin et al., 2006). With Figure 8. Shoreface erosion on Parramore Island. The rapidly retreating ocean shoreline is cutting into the island’s maritime accelerating sea-level rise, it appears likely that large areas of forest. Photo taken in August 2006 by C.H. Hobbs, III. existing salt marshes may revert to open water over the next 50 to 100 years (Erwin et al., 2006). of the large ebb-tidal delta at Wachapreague Inlet; and 4. continuing relative sea-level rise. Chesapeake Bay Mouth The Virginia Coast Reserve / Long Term Ecological Research Site (VCR/LTER) on the barrier and back-barrier system of southern Delmarva has been the focus of numerous The mouth of Chesapeake Bay is a dynamic area with integrated geological and ecological studies. The VCR/LTER shoals and channels both inside the bay and on the adjacent research activities “focus on the mosaic of transitions and steady- inner shelf (Figure 9). The net transport of seawater, suspended state systems that comprise the barrier-island/lagoon/mainland sediment, and sand along the bottom is from the shelf into the landscape of the Eastern Shore of Virginia” (Virginia Coast lower bay, as traced by drifters (Harrison et al., 1967), and Reserve, 2007). Primary study sites are located on Parramore inferred from sediment budgets (Meade 1969, 1972; Hobbs et Island, Hog Island, and mainland marshes near Nassawadox, al., 1992). Within the main stem of the lower bay, major shoals Virginia. have developed on the interfluves between the modern channels in the bay, the courses of which tend to follow the lowstand The marshes behind the Virginia barrier islands exist in a channels of the Susquehanna, York, and James Rivers. tenuous balance between sediment accumulation and sea-level Convergence of sand transport creates an extensive shoal rise. The elevation of the marsh surface with respect to the complex on the northern flank of the bay mouth off Cape Charles, elevation of mid-tide, the rate of relative sea-level rise, sediment and a lesser shoal on the southern flank off Cape Henry. This outwash from the mainland, and capture of suspended sediment massive accumulation of sand extends at least 20 km (12 mi) from the coastal ocean (transported in through the inlets) work out onto the shelf, and at least 15 km (9 mi) into the lower bay. together to control the net rate of marsh accretion (Oertel et al., As sea level rose through the Holocene, the ancestral equivalent 1989). Further, it is important to differentiate between marsh of the bay-mouth shoal system left a trailing sand body that is accretion resulting from short-term events and from long-term accumulation of suspended sediment. York River Within the back-barrier marshes, several physical processes control sedimentation onto the marsh surface. On daily time James River scales, flocculation of suspended material to produce larger Cape Charles Chan. Hampton Chesapeake particles, and the dramatic reduction of water velocity and Channel Thimble Shoal Chan. The turbulence by marsh grass enhance deposition of fine sediments. Horseshoe Will. Flocculation of fine particles may account for as much as 75% Spit Cape Charles of the inorganic material deposited within 8 m (26 ft) of tidal Little Creek creeks (Christiansen et al., 2000). Concentrations of suspended Middle Ground Smith Island sediment at the marsh edge usually are greater on rising tides Fishermans Is. than falling, indicating net deposition in the marsh during high Nautilus Shoal tide and early in the ebb phase. The greatest rate of deposition Cape Henry is locally near the creek edge, producing low levees along the Virginia banks. Beach

Location within the back barrier also is critically important Sandbridge for marsh accretion and expansion. Not surprisingly, sites adjacent to open water are more susceptible to marsh loss Figure 9. Topographic and bathymetric features of the around the edges because of wave attack (Erwin et al., 2004). In Chesapeake Bay mouth. Zoomed view from Figure 3. 351 The Geology of Virginia easily traced across the shelf. These shoal-retreat massifs appear very shallow, shoal system, but no island. In the interpretation as prominent bathymetric features on the shelf seaward of each of Oertel and Overman (2004), Fishermans Island became of the major Mid-Atlantic estuaries (Swift, 1975). emergent as waves shallowed and refracted across the shoals, The bay-mouth sand bodies are a potentially valuable sweeping sand landward. This process initially generated linear resource for beach nourishment, if the sand is of the appropriate bars parallel to shore that then migrated landward to create the grain size. Most of the surficial sediment in the bay mouth is fine ridges and hummocks of the modern island (Figure 10). The to very fine sand derived from outside the bay during the late unique crescentic shape, with beach ridges that wrap almost 360 Holocene (Meisburger, 1972). However, sand for nourishment degrees around the island, sets Fishermans Island apart from the of recreational beaches usually needs to be medium sand, or at other Virginia barriers. least the coarse end of fine sand. In contrast to the shoals, coarse, Together, these studies indicate that the Bay Mouth Segment gravelly sand is exposed in Thimble Shoals Channel, which of the Virginia coast plays an important role in the sediment is the southernmost channel into the bay (Figure 9). Thimble transportation and deposition processes of the inner and middle Shoals Channel aligns with the lowstand channel of the James continental shelf. River, and tidal currents are likely excavating the coarse sand from Pleistocene or older fluvial deposits. Southeastern Virginia Mainland Process studies of sediment trapping and accumulation in the shoal system (Ludwick, 1970, 1975, 1981) documented As noted previously, the shore of southeastern Virginia is deposition rates near Thimble Shoal Channel that are very high part of the large coastal compartment that extends from Cape compared to other sections of Chesapeake Bay. The Tail of Henry at the mouth of Chesapeake Bay south to Cape Lookout, the Horseshoe, a shoal built on an interfluve just north of the North Carolina. Stepping back from the shore, the mainland channel, is aggrading on its southern flank, in effect migrating landward of Currituck Sound and Back Bay is a low-elevation, south (Ludwick, 1981). Similarly, sand waves 5 km (3 mi) north low-relief plain extending west 50 km (31 mi) to the Suffolk of the Tail of the Horseshoe are asymmetrical in shape indicating Scarp (Figure 11). Individual late Pleistocene shorelines trace movement to the south (Ludwick, 1981). across this plain, and intersect the modern coastline obliquely. The Inner Middle Ground shoal, which includes Nine-Foot The relative high ground of the Virginia Beach headland is Shoal on its northern flank, occupies the north-central section of created by several of these Pleistocene shoreline ridges and the bay mouth and spans about 40% of its width. This system is part of a cuspate foreland similar in shape and location to the a classic example of a bay-mouth shoal developed with mutually modern Cape Henry (Allen et al., 2012). South of Virginia evasive dominant flood-tidal and ebb-tidal channels (Ludwick, Beach, the U.S. Navy facility at Dam Neck and the northern part 1970). For this type of shoal, the high-relief margins and cross- of Sandbridge each sit astride Pleistocene ridges. The section cutting channels alternately protect one face of the shoal while from Sandbridge south to False Cape is the low trough between allowing high-velocity tidal flow during the flooding and ebbing ridges. Here the coast is a narrow barrier spit backed only by the phases. Consistent with the overall south by west migration of wetlands and lagoons of the northernmost end of the Currituck sand in the bay mouth, the main body of Inner Middle Ground Sound system. migrated 1.9 km (1.1 mi) southward in the 124 years preceding The shore of southeastern Virginia has been the site of the study by Granat and Ludwick (1980). many studies of shoreline processes, especially as they affect the Seismic profiles show that the modern bay-mouth shoal economically important beaches. Besides the physical setting, system is a distinct stratigraphic unit created during the late varying combinations of beach and shoreface processes interact Holocene when sea level has been close to its present elevation, to affect the response of the shore zone (Harrison and Krumbein, i.e., within the last several thousand years (Colman et al., 1964). In some cases, response may lag active process by as much 1988). Large-scale cross-bedding within the body of the shoal as 12 hours. Beach profiles show direct evidence of responses; shows southward and westward growth into Chesapeake Bay. Goldsmith et al. (1977) reported on repeated surveys of 18 This migration is supported by the substantial supply of sandy beach profiles spaced between Cape Henry and the Virginia– sediment from the shelf into the bay (Hobbs et al., 1992). Once North Carolina border collected from September 1974 through within the bay mouth, the sediments are reworked by tidal December 1976, and reviewed profile data going back to 1956. currents, with much recycling but net movement south and west. They found narrow, erosional beaches in the center of the area, Cape Charles, the southern tip of the Delmarva Peninsula, is Dam Neck, Sandbridge and Back Bay National Wildlife Refuge, also the point of convergence of two littoral transport pathways and wider accretional beaches at the north and south ends of the (Oertel and Overman, 2004). The dominant flow is southward study area, Fort Story and False Cape State Park, respectively. on the Atlantic side, moving sand past Smith Island toward During the study period, the maximum rate of beach accretion Nautilus Shoal just outside the bay mouth (Figure 9). A smaller was 18.9 m3 per meter of beach front per year at Fort Story, and volume also moves southward along the western shore of the the maximum rate of erosion was 11.6 m3 per meter per year at peninsula, the Chesapeake Bay side, and is driven mostly by Sandbridge. They attributed this pattern to a central nodal zone strong northwesterly winds that are common during the autumn where the direction of net longshore transport shifts from north and winter. to south. They concluded, “The Virginia Beach commercial area Immediately landward of this zone of convergence, would be erosional without the extensive sand nourishment Fishermans Island has developed as an emergent barrier island which is necessary for the maintenance of the commercial (Oertel and Overman, 2004) since the early 1800s (Boule, beaches.” 1976a, b, 1978). Prior to 1800, charts show an extensive, The southern portion of the shoreline of southeastern Virginia 352 Atlantic Coast and Inner Shelf

Cape Charles Smith Island

Fishermans Island

Nautilus Shoal Chesapeake Bay Bridge-Tunnel 2 km

Figure 10. Fishermans Island, an emergent barrier island (Oertel and Overman, 2004) along the north flank of the Chesapeake Bay mouth. Image generated from Google Earth; USDA Farm Service photographs from June 2011. is part of Currituck Spit, the long barrier spit that extends from the main inlet channel was 400 m (1,300 ft) wide and 7 m (23 Sandbridge, Virginia, to Oregon Inlet, N.C., and constitutes the ft) deep in the channel axis, and prograded from north to south northern segment of the Outer Banks. As a prominent section (McBride et al., 2004). of the Mid-Atlantic coast, the history of the Outer Banks is Much of the ocean shoreline of southeastern Virginia documented by charts dating back to Colonial times (Hennigar, has been managed through construction of sea walls and/or 1977). Although there are geomorphic indications of numerous nourishment. Three separate areas, the resort area north of Rudee former inlets, only “Old Coratuck” (or “Old Currituct”) Inlet, Inlet, the federal facility at Dam Neck, and Sandbridge, have which opened in 1657 and closed in 1731, has been known been nourished at various times during the past half century. in historic times (Hennigar, 1977; Prow, 1977; Robinson and Between 1951 and 2003, almost 17.5 x 106 m3 (23 x 106 yd3) of McBride, 2006). Indeed, according to Sharpe (1961), Currituck sand were placed during approximately fifty separate projects for was a major port. In 1728, while surveying the border between a total cost of about $150 million (Western Carolina University, Virginia and North Carolina, William Byrd described the inlet 2007). Most of the projects were annual renourishment along as follows: “It was just Noon before we arrived at Coratuck James Inlet, which is not so shallow that the Breakers fly over it with a River horrible Sound, and at the same time afford a very wild Prospect. Suffolk Scarp ... leaving an opening of not quite a mile, which at this day is not Poquoson practical for any vessel whatsoever. And as shallow as it is now, Hampton it continues to fill up more and more, both the Wind and the Great Dismal Waves rolling in the Sands from the Eastern Shoals.” Swamp Norfolk Recent work in the former Currituck Inlet (McBride et al., 2004; Robinson and McBride, 2004, 2006) documented the Scarp dynamic nature of inlets and the Outer Banks barrier. The inlet Hickory was open before 1585. Between 1713 and the closure of Old } Cape Currituck in 1731, a new inlet, New Currituck Inlet, opened Henry about 10 km (6 mi) south, and progressively captured the tidal Va. Beach Pungo Ridges headland prism of the back barrier. By analyzing vibracores from the Sandbridge relict flood-tidal delta, McBride et al. (2004) identified four Back Bay sedimentary facies that record the history of the inlet. From Knotts Is. oldest to youngest, these sedimentary environments are the 1. estuary; 2. active, subtidal, flood-tidal delta; 3. diminishingly False Cape active, intertidal, flood-tidal delta; and 4. marsh. Studies of the Old Currituck foraminifera also show salinity in the marsh decreasing with time Inlet as seawater exchange through the inlet diminished (Robinson Figure 11. Topographic features of the southeastern Virginia and McBride, 2006). Ground-penetrating radar profiles show coast. Zoomed view from Figure 3. 353 The Geology of Virginia

5,600 m (18,480 ft) of the resort area. Much of this material was farther offshore. The oldest of these channels trends northwest- obtained from maintenance dredging of Rudee Inlet, occasional southeast, whereas the other two are oriented more north- dredging of entrance navigation channels to Chesapeake Bay, south, nearly parallel with the modern shoreline, and possibly and, early on, upland sources. indicative of a trellis drainage pattern. Since 1996, Sandbridge Shoal, 5.5 km (3.4 mi) offshore, has been the source of sand to nourish both Dam Neck and Inner-Shelf Sediments Sandbridge. Through 2003, over 3.5 x 106 m3 (4.6 x 106 yd3) of sand have been excavated from the shoal and placed onto In the present coastal zone of the Mid-Atlantic, nearly the beaches (Diaz et al., 2006). Monitoring the beach has been all coarse clastic material (sand and gravel) carried by rivers important for determining the effectiveness of nourishment draining the Piedmont and Appalachians is deposited at the head projects. Hardaway et al. (1998) reviewed previous studies of of the tidal estuarine portion of the river (Meade, 1969, 1972). shoreline change, and Wikel et al. (2006) updated the recent Finer-grained particles (silt and clay) discharged by the rivers history. Sand trapping by the jetties at Rudee Inlet has produced are held in suspension until flocculation and rapid deposition local beach accretion. Also, the area immediately south of occur in the zone of maximum turbidity, which coincides with Rudee Inlet benefits from its position relative to the sediment the initial increase of dissolved ions (salinity) in the upper reach transport nodal zone whereby, locally, sand is transported north of the estuary (Nichols, 1972; Gibbs, 1987; Nichols et al., 1991). into the headland area. Farther south below Sandbridge, where This means that the supply of new sand to the upper reaches the shoreline is backed only by marsh or lagoon, the long-term of the estuaries is displaced spatially from the ocean coast by history shows persistent erosion and shoreline retreat. more than 100 km (60 mi). Consequently, nearly all of the The search for beach-nourishment sands has encompassed sand moved about in the coastal zone and on the shelf has been surveys employing seismic profiling and vibracoring to evaluate eroded and reworked from previously deposited sediments. The both the modern processes and the shallow stratigraphy (the modern Mid-Atlantic shelf is effectively sediment-starved, and upper 20 m) of the inner shelf (Shideler and Swift, 1972; Swift et al. (1971) applied the term palimpsest – derived from Shideler et al., 1972; Williams, 1987a, 1987b). The surficial unit the scraping off and reuse of parchment in Medieval times – of the inner shelf is a discontinuous sand sheet formed during to describe the continual reworking of existing sediment on the the Holocene, Unit D of Shideler et al. (1972). This sand sheet shelf. is relatively thin overall, but is swept by waves and currents to Most of the surficial sediments of the Virginia shelf out to create low shoals, and a few larger shoals locally. Beneath the water depths of 60 m (200 ft) are arkosic to subarkosic sands sand sheet, a series of filled near-surface channels are cut into (Milliman, 1972; Milliman et al., 1972; Amato, 1994). That the underlying upper Pleistocene coastal and shelf sequence. is, the sands are mineralogically and texturally mature, with a These channels influence both the morphology and surficial moderate amount of finer-grained particles, especially farther sediment type of the modern inner shelf (Chen, 1992; Chen et al., offshore in deeper water. Somewhat anomalously, a narrow 1995; Hobbs, 1997; McNinch, 2004; Browder, 2005; Browder strip of “subarkosic to arkosic, fine-grained sediments” appears and McNinch, 2006). These channels were formed as the late along the inner shelf; Milliman et al. (1972) did not have a clear Wisconsinan lowstand drainage network, equivalent in time to explanation for these sediments, and interpreted the deposit as the Cape Charles paleochannel of the Susquehanna River, and “nearshore (fluvial?).” filled as sea level rose during the Holocene. South of the Chesapeake Bay mouth, along the southeastern Geophysical surveys of the southern reach of the Virginia Virginia coast into North Carolina, the surficial sediments are coast and the northern North Carolina coast show the connection predominantly shells, sand, and gravel (Shepard, 1932). Hobbs between subsurface strata and shoreface processes and features (1997) provided more detail on the distribution of surface (McNinch, 2004). Shore-oblique sand bars occur adjacent to sediment types than earlier studies based on a set of 380 grab gravel outcrops, and those outcrops are the surface expressions samples collected from the Chesapeake Bay mouth to the of underlying fluvial channels filled with relatively coarser Virginia–North Carolina border, and from the shoreline east sediments. After energetic conditions, large-scale sandbars about 60 km (37 mi) into maximum water depths of 38 m (126 recur in the same locations, and the sandbar-gravel outcrop ft). Sands and granules are the dominant surface sediment. pairs coincide spatially with erosional hotspots on the beach However, two nearshore pockets have a slightly elevated (McNinch, 2004; Schupp et al., 2006). fine-grained component. The first is immediately south of the Browder (2005) and Browder and McNinch (2006) linked Chesapeake Bay mouth, and likely is the result of deposition the antecedent geology with nearshore morphology in the from the suspended-sediment plume transported out of the Bay. Sandbridge, Virginia, and the North Carolina study areas. They The second is farther south, in the vicinity of False Cape, and mapped four southwest–northeast oriented channels crossing likely represents an outcrop of relict back-barrier, lagoonal the nearshore zone at Sandbridge. The smaller channels of the sediments. Other small areas of fine-grained sediments on the set, which are about 100 m (330 ft) wide, may be analogous southern Virginia shelf are associated with local topographic to the “Cape Charles Aged Paleochannels” of Chen (1992) and depressions or outcrops of older strata. Chen et al. (1995), and the small channels depicted by Hobbs Consistent with the maturity of the sediments on the Virginia (1997). These channels could be either old upland drainage or shelf, the heavy mineral assemblage of the fine-sand fraction tidal inlet channels. The origin of the larger, 800 m (2,600 ft) is generally of the amphibole–garnet–kyanite suite (Swift et wide, channel in the set is more problematical. Hobbs (1997) al., 1971). Garnet, derived from erosion of crystalline rocks noted three channels of similar scale but different orientation and resistant to weathering, constitutes 16–30% of the heavy- 354 Atlantic Coast and Inner Shelf mineral fraction (Milliman et al., 1972; Amato, 1994). Locally, Relative Sea Level offshore of the Chesapeake Bay mouth, out to the 60 m (200 ft) isobath, garnet concentrations may be as high as 45%. However, Change in sea level, more properly in relative sea level, within the bay mouth the garnet fraction is relatively low, is a major factor in the evolution of the Mid-Atlantic coast. possibly because of dilution by the large volume of arkosic fine Relative sea level is the vertical position of the sea relative to sand transported into the area. Another large section of the shelf fixed landmarks on the freeboard of the continent; it canbe off Delmarva, between the Delaware–Maryland and Maryland– thought of as the effective sea level. Two major sets of processes Virginia borders, has relatively high garnet concentrations. work together to change relative sea level: eustatic processes, From a survey between Cape Henry and Cape Hatteras, and regional or local tectonic processes. Eustatic processes Swift et al. (1971) defined three heavy-mineral provinces influence the level of the sea globally. Variation in the volume of across the inner shelf. Two zones dominated by the amphibole– water transferred between the world’s ice caps and oceans is the garnet–kyanite suite appear on the beach and farther offshore, dominant eustatic process. The polar ice sheets wax and wane but are separated by an amphibole–epidote–kyanite suite in the in cycles of approximately 20 and 40 ky, and in major cycles nearshore. This zonation correlates spatially with grain-size of 100 ky that produce sea-level changes on the order of 120 provinces that respond to the hydraulic regimes of the beachface m (395 ft). Other physical processes, such as those associated and shoreface. A less distinct variation appears alongshore, with sea-floor spreading, alter the size of the ocean basins and with garnet and opaque minerals increasing and amphibole thus the level of the sea. However, those processes operate over decreasing to the south. millions of years, time scales longer than cycles in the growth While earlier studies were primarily sedimentological, and decay of ice caps. Grosz and Eskowitz (1983) considered the economic potential Changes in global climate and total volume of glacial ice of heavy-mineral deposits on the Virginia shelf. Other studies can be reconstructed from many different preserved geologic (Berquist and Hobbs, 1986, 1988a, 1988b, 1989; Ozalpasan, records. Some of the most continuous and detailed time series 1989; Berquist, 1990; Berquist et al., 1990; Grosz et al., 1990; are from cores drilled in deep-ocean sediments and thick glacial Dydak, 1991) specifically addressed the economic heavy ice, such as on Antarctica and Greenland. Many of the ice cores minerals of the inner shelf, generally within 10 km (6 mi) of have annual layers of snow that provide a chronology analogous the shoreline. The minerals of potential interest are the titanium- to that of tree rings. These cores yield detailed records of bearing minerals ilmenite, leucoxene, and rutile, along with changes in temperature and atmospheric gases, including the zircon and monazite. At least four areas satisfy Garner’s (1978) greenhouse gases carbon dioxide and methane, back through the criteria for threshold levels of economic heavy minerals: off Hog Island, and off Smith Island, in the southern half of the most recent ice age (roughly 12 to 71 ka, or thousand years ago) Eastern Shore; off Virginia Beach just south of the Chesapeake to the previous interglacial warm period (about 71 to 130 ka). Bay mouth; and off False Cape near the Virginia–North Carolina Recent work with extremely long ice cores from Antarctica has border (Berquist et al., 1990). yielded records through multiple glacial–interglacial cycles as far back as 800 ka (Jouzel et al., 2002, 2007). Changes in air temperature over the ice sheet are quantified by analyzing the EVOLUTION OF THE VIRGINIA COAST ratios of stable (i.e., not radioactive) isotopes of oxygen (16O and 18O) and hydrogen (1H and 2H) in the water molecules of the ice. A human lifetime affords only the briefest snapshot in The climate records derived from deep-ocean sediment the evolution of a coastline. To assemble a more complete cores also are based on stable isotopes of oxygen, as preserved history, coastal geologists evaluate preserved evidence—in the form of historical maps, landforms, and stratigraphy—of in the calcium carbonate shells of microscopic marine organisms such as foraminifera. The 18O/16O ratio in the CaCO of the processes operating over hundreds, thousands, and millions of 3 years. As introduced previously, the larger-scale and longer- shells records changes through time of the ocean temperature term geologic history of the Virginia continental margin and the isotopic composition of the seawater. Stable oxygen stretches back nearly 220 My to the initial tectonic rifting that isotopes in seawater trace global ice volume, and hence are a separated North America from Africa and opened the Atlantic proxy for global sea level. Numerous paleoceanographic studies Ocean basin. The sedimentary processes operating since that have documented the empirical relationship that a change 18 16 time provided sediment to the coastal zone, either as an influx of 0.1 part per thousand in the O/ O ratio (expressed as 18 of new siliciclastic sediment eroded from the Piedmont and δ O ‰) is equivalent to 10 m of eustatic sea-level change Appalachians or by reworking of existing sediments on the (e.g., Fairbanks and Matthews, 1989), although the details are still continental margin. The penultimate stage of this evolution being evaluated (e.g., Cutler et al., 2003). This response occurs 16 has played out since the earliest Pleistocene (the last 2.6 My) because the light isotope O is preferentially removed from the as global climate shifted into a mode that produced the Ice ocean during the evaporation of water. If the 16O-rich water is Ages. During this period, global sea level cycled between precipitated as a snow layer and stored as glacial ice, it decreases highstands during which shorelines were constructed and the the δ18O of that ice; at the same time, it effectively enriches the shelf aggraded, and lowstands dominated by downcutting of the remaining seawater with 18O. The compositional change of the major river systems. Finally, during the Holocene transgression seawater is then preserved in the δ18O of foraminiferal shells. of the last 12,000 years, the modern coast has been shaped by Both the ice-core and deep-ocean records show cyclic rising sea level, and by wave and tide energy that has transported variations in global climate that are driven by changes in the and deposited sediments to create the barrier islands, capes, and orbital geometry of the Earth around the Sun (Milankovitch mainland shorelines. cycles), and are amplified by atmospheric greenhouse gases, 355 The Geology of Virginia

especially CO2. Detailed correlations between the ice-core and Change in relative sea level is the sum of the eustatic, deep-ocean data show that these mostly independent records are isostatic, and tectonic processes. The change in relative sea tracking the same global climate signal. This synchrony is so level can vary from place to place because although the eustatic consistent worldwide that discrete periods of geologic time are component is uniform around the world ocean, the isostatic- now defined as Marine Isotope Stages (MIS). The cycles have tectonic component can differ substantially between regions. been enumerated with the convention that odd-numbered stages Relative vertical motion can be dramatically different over indicate interglacial (warm) intervals and even-numbered stages a small distance, such as in Alaska where two sites can be on indicate glacial (cool) stages (Figure 12) (Hays et al., 1976; opposite sides of an active fault. Martinson et al., 1987; Lisiecki and Raymo, 2005). Beyond the global to regional effects of eustatic sea For the late Pleistocene, MIS 2 was the most recent level and tectonics, local factors controlling the response glacial episode—coinciding with the maximum extent of the of a section of the coast include the topography, antecedent Wisconsinan Glaciation and the minimum sea-level lowstand geology, and sediment supply. Even during an overall rise in at 21 ka. MIS 3, equivalent to the Wisconsinan Interstadial of sea level, different sections of a coast may experience different glacial literature, was a mild warming during which global sea rates of rise or even a relative fall of sea level depending upon level did not reach the present elevation. Similarly, MIS 4 was the interplay of these factors. For example, if eustatic sea a mild cooling that was not as extreme as the glacial maximum level is rising, the land is subsiding, and/or sediment supply of MIS 2. The last full interglaciation was MIS 5 from 130 to is relatively low, the shoreline or barrier islands will migrate 71 ka. MIS 5 has a characteristic roller-coaster step-down from landward, producing a transgression. Conversely, if eustatic the peak temperature and sea level at 123 ka, known as the sea level is falling, the land is being uplifted, or sediment input Sangamonian Interglaciation, into the mild glaciation of MIS 4. to the shoreface is substantial, the shoreline migrates seaward, These substages of MIS 5, with three peaks and two intervening producing a regression. Sediment supply, in particular, may vary lows, are designated with letters from 5e at 123 ka through 5a at considerably along a stretch of coast—a spit that is the receiving 82 ka (Figure 12). end of longshore transport may be prograding (extending) while Separate from the level of the ocean moving up and down, at the same time a sediment-starved downdrift area may be tectonic processes result in regional or local movements of the eroding and retreating rapidly landward. This is the case with the Earth’s crust. If an area of land is uplifted relative to the elevation progradation of southern Assateague Island and the rapid retreat of the ocean, the local sea level appears to fall; conversely, if of the downdrift barrier islands within Chincoteague Bight. an area subsides, sea level appears to rise (Figure 13). These In the Mid-Atlantic region, subtle tectonic processes add processes may act rapidly due to major earthquakes, but more to the eustatic component with the result that the rate of sea- typically they act slowly, such as the progressive depression of level rise as determined from long-term tide gages generally is the land surface under and near growing ice caps, or as rising between 3.0 and 4.4 mm per year (1 to 1.5 feet per century) sea level floods and weighs down upon the continental margin. (Zervas, 2001). The greatest present rate of rise in the area is at This vertical adjustment of the Earth’s crust is termed isostasy. Hampton Roads, within the faulted and subsiding Chesapeake Because the Earth’s mantle flows slowly, the response of vertical Bay Impact Crater. Because the Mid-Atlantic continental margin movement typically lags the change in loading at the surface and has such a low slope, in most areas less than one degree, a 1-m may continue to adjust for thousands of years. (3.3-ft) rise of sea level will result in 50 to 120 m (165 to 395 ft) of landward migration of the shoreline (Zhang et al., 2004). As

+20 +? m sea level rises and the high-energy shoreline moves landward, +9 m + 9? m Interglacial, +3 m modern warm with 0 any low-lying areas behind the shoreline are progressively sea level Last full high sea level inundated to create the back-barrier lagoon and marsh complex. -20 interglacial, Sangamonian 5e 7 -40 5a 5c A Hierarchy of Processes in Time and Space Previous glacial, -60 Wisconsinan 5b 5d Interstadial Illinoian -80 In evaluating the morphology and distribution of barrier LGM Stage 5 -100 islands worldwide, and applying that categorization to the 3 4 barrier islands of the Georgia Bight, Hayes (1994) developed Sea level (m) relative to modern -120 Glacial, 6 cold with 2 a hierarchical list of controlling physical factors (Table 2). This -140 low sea level 0 20 40 60 80 100 120 140 160 180 200 list of basic controls applies equally well to the Virginia coast: Time (thousands of years BP) all of the Virginia coast shares the same tectonic setting on the trailing edge of the westward-drifting continent, all of the coast is microtidal, with a tidal range less than 2 m, and all of the coast Figure 12. Marine oxygen isotope record of global ice volume as a proxy for sea level through the past 200,000 years showing the has limited input of new sediment (with the exception of the major glacial and interglacial episodes (data from Martinson et convergence of sediment transport at the northern flank of the al., 1987). Marine isotope stages are identified by numbers under Chesapeake Bay mouth). However, the set of important factors the curve. Abbreviation LGM is Last Glacial Maximum, which influencing the coastal morphology expands when considering is the Wisconsinan Glaciation. Dashed lines above the curve indicate highstand deposits in the Mid-Atlantic region associated the details of the local setting and geologic history. with the global periods of reduced ice volume (Wehmiller et al., A modified list specific to the Virginia coast includes the 2004; Mallinson et al., 2008; Parham et al., 2013). following factors and processes, in approximate order from large 356 Atlantic Coast and Inner Shelf spatial and long temporal scales to processes operating today. Foundation of the Coastal Zone • structural setting of the continental margin (trailing-edge) The central Virginia Coastal Plain and shelf lie within the • long-term vertical tectonic movement (post-rifting Salisbury Embayment, a sedimentary basin located between subsidence) the South New Jersey Arch and Norfolk Arch (Richards, 1967, • foundation of Cretaceous and early Tertiary marine and 1974; Ward and Strickland, 1985). These structural highs marginal-marine sediments and the intervening basin directly influenced Tertiary and • basin created by the Eocene Chesapeake Bay Impact Quaternary stratigraphic development along the Mid-Atlantic Crater coast. Deposition in the Salisbury Embayment has produced • post-Eocene infill of the Impact Crater a seaward-thickening wedge of sediments beneath the coastal • positions of the major rivers and their broader valleys plain, shelf, and continental slope. The older, deeper sediments through the late Cenozoic in this sequence reflect continental depositional environments • episodic delivery of coarse clastic material to the (fluvial and deltaic) that dominated the early stages of the post- Coastal Plain and shelf rift history. These were followed by inundation by the ocean • major changes of sea level through the Pliocene and and deposition of marine and marginal-marine sediments since Pleistocene, since 5 My ago the Late Cretaceous. The 200-My view of the Atlantic margin • last interglacial-glacial cycle of sea level, from 130 ka may be described as the as the up-building and out-building to present of sediments on the continental shelf and slope over subsiding • regional post-glacial isostatic adjustment Triassic and Jurassic strata (Uchupi, 1970). • continuing and accelerating Holocene sea-level rise During the past ~160 My, the persistent transfer of mass

Initial relative sea level Continent Ocean Shoreline

RSL-1

TRANSGRESSION REGRESSION

Global sea-level rise Global sea-level fall Continent Ocean Continent Ocean RSL-2 RSL-1 RSL-1 RSL-2

OR OR Tectonic subsidence Tectonic uplift

Continent Initial land surface Ocean Continent Ocean RSL-2 RSL-1

RSL-1 Initial land RSL-2 surface

Figure 13. Schematic representations of the processes associated with changes in relative sea level. Left column of panels: From the initial sea level relative to a fixed position of the shoreline on the continent (marked by flag), a transgression may be caused by a rise of eustatic (global) sea level and/or tectonic subsidence of the land surface. Right column: A regression may be caused by a fall of global sea level and/or tectonic uplift of the land surface. 357 The Geology of Virginia

Table 2. Basic controls of barrier island morphology. Adapted Effects of the Chesapeake Bay Impact Crater from Hayes (1994). Basic Controls of Barrier Island Morphology The last major greenhouse climate for the planet was during the early Eocene (56–48 Ma). At that time, the continents and 1 Large-scale tectonism oceans were arranged differently than today, Antarctica was barrier islands are most common on trailing-edge, not yet completely isolated over the South Pole, global oceanic and atmospheric circulation were different, atmospheric CO depositional coasts of continents 2 concentrations were 2.5 times pre-Industrial levels, and there 2a Sediment supply and relative sea-level changes was minimal glacial ice globally. The oceans flooded well onto determine whether the barrier islands are transgressive the edges of the continents worldwide, the entire Gulf of Mexico or regressive Coast Plain was fully marine, and in the Mississippi Embayment estuarine conditions extended north of the modern location of 2b Hydrodynamic regime Memphis. Along the Mid-Atlantic coast, the present Coastal Plain and shelf were flooded by 100 to 500 m of seawater, and morphology changes with the ratio between tidal- the shoreline was on the Piedmont and possibly lapped against energy flux and wave-energy flux the Blue Ridge. 3 Local tectonic influences and local geologic history In the late Eocene, about 35 My ago, a high-velocity generally longer-term processes (thousands to bolide—either a comet or asteroid—flew into this setting, split millions of years) into two pieces coming through the atmosphere, and impacted the Atlantic shelf to create the Chesapeake Bay Impact Crater 4 Inlet switching and related processes (CBIC) in Virginia and the Toms Canyon Impact Crater off New generally active coastal processes (years to centuries) Jersey (Poag et al., 1994; Poag, 1997; Poag et al., 2004). The cataclysmic collision that created the CBIC excavated 1,000 from the eroding Piedmont and Appalachians to the continental m (3,300 ft) of Eocene and older sediments and sedimentary shelf and slope has produced a slow net uplift of the Virginia rocks, and nearly as much of the underlying crystalline bedrock. Coastal Plain landward from a hinge line near and approximately The impactor may have penetrated 8 km (5 mi) into the crust, parallel to the present shoreline (Owens and Gohn, 1985; Poag, and produced a crater 85 km (53 mi) in diameter centered near 1985; Ward and Strickland, 1985; Ator et al., 2005). This vertical the town of Cape Charles on the Eastern Shore. The crater has movement was enhanced by the general long-term subsidence of a smaller inner basin and peak ring about 35 km (22 mi) in the continental margin as the bedrock cooled after rifting and diameter. The huge depression formed by the impact was re- the structurally weak transition between continental and oceanic filled over the next hours and days by an in-rush of breccia (large crust sank beneath the added mass of deposited sediments pieces of broken rock), and sediments and debris swept in by the (Gardner, 1989). accompanying tsunami. The outer rim of the CBIC encompasses Besides the flexure of the crust in response to unloading and most of the Virginia section of Chesapeake Bay, extending north loading, two additional large-scale processes recently have been of the mouth of the Rappahannock River and south to Cape recognized that influence the vertical position of the margin. Henry. The western edge of the outer rim aligns with the Suffolk Passive (trailing-edge) margins worldwide commonly have Scarp on the James–York and Middle Peninsulas (Figure 3); the coastal staircase sequences that Pedoja et al. (2014) interpret as eastern edge is under the shelf about 25 km (15.5 mi) east of the result of tectonic compression of the continent, which raises Wreck Island. it relative to the ocean. Regionally along the U.S. Atlantic Coast, Because the Chesapeake Bay Impact Crater was discovered differential warping of the shoreline and shelf strata created only in the early 1990s (Powars et al., 1993), the effect of the crater during the mid-Pliocene highstand at ~3 million years ago (Ma) on the regional geologic history has just recently been realized is explained by Rowley et al. (2013) as expressing dynamic and evaluated. In some cases the influence is only inferential, topographic changes driven by deep-seated mantle convection. whereas in others the position and structure of the crater provide Even as the Atlantic margin was subsiding, global sea level reasonable explanations for otherwise anomalous features noted dropped in stages through the Tertiary as plate tectonics moved by earlier researchers. The impact completely disrupted the Antarctica toward the South Pole, global climate cooled, and regional strata that form the deep aquifers, and the bowl of the large ice sheets developed near both poles. The net result has crater was filled with a brine that has 1.5 times the salinity of been flooding of the margin by the ocean for much of the time seawater. Extensive drilling by the USGS and the Virginia State from the late Cretaceous into the Pliocene, from approximately Water Control Board has provided a re-assessment of the main 100 Ma to 3 Ma, punctuated by major drops in global sea level aquifers, and has confirmed that the brine is associated spatially associated with periods of growth of the Antarctic Ice Sheet, and by process with the CBIC (Powars and Bruce, 1999; Powars, starting in the early Oligocene, about 28 Ma (Zachos et al., 2000). The stratigraphic and geophysical evidence also indicate 1992; Barrett et al., 2006; among others). As noted previously, compaction and subsidence within the crater that continues to contemporaneous sea level marks the boundary between the the present. The subsidence partially explains the rapid rate of shelf and coastal plain provinces; during the lowstands of glacial relative sea-level rise, especially in the Hampton Roads area, maxima, the continental shelf is emergent and, hence, part of the and the accentuated, arcuate paleoshoreline of the Suffolk coastal plain. Scarp. The courses of the James and York Rivers turn abruptly to the northeast, toward the crater center, as they cross the outer 358 Atlantic Coast and Inner Shelf rim in their lower reaches. And the epicenters of the few small each highstand. In the central and southern Virginia Coastal earthquakes that have occurred beneath the Virginia Coastal Plain, deposition during the middle Pleistocene highstands is Plain are located close to the outer rim of the crater, indicating preserved as the Windsor, Charles City, Chuckatuck, and Shirley continued slip on the faulted basement or listric slump blocks Formations; the toes of the associated scarps range in present within the crater (Johnson et al., 1998). elevation from 30 m (100 ft) for the Surry Scarp and Windsor Formation, to 15 m (50 ft) for the Kingsmill Scarp and Shirley Sea-Level Trends Since the Mid-Pliocene Formation (Johnson et al., 1987). The youngest major scarp is the Suffolk Scarp, which separates the upper Pleistocene Tabb Climate and sea level worldwide since the middle of the Formation from the upper–middle Pleistocene Shirley Formation Pliocene have progressed in three broad episodes: generally (Peebles, 1984; Peebles et al., 1984; Johnson et al., 1987). warm climate and high sea level from 3.5 to 3.0 Ma; transition into the Ice Ages starting at 2.6 Ma; and a fundamental switch Courses of the Regional Rivers and into high-amplitude cycles near 1.0 Ma (Figure 14). Extension of the Eastern Shore Through most of the early and middle Pliocene, global climate was equable, and warmer than the Holocene interglaciation by The southern half of Chesapeake Bay owes its modern as much as 2–3°C (3.5–5.5°F). Sea level was higher than present configuration to the interplay of the topographic depression of by 20 m (65 ft) or possibly 30 m (100 ft) (Miller et al., 2012), the CBIC with the regional rivers responding to changing base and fluctuated by a few tens of meters with the 41-ky and 23- level, and large-scale coastal sediment transport (Colman and ky Milankovitch orbital cycles (Lisiecki and Raymo, 2005). Hobbs, 1987; Colman and Mixon, 1988; Colman et al., 1990; A relatively rapid and deep drop at 3.3 Ma preceded the final Hobbs, 2004). The geologic evidence suggests that prior to and peak Pliocene highstand from 3.2 to 3.0 Ma. This last highstand during the middle Pleistocene, the Potomac, Rappahannock, correlates with the maximum transgression of the upper units of York, and James Rivers flowed in separate valleys southeast the Yorktown Formation in Virginia and North Carolina (Krantz, from the Piedmont across the Coastal Plain. With the progressive 1991). This event flooded onto the edge of the Piedmont, and drop of sea level through the Pleistocene, the Delaware and cut an ocean shoreline, expressed as the Chippenham Scarp Susquehanna Rivers carved out the valleys to create the main near Richmond and the Thornburg Scarp near Fredericksburg stems of Delaware and Chesapeake Bays. During the lowest of (Mixon et al., 1989; Mixon et al., 2000), that approximately the lowstands, the ocean shoreline was near the modern shelf follows the Interstate 95 corridor near the Fall Zone (Figure 1). edge, and the rivers transported sediment from the uplands to Present elevations of the toe of the Chippenham and Thornburg the narrow and steep shelf, and into the submarine canyons scarps are around 75 m (245 ft). Between 3.0 and 2.6 Ma, only a of the continental slope. The canyons of the Middle Atlantic few fairly brief peaks occurred that may have been high enough Bight were most active during these glacial maxima, carrying to allow marine flooding in the Albemarle Embayment of North terrestrial sediments from the shelf into the deep ocean basin Carolina and southeastern Virginia to deposit the Chowan River and aggrading the continental rise. Channel cutting and Formation. sediment transport would have been most vigorous during the Large-scale glaciation in the Northern Hemisphere initial phases of deglaciation as tremendous volumes of glacial commenced at 2.6 Ma, dropping sea level worldwide. The meltwater and outwash sediments were discharged down the significance of this transition is recognized by the International Hudson, Delaware, and Susquehanna River systems. Commission on Stratigraphy as the newly defined Gelasian Four main channels of the combined Susquehanna-Potomac Stage of the Pleistocene from 2.6 to 1.8 Ma (Figure 14) (Cohen River system have been mapped under Chesapeake Bay, the et al., 2013). This climate mode was dominated by the 41-ky southern half of the Virginia Eastern Shore, and the inner shelf orbital cycle of obliquity (Lisiecki and Raymo, 2005), which (Colman and Hobbs, 1987; Colman and Mixon, 1988; Colman generated sea-level changes on the order of 80–100 m (260–330 et al., 1990; Oertel and Foyle, 1995; Hobbs, 2004). In age from ft). Global climate continued to cool overall from 2.5 to 1.5 Ma. oldest to youngest, and location from north to south, these The last major climate shift started near 1 Ma, and was paleochannels are the Exmore, Belle Haven, Eastville, and Cape operating fully by 800 ka, with large-amplitude oscillations Charles channels (Figure 15). During sea-level highstands, dominated by the 100-ky cycle of orbital eccentricity, while direct wave attack by the Atlantic Ocean eroded the seaward retaining the 41-ky and 23-ky cycles. This climate mode edge of the older fluvial and deltaic deposits that form the central produced some extreme glaciations through the past 750 ky, core of Delmarva, and released sand into the regional, usually including the two most recent at 21 ka (the Wisconsinan) and south-flowing, longshore-transport system. The littoral transport 160 ka (the Illinoian). Full-amplitude sea-level fluctuations extended the southern Delmarva Peninsula (the Virginia Eastern through the Middle and Late Pleistocene were typically 120 m Shore) as a progression of major spits (Mixon, 1985; Oertel and (395 ft), to as large as 150 m (495 ft). Foyle, 1995; Hobbs, 2004), each forming during an interglacial The land surface of the Virginia Coastal Plain was sculpted highstand. Each major spit prograded to fill the northernmost by the interaction of these major episodes in global sea level valley in that highstand cycle (Foyle and Oertel, 1997), which cutting into a slowly uplifting continental margin. The staircase deflected the courses of the rivers during the following lowstand. sequence of terraces—plains of consistent elevation with a Through time, the main river system migrated south, gentle seaward slope—is punctuated by the abrupt changes in sequentially occupying each of the pre-existing valleys, elevation at the scarps marking the maximum transgression of which were the topographic lows most likely to capture the 359 The Geology of Virginia

2.0 2 Dominant Dominant

{ 41-ky cycles 100-ky{ cycles 0 2.5

Peak of Present

-2 Interglaciation 3.0 O ( ‰ )

for Reference 18 -4 d 3.5 -6

4.0 Benthic -8 emperature equivalent ( ° C)

T 4.5 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 Time (millions of years) PLIOCENE PLEISTOCENE TE LA

Zanclean Piacenzian Gelasian Calabrian Middle HOLOCENE MIOCENE 5.3 Ma 3.6 Ma 2.58 Ma 1.8 Ma 781 ka 126 ka Geologic Time

Figure 14. The Lisiecki and Raymo (2005) stack of marine stable isotope records as a proxy measure of global climate, ice volume, and eustatic sea level from the beginning of the Pliocene through the Holocene. Chronostratigraphic Ages, with dates of the boundaries, presented at the bottom are from the International Commission on Stratigraphy (Cohen et al., 2013). drainage. First the Susquehanna merged with the Potomac of the ancestral Potomac River may have extended beneath River, and flowed through the Exmore paleochannel (Hobbs, the northern section of the peninsula (Figure 15) and pre-dated 2004). Then the combined Susquehanna–Potomac system Chesapeake Bay in its current configuration. His inference was moved progressively into the valleys of the Rappahannock and based in part on the alignment of the mouth of the Potomac York Rivers, corresponding to the Eastville and Cape Charles River estuary with Washington Canyon at the edge of the shelf. paleochannels, respectively. The lowstand channel of the James Additional supporting observations included the occurrence of River appears to have remained separate, and is distinct from coarse gravel and cobbles up to 25 cm diameter found in pits the Cape Charles paleochannel near the Chesapeake Bay mouth. on the mainland west of Cedar and Wallops Islands. He also If each of the four known paleochannels of the Susquehanna– noted that gravel is found only on the beaches of the islands Potomac is correlated with the maximum glacial events of the in the “peculiar indentation of the southern Delmarva shoreline middle to late Pleistocene, the extension of the southern half of of Wallops–Cedar islands,” that is, Chincoteague Bight. The the Eastern Shore peninsula may have started at about 350 ka lithology of the cobbles and gravels has a strong affinity to (Figure 16). However, the ages of these lowstand channels have the Blue Ridge or Ridge & Valley provinces in the Potomac not been established with any direct geochronological data. All drainage basin. Harrison considered the recently discovered interpretations to date are based on inference, ages assigned to Naylors Mill (or Salisbury, Maryland) paleochannel (Hansen, the overlying units, and the assumption that the deepest incisions 1966) to be a tributary to the main Potomac valley, although occurred during the maximum glacial episodes. The initial no evidence directly supports a correlation in space and time interpretation by Colman and Mixon (1988) correlated the Exmore between the two. channel with either MIS 12 at 440 ka, or MIS 8 at 260 ka. Because Recent seismic surveys in Chincoteague Bight, south of of its stratigraphic position, all interpretations associate the Cape Assateague Island, found at least two large paleochannels off Charles channel with the last glacial maximum of MIS 2. Wallops and Assawoman Islands (Figure 17) and extensive However, the sequence of events presented above does channelization into Tertiary strata below the apparent not account for vertical uplift and subsidence of tens of meters Quaternary coastal and shelf deposits (Wikel, 2008; Krantz et associated with glacial isostatic adjustment that has been al., 2009; Krantz et al., unpublished data). From boreholes on recognized only recently. A modified, and much shorter, timing the Eastern Shore, Mixon (1985) mapped a saddle on the top of of events has been proposed by Scott et al. (2010) that correlates the Yorktown Formation between the towns of Hallwood and the Exmore channel with MIS 6 at 160 ka, and the Cape Charles Bloxom, Virginia. Powars (2011) updated this interpretation, channel with MIS 2 at 21 ka; the Belle Haven and Eastville identified likely fluvial sediments in the infilling unit, and channels are then associated with the moderate glacial events of informally named this feature the Persimmon Point paleochannel MIS 5d and MIS 4, respectively. (Figure 15). The paleochannel offshore of Assawoman Island Before the discovery and mapping of the Susquehanna aligns well with the Persimmon Point channel onshore. It is paleochannels, Harrison (1972) speculated that a paleovalley likely that these channels were produced by the Potomac River 360 Atlantic Coast and Inner Shelf

76° 75° DELAWARE 39° 0 25 mi BAY

0 25 km

CHESAPEAKE BAY DELMARVA PENINSULA DEL ATLANTIC MD OCEAN

POTOMAC R.

RAPPAHANNOCK R. MD 38° VA Interpreted Potomac River channel path by PP Harrison (1972) 100 m

? 200 m

YORK R.

WASHINGTON EX CANYON BH EV 500 m

JAMES R. CC NORFOLK CANYON 37°

JR James River path CAPE HENRY by Harrison (1972)

Mapped channels: PP EX BH EV CC (oldest) Persimmon Point Exmore Belle Haven Eastville Cape Charles (youngest)

Figure 15. Schematic map depicting the Pleistocene channels of the combined Susquehanna–Potomac River system crossing the Delmarva Peninsula and the continental shelf. The channels were excavated during lowstands of sea level and the peninsula lengthened during highstands. All positions are approximations. Adapted from Harrison (1972), Colman and Mixon (1988), Colman et al. (1990), Oertel and Foyle (1995), Hobbs (2004), and Powars (2011). and/or a combined Susquehanna–Potomac system. Again, (Johnson, 1972, 1976; Peebles et al., 1984; Mixon et al., 1989), the age of these channels is speculative at this time, but at a which is subdivided into the Sedgefield, Lynnhaven, and minimum, these features likely are middle Pleistocene, older Poquoson Members. The higher plain west of the Suffolk Scarp than about 400 ky (Figure 16). Geomorphic and stratigraphic is underlain by the upper–middle Pleistocene Shirley Formation evidence also suggest an earlier sequence of large-scale spit and older middle Pleistocene units (Peebles et al., 1984; Mixon progradation, similar to the extension of the southern half of et al., 1989). Hobbs (2004) reviewed the history of the regional the peninsula, that was associated with closing off the Potomac stratigraphic nomenclature. River valley and rerouting the drainage to the south (Krantz, The Suffolk Scarp itself is an erosional shoreline in personal observation; noted by Oertel et al., 2008). Virginia, exposed to direct ocean wave attack at least twice during late Pleistocene highstands. To the south in North The Late Pleistocene of Southeastern Virginia Carolina, this same shoreline becomes depositional as the Chowan Spit Complex, created during more than one highstand event (Parham et al., 2013); this composite spit feature is the The outer Coastal Plain of southeastern Virginia preserves “Suffolk Sand Ridge” of Oaks and Coch (1973) (Figure 18). The an intricate record of Pleistocene sea-level events. In the classical most pronounced break in elevation on the Pamlico Terrace is physiographic interpretation of Cooke (1930, 1936), this area the Hickory Scarp, 32 km (20 mi) east of the Suffolk Scarp and includes the Pamlico (0 to 6 m elevation; 0 to 20 ft), Talbot (6– 25 km (15.5 mi) inland from the ocean shoreline. The Hickory 14 m; 20–46 ft), and Wicomico (14–29 m; 46–95 ft) Terraces. Shoreline is composed of the seaward scarp and the landward Detailed study locally by Oaks and Coch (1973) and regional Fentress Rise, which has the geomorphic signature of a chain synthesis by Oaks et al. (1974) identified individual shoreline of barrier islands (Figure 11); this feature is mapped as part of features on these primary terraces (Figure 18). The Suffolk the Sedgefield Member of the Tabb Formation (Mixon et al., Scarp is a prominent paleoshoreline, about 60 km (37 mi) inland 1989). These Pleistocene islands form the seaward edge of the from the present ocean coast, that separates the lower-elevation lowlands now occupied by the Dismal Swamp that continue Pamlico Terrace from the two higher terraces. The Smithfield westward to other exposures of the Sedgefield Member along barrier sits on the crest of the northern part of the Suffolk Scarp the Suffolk Scarp. A series of lower-elevation, relatively narrow (Peebles et al., 1984), implying that during the sea-level cycle sand ridges lie seaward of the Hickory Shoreline; in Virginia that created this shoreline initially, the topographic break marked these include the Land of Promise, Oceana, and Pungo Ridges the landward edge of the ocean shoreface. (Figure 18). Geomorphically and stratigraphically, these smaller The surficial deposits of the mainland east of the Suffolk ridges also have the characteristics of high-energy, ocean barrier Scarp are placed in the upper Pleistocene Tabb Formation islands. 361 The Geology of Virginia

Modern sea level High SPECMAP d18O Stack

7 9 11 5 13 2 15 17 3 16 4 6 8 10 12 14

Global sea level Low 0 Cape Charles100 200 300 400 500 600 700 Eastville Belle Haven Exmore Age (thousands of years) (no age estimate)Persimmon Point

Ex BH PP CC Ev

? ? ? ?

Figure 16. Paths of the known middle to late Pleistocene channels crossing beneath the modern Virginia Eastern Shore (Colman et al., 1990; Oertel and Foyle, 1995; Powars, 2011). A simple chronology for the Cape Charles through Exmore channels correlates incision with each of the four full glaciations of the past 400 ky. At least two additional large paleochannels, possibly of an ancestral Potomac River, were discovered off Wallops and Assawoman Islands (Krantz et al., 2009; Krantz et al., unpublished data). The ages of these shoreline features have been debated Some of the difficulty of interpreting the stratigraphy in for the past 40+ years, but they generally are considered late southeastern Virginia is that equivalent lithofacies (sediments Pleistocene, and presumably represent stepwise regression at deposited in a particular environment) that were deposited the end of the last interglacial episode. Shoreline deposits of the during separate sea-level events are stacked into what may Sedgefield Member that create the Fentress Rise and a narrow appear locally to be a single, conformable sequence. A prime constructional shoreline against the Suffolk Scarp sit between 6 example of this stacking comes from Gomez pit, dug into the and 7.5 m (20–25 ft) above modern sea level. These elevations Hickory Shoreline barrier complex. The deposits above the seem to make the Sedgefield Member a good candidate for the upper Pliocene Chowan River Formation at this site seem to Sangamonian or MIS 5e highstand at 127–120 ka, based on a show a single transgressive cycle progressing upward from prominent transgression worldwide to +6 to +8 m (20 to 26 ft) fluvial through estuarine/lagoonal to barrier-island facies. (Figure 12). However, early studies employing uranium-series However, amino-acid racemization of mollusk shells indicates radiometric dating of coral fragments from barrier-island and three separate periods of deposition within this sequence back-barrier strata exposed in quarries (Womack, Moyock, and corresponding approximately to late MIS 5, possible MIS 7, and Gomez pits) where sand is excavated from the Fentress Rise older middle Pleistocene ages (Wehmiller et al., 1988; Mirecki yielded ages of 70 to 85 ka (Cronin et al., 1981; Mixon et al., et al., 1995). As discussed in the next section, the barrier-island 1982; Szabo, 1985). Wehmiller et al. (2004) confirmed these sands at the top of this sequence may be younger still, by as younger-than-expected ages using a more advanced analytical much as 20 ky. method for U-series dating. These dates make the prominent This configuration of geomorphic and stratigraphic units Hickory Shoreline in southeastern Virginia late MIS 5 (substage in the outer Coastal Plain, and the previously confounding 5a); globally, MIS 5a was followed shortly by the moderate chronologies that appeared not to match the global record of cooling of MIS 4 and a drop of sea level by 50–60 m (165–195 sea-level events, may have been reconciled with dates from a ft) (Figure 12). new method—optically stimulated luminescence (OSL)—and Based on marine isotope records of global ice volume a realization that vertical movement of the crust in the Mid- confirmed independently by U-series dates of coral terraces Atlantic region has been on the order of many tens of meters. on Pacific islands, New Guinea and Barbados, global sea level during MIS 5a was approximately 20 m (65 ft) below present Effects of Isostatic Adjustment in the Mid-Atlantic (Lambeck and Chappell, 2001; Potter and Lambeck, 2003). Three questions arise from the finding that the Hickory Shoreline has Recent papers from several coastal geology research a MIS 5a age: 1. Why is the substage 5a shoreline 26 m (85 ft) groups have dramatically revised our collective view of the higher than expected along the Mid-Atlantic coast? 2. What age late Quaternary history of the Mid-Atlantic coast. Mallinson et are the younger shorelines seaward of the Hickory Scarp? and al. (2005) and Parham et al. (2007) demonstrated that multiple 3. Where is the MIS 5e shoreline that is so prominent worldwide? upper Quaternary transgressive-regressive cycles are preserved 362 Atlantic Coast and Inner Shelf

South Seismic Line AI-05.05 North A 0 0 10 10

20 20

30 30

Depth (m) 40 40

50 50

60 60 Quaternary Seafloor Tidal inlet (filled) coastal deposits Interpreted Section 0 0

10 } 10

20 20 Paleochannel 30 30

Depth (m) 40 40

50 Tertiary shelf deposits 50 0 200 400 m 60 60

South Seismic Line AI-05.10 North 0 0

10 10

20 20

30 30

Depth (m) 40 40 600 m of profile not shown 50 50

60 60 Seafloor Interpreted Section 0 0

10 10

20 20 Paleochannel 30 30

Depth (m) 40 40

50 50 Tertiary shelf deposits 0 200 400 m 60 60 38°00'

CHINCOTEAGUE B BAY

Chincoteague Island

Assateague Island Toms Cove

allops IslandAI-05.05 W

Persimmon Point AI-05.10

37°45' ATLANTIC OCEAN

10 km

75°30' 75°15'

Figure 17. A. Seismic sections of two paleochannels off Wallops Island (section AI-05.05) and off Assawoman Island (section AI- 05.10) in the Chincoteague Bight with interpretation of primary stratigraphic units (Krantz et al., 2009; Krantz et al., unpublished data). B. Map of the northern Chincoteague Bight showing tracklines and seismic sections presented in A.

363 The Geology of Virginia

were obtained many years ago from correlative estuary-margin shorelines of the Kent Island Formation around Chesapeake Bay (Owens and Denny, 1979; Olsson et al., 1988), and associated with Mockhorn Island between the Virginia Atlantic barrier islands and the mainland of the Eastern Shore (Finkelstein, 1988; Finkelstein and Kearney, 1988) (location shown in Figures 3 and 5). The seemingly anomalous Wisconsinan age for Mockhorn Island was dismissed because the elevation is 60 m (197 ft) above the “correct,” eustatic sea level (Colman et al., 1989; cf., Finkelstein and Kearney, 1989). Even so, these dates and elevations agreed generally with early sea-level curves for the Atlantic and Gulf coasts (Fairbridge, 1961; Milliman and Emery, 1968); although re-evaluation of MIS 3 and MIS 5a highstands preserved on the northwestern Gulf of Mexico shelf by Simms et al. (2009) places those events well below present sea level. The OSL ages of shoreline features by Mallinson et al. (2008), Scott et al. (2010), and Parham et al. (2013) dramatically changed the interpretation of the regional sea-level history. Included in these findings of are documentation of: • Few preserved MIS 5e deposits because of erosion and burial during subsequent MIS 5c and 5a highstands Figure 18. Late Quaternary scarps and shoreline ridges in • Occupation of the Suffolk Shoreline during the southeastern Virginia and northeastern North Carolina, from Oaks et al. (1974). highstands of MIS 5c and 5a, with relative sea level of at least 9 m (30 ft) above present as sedimentary sequences beneath Albemarle Sound and its tidal • Erosional removal, or partial erosion and depositional tributaries, as interpreted from high-resolution seismic surveys burial, of MIS 5e and 5c deposits by the MIS 5a supplemented with cores. Then Mallinson et al. (2008) reported highstand OSL dates for several of the shorelines on the outer Coastal Plain • Construction of the Chowan Spit Complex during the of North Carolina that are either connected to or correlative with highstands of MIS 5c and 5a shorelines in southeastern Virginia. The OSL method obtains • Formation of two MIS 5a shorelines in North Carolina, dates directly from the sand of the ridges, and is independent either by constructive seaward progradation and/or a of the more standard carbon-14 or uranium-series radiometric second highstand dates (Szabo, 1985; Muhs et al., 2002), or geochemical methods • MIS 3 ages for Land of Promise, Pungo, and Powells such as amino-acid racemization (Wehmiller et al., 1988, 2010). Point shorelines In a parallel study, Scott et al. (2010) obtained OSL dates from • MIS 3 age for the seaward side of the Hickory Shoreline key shoreline features in southeastern Virginia, including Pungo • Relative sea level during MIS 3 at least 3 m (10 ft) Ridge, and on the Virginia Eastern Shore. Parham et al. (2013) above present extended the stratigraphic and chronologic study from North • An initial late Holocene transgression dated between 3 Carolina into southeastern Virginia, and offered a synthesis and 2.5 ka, preserved as a series of beach ridges behind of the entire regional system. These studies also benefitted the modern beach at Kitty Hawk, North Carolina tremendously from the greatly improved topographic resolution provided by LiDAR (light detection and ranging) to evaluate the Other studies in the Chesapeake Bay region (Pavich et al., geomorphology and identify subtle (and not so subtle) shoreline 2006, 2010; DeJong et al., submitted manuscript 2014) support features. MIS 3 ages for extensive estuarine deposits of the Kent Island As discussed above, the conclusion that the Hickory Formation (Owens and Denny, 1979) above modern sea level. Shoreline barrier islands formed in MIS 5a left open the ages On the Atlantic side of Delmarva, shorelines in Maryland and of the four lower-elevation beach ridges seaward of the Hickory Delaware previously interpreted as late MIS 5 (Demarest and Scarp. Previously it was assumed that these shorelines marked Leatherman, 1985; Toscano, 1992; Toscano and York, 1992) stillstands during the regressive phase at the very end of MIS recently have been dated with OSL as MIS 3 (Pavich and Smoot, 5. The conventional wisdom was that as difficult as it was to personal communication, 2014). These shorelines are correlative reconcile a late MIS 5a shoreline at 6 m (20 ft) above present with those of the Wachapreague Formation on the Virginia sea level, when the global record indicates sea level 20 m (65 Eastern Shore dated by Scott et al. (2010) as MIS 3. Farther ft) below present, it was nearly impossible to justify shorelines north, the New Jersey shelf preserves two marine sequences above present sea level that were created during the Wisconsinan of MIS 3 age that approached present sea level (Wellner et al., Interstadial, MIS 3 from 57 to 29 ky ago. The global record has 1993; Wright et al., 2009), although no emergent MIS 3 features the highest MIS 3 highstands at 40 to 60 m (130 to 197 ft) below have yet been identified. present—that is, these shorelines should be under water, half The significance of these results is that the outer Coastal way out on the continental shelf. Ironically, radiocarbon dates Plain and shelf of the Mid-Atlantic region have a very different 364 Atlantic Coast and Inner Shelf preserved history of the sea-level oscillations of the late Holocene Sea-Level Rise Pleistocene than other regions globally. Based on the marine isotope record as a proxy indicator of global ice volume and sea The most dramatic eustatic changes in sea level result from level, as supported by uranium-series dates on late Pleistocene the freezing and thawing of the polar ice caps. Since the last coral terraces in tropical and subtropical areas, the highstand glacial maximum between 22 and 18 ka, sea level has risen shorelines along the Mid-Atlantic coast for late MIS 5 and MIS about 125 m (410 ft) (Peltier, 2002). For much of the 20th 3 are anomalously high, by at least 29 m (95 ft) for MIS 5a and century, it was assumed that sea level rose quickly but steadily possibly by >60 m (195 ft) for MIS 3. from its minimum until about 5 ka, then slowed significantly The picture that emerges from these elevations of highstand (for example, Milliman and Emery, 1968). More recent work deposits for late MIS 5 and MIS 3 in the Mid-Atlantic is that the (Fairbanks, 1989; Nikitina et al., 2000; Peltier, 2002) indicates entire region underwent considerable isostatic adjustment after that sea level has risen in a series of steep steps with intervening the MIS 6 glacial event (~160–140 ka, the Illinoian glaciation) plateaus that might have included minor regressions. The (Peltier, 2002, 2004). When uneven forces are applied to periods of exceptionally rapid sea-level rise appear to have been the crust, by loading or unloading of mass, the upper mantle in response to major pulses, or releases, of meltwater from the (specifically the asthenosphere) responds by flowing, slowly. Northern Hemisphere ice sheets. During the most prominent of Along the east coast of North America, when glaciers covered these meltwater pulses at about 12 and 9.5 ka, sea level rose eastern Canada, New England and the Gulf of Maine, the Mid- 10 to 20 times faster than at present, and transgression of the Atlantic sat astride the crest of the glacial forebulge (Figure shoreline across the Coastal Plain would have been clearly 19). As the ice sheet melted and retreated to the northwest, the evident within a few decades. As sea level rose above about crust in New England rebounded and the forebulge in the Mid- 18 m (60 ft) below the present level, between 8.2 and 7.2 ka, Atlantic relaxed and migrated northward. Meanwhile, with the brackish waters reached northern Chesapeake Bay (Bratton et glacial meltwater flooding back into the ocean, sea level began al., 2003). rising, slowly at first and then rapidly. The rate of sea-level rise slowed considerably by about 6 The eustatic rise of sea level from the glacial maximum of ka (Bratton et al., 2003), when the last vestiges of the major ice MIS 6 to the peak highstand of substage 5e proceeded as rapidly sheets covering Canada and Scandinavia had melted completely. as the equivalent transition from the Last Glacial Maximum Substantial evidence from low-latitude sites indicates a mid- of MIS 2 up to the present interglacial highstand. As sea level Holocene highstand, between 5 and 1.5 ka, with relative sea during MIS 5e peaked and cut a shoreline notch into the Coastal level 1 to 3 m (3.3 to 10 ft) higher than present (Grossman et Plain, much of the previous elevation of the forebulge had al., 1998; Morton et al., 2000; Blum et al., 2001; Peltier, 2002). relaxed and, measured from the center of the Earth, the land In quantitatively modeling Holocene sea level, Peltier (2002, surface in Virginia was approximately at the same position as 2004) considered the slow, complex, viscous flow of the Earth’s today. However, through the remaining 40 ky of the MIS 5 mantle in response to changes in load on the crust from melting warm interval and about 25 ky of MIS 3, the isostatic subsidence ice sheets and the increasing volume of ocean water, and the continued, although at a slower rate. In addition, the mass of time lags between the change in load and response of the crust. the seawater that inundated the continental margin also pressed Some of the earliest work on Holocene sea level along the down on the continental shelf and outer Coastal Plain. Virginia coast was that of Harrison et al. (1965), who evaluated The result for the Mid-Atlantic margin was a total depression cores and seismic lines collected for construction of the of at least 26 to 29 m (85 to 95 ft) through the end of MIS 5a at Chesapeake Bay Bridge-Tunnel. They interpreted what appeared 71 ka (Mallinson et al., 2008; Parham et al., 2013). The relative to be anomalously shallow depths of a buried Susquehanna River lowering of the land surface of the outer Coastal Plain allowed channel, and the occurrence of intertidal peats and underlying the middle and late MIS 5 highstands to flood the Mid-Atlantic shell beds with 14C dates from 1,170 to 1,900 years BP as to about the same elevation as the peak MIS 5e highstand, even much as 1.5 m (5 ft) above sea level. From these observations, though eustatic sea level was lower by 20 to 30 m (65 to 100 ft). Harrison et al. (1965) postulated regional uplift in the southern The land surface continued to subside, possibly an additional Chesapeake Bay during the late Pleistocene or Holocene. 30 m (100 ft) until almost the end of MIS 3, allowing flooding Although no more recent studies have confirmed Holocene of the Coastal Plain higher than present sea level. Then at 35 or coastal deposits above present sea level on the Virginia coast, 30 ka, the rapidly growing Laurentide Ice Sheet again pressed several geomorphic features suggest an earlier highstand event. down on the mantle, forming a forebulge that uplifted the Mid- The hypothesis of vertical displacement in the Holocene Atlantic region, causing a rapid drop of relative sea level, and was supported by Newman and Rusnak (1965), who obtained reinitiating incision of the major rivers (Bierman et al., 2002; radiocarbon dates from buried tidal-marsh peats near Reusser et al., 2003; Pavich et al., 2006, 2010). Wachapreague. Their oldest date was 5,120 ± 145 years from a The implication of these findings and interpretations is sample 6.1 m (20 ft) below the marsh surface; another sample that sea level along the Mid-Atlantic coast will continue to rise was almost 800 years younger (4,350 ± 75 years), but from a relative to the land well into the future, even without any further depth of 7.2 m (24 ft), roughly a meter deeper than the older melting of glacial ice. However, most recent observations sample. From additional work near Wachapreague, Newman indicate that the rates of melting of montane glaciers worldwide and Munsart (1968) concluded that the barrier islands and the and sections of the Greenland and Antarctic ice sheets have back-barrier lagoons have existed for at least 5,500 years. They accelerated dramatically, resulting in a rate of sea-level rise that also investigated Italian Ridge on Parramore Island, which is a has approximately doubled in the last decade (IPCC, 2013). prominent ridge rotated about 20º clockwise and set back 300 365 The Geology of Virginia

GLACIAL PHASE South Glacial ice North Uplifted glacial forebulge

LS end of Stage 6

Present SL

LS end of Stage 6 Isostatic subsidence LGM Sea Level (-125 m)

EARLY INTERGLACIAL Glacial forebulge collapses

LS end of StageLS 6 during Stage 5e

Present SL Stage 5e Sea Level (+8 m) LS end of Stage 6 Post-glacial rebound LGM Sea Level (-125 m)

LATE INTERGLACIAL Continued subsidence

LS end of StageLS 6 during Stage 5e Present SL

LS during Stages 5a & 3

Continued rebound Stage 5a Sea Level (-20 m)

LGM Sea Level (-125 m)

Georgia Bight Mid-Atlantic Transition New England & (FL-GA-SC) (NC-VA-MD-DE) (NJ-NY) Gulf of Maine

Figure 19. Regional isostatic response of the Atlantic coast after the Illinoian glaciation (MIS 6). This representation is based on the glacial isostatic adjustment model of Peltier (2004), and the ages and elevations of late MIS 5 and MIS 3 shorelines above modern sea level by Wehmiller et al. (2004), Mallinson et al. (2008), and Parham et al. (2013). The land surface of the Mid-Atlantic rode high on a forebulge during the MIS 6 glaciation. During the early interglacial period, the glacial forebulge collapsed and retreated northward. The land-surface elevation during MIS 5e would have been roughly equivalent to the present. Through the later part of the interglaciation, with continued subsidence of the forebulge and hydroisostatic depression of the continental margin, the land surface was lowered by an additional ~25 m through MIS 5a, and possibly by as much as 60 m through late MIS 3. Forebulge uplift was re- initiated at 35–30 ka, coincident with a phase of rapid growth of the Laurentide Ice Sheet (Pavich et al., 2006, 2010). [Abbreviations used in the figure: LS, land surface; SL, sea level; LGM, Last Glacial Maximum.] to 700 m (985 to 2300 ft) from the present shoreline (Figure 2.0 and 3.6 mm/yr (0.08 and 0.15 in), or 0.5 to 2.1 mm/yr (0.02 7). They suggested that Italian Ridge represents a prograding to 0.08 in/yr) faster than for the period beginning at 4.6 ka. Their constructional shoreline from an earlier Holocene period of analysis of cores taken in the back-barrier region indicated that emergence. Coupling the high sea level suggested by Italian “extensive marsh growth occurred after approximately 1.6 ka,” Ridge with a several-meter fall of sea level between 5,100 and when the rate of sea-level rise slowed. 4,400 ka, they inferred either a brief regression or a “cessation In an attempt to reconcile the sea-level history of the Eastern of crustal uplift.” Either of these situations requires that sea level Shore with other localities farther north, Van de Plassche (1990) was a meter or more above present several thousand years ago added new radiocarbon dates and reviewed all existing dates and followed by a brief regressive phase, a view supported by (Figure 20). He discounted the drop in sea level between 5.1 and Kemerer (1972). Assateague Island is similarly offset seaward 4.4 ka reported by Newman and Munsart (1968), and concluded of an older chain of barrier islands, including Chincoteague that late Holocene sea level in the area rose as rapidly and Island (discussed by Krantz et al., 2009). continuously as all sites to the north, including the extensive Finkelstein and Ferland (1987) related the relative rise of sea set of dates from the Delaware coast (e.g., Belknap and Kraft, level and/or a local deficiency in sediment to the Holocene retreat 1977; compiled by Ramsey and Baxter, 1996). However, these of the Virginia barrier islands. Their sea-level curve for the past assessments of late Holocene sea level were based entirely on 5,000 years shows a slightly increased rate of rise between about radiocarbon dates, mostly of basal marsh peats, and not dates 3.8 and 2.2 ka, when sea level rose from about -5.5 to -2.7 m on geomorphic shoreline features composed of sand, which now (Figure 20). They stated that the present rate of rise was between can be dated with OSL. Additionally, the outer rim of the CBIC 366 Atlantic Coast and Inner Shelf runs beneath Wachapreague and Parramore Island, so it is not Radiocarbon age (thousands of years BP) unreasonable to expect tectonic movement during the past 6,000 6 5 4 3 2 1 0 0 years. + Newman and Rusnak, 1965 ++ + The response to sea-level rise as influenced by antecedent + + -2 geology and topography plays out differently in the northern + Finkelstein and Ferland, 1987 versus southern sections of the Delmarva coast. Most of the -4 Virginia barrier islands are anchored on the interfluves of the lowstand drainage network, and most of the tidal inlets and back- -6 barrier tidal creeks align with the lowstand valleys (Oertel et al.,

1992; Oertel and Foyle, 1995). In this setting, the updrift, main Elevation (m), -8 body of the barrier island sitting on the interfluve is relatively

stable, whereas the inlets and the downdrift spit end of the islands relative to modern sea level -10 have higher and more variable erosion rates. Farther north, Virginia Eastern Shore along the wave-dominated coast, at sites where Pleistocene or -12 earlier Holocene shorelines intersect the modern ocean coast, 6 5 4 3 2 1 0 the topographic high of the older ridge and the abundant sand Calibrated age (thousands of years BP) that can be mined by wave action and redistributed into the 0 shoreface will stall the transgression at least locally (Riggs et al., 1995). These effects have dramatically slowed shoreline -5 retreat at five sites along the northern Delmarva coast: central Assateague Island, Fenwick headland, Bethany headland, Rehoboth headland, and sections of Cape Henlopen (Honeycutt, -10 2003; Honeycutt and Krantz, 2003). -15 Peat sample

Into the Near Future Elevation (m), -20 Basal peat sample

According to the vast majority of scientific studies, as relative to modern sea level presented in the 2013 IPCC report and the 2014 National -25 Delaware Climate Assessment, the near future will be a time of rapid climate change, with potentially dramatic consequences in 12 10 8 6 4 2 0 Virginia’s coastal zone. An accelerating rate of sea-level rise and Calibrated age (thousands of years BP) an increase in the severity of tropical storms, in particular, are Figure 20. Holocene sea-level curve for the Virginia Eastern likely to have major impacts. Shore (upper panel), modified from Van de Plassche (1990). A Many indicators suggest strongly that the global climate more extensive, regional sea-level curve for the Delaware coast system has crossed a critical threshold. The Greenland and West (lower panel), drawing mainly from the work of Kraft and co- Antarctic Ice Sheets are the two largest ice sheets that presently workers, as compiled by Ramsey and Baxter (1996). The shaded inset in the lower panel is the equivalent age and elevation range are threatened by warming, and that will affect sea-level rise as the upper panel. through the 21st century. Just in the past decade, the rate of melting of the Greenland Ice Sheet has been six times greater heat, and enhances melting. During an average summer through than during the preceding decade, and that of the Antarctic Ice the 20th century, about 10% of the surface of the Greenland Ice Sheet has increased by a factor of nearly five (IPCC, 2013). This Sheet would melt in lower elevations around the outer edge. The accelerated rate of ice loss has caused an accelerated rate of sea- percentage of surface melting has increased in the past decade, level rise as the meltwater flows into the ocean. and in the summer of 2012, 97% of the ice surface melted If the Greenland Ice Sheet were to melt completely, sea level (Keegan et al., 2014); the last similar event was in 1889. This globally would rise by 5 to 7 m (16 to 23 ft) (Gornitz, 2013). led Keegan et al. (2014) to conclude that by 2100, widespread Unfortunately, several major processes are acting to degrade the melting of the Greenland Ice Sheet could be an almost annual ice sheet, some of which operate in positive-feedback loops. In event, very likely leading to an extreme reduction of the ice general, the processes that cause disintegration act more rapidly mass. than those that build a large ice sheet. Around the outer margins The potential rise in sea level resulting from the melting of where the ice sheet meets the sea, even slightly warmer water the West Antarctic Ice Sheet also is about 6 m (20 ft) (Williams et erodes the leading edge of the seaward-flowing glaciers so that al. 2005, rev 2011). Recent studies (Rignot et al., 2014; Joughin et the ice flows more quickly and calves rapidly. On the surface al., 2014) concluded that although it will take hundreds of years of the ice cap, increased melting produces ponded water that for the ice sheet to melt completely, the melting is in progress, absorbs solar heat and causes further melting. The meltwater is irreversible, and is contributing significantly to sea-level rise. streams downward through crevasses in the ice, reaching For West Antarctica, slightly warmer circumpolar seawater bedrock, lubricating the base of the ice, and speeding the ice flowing under the ice shelf is degrading the buttress of grounded flow. Also, as the snow cover on the ice surface melts, dark glacial ice. As the ice shelf weakens, it is less able to hold back grains of dust and soot from forest fires are concentrated, which the seaward flow of the terrestrial ice sheet. This disintegration reduces the albedo (reflection of sunlight), increases retention of of the West Antarctic Ice Sheet was forecast by Mercer (1978),

367 The Geology of Virginia who wrote that the stability of the West Antarctic Ice Sheet is which already are on the threshold of “collapse” (Gutierrez et “reasonably secure” against all mechanisms except climatic al., 2007). Notably, Wallops Island, site of the Mid-Atlantic warming, and that warming sufficient to cause the “deglaciation Regional Spaceport, is one of the threatened islands. The recent of West Antarctica would not occur in the foreseeable future repair and installation of a substantial revetment and placement without Man’s injection of massive amounts of industrial CO2 of sand to nourish the beach will provide a temporary protective into the atmosphere.” buffer, but, with rising seas, storms will remove the sand and The same 2013 IPCC report also found that the top 75 m storm surge will overtop the revetment. The other barrier islands (250 ft) of the world ocean has warmed about 0.1°C each decade of the group already are very low and narrow, frequently washed since 1971 and that much, but not all, of the deeper ocean has over during storms, and migrating landward rapidly. In this area, warmed as well. The steric (thermal) expansion of seawater as a an ocean shoreline abutting the mainland is likely in the next result of the warming contributes to sea-level rise. Through the centuries, as has occurred previously in the late Pleistocene. 21st century, 30 to 55 percent of the rise in global mean sea level Sites in Chesapeake Bay also will experience potential may be due to the thermal expansion of the water (IPCC, 2013). problems. As with low-lying islands worldwide, Tangier and The hydrologic cycle, global atmospheric circulation, and Smith Islands, with no natural elevation greater than 1.8 m (6 the power of tropical storms (hurricanes, cyclones, and typhoons) ft) above sea level, are at risk of disappearing by 2100. Low- are driven by heat from the Sun, especially heat absorbed into elevation portions of the mainland, such as in Accomack, the surface ocean. A recent report by the Geophysical Fluid Matthews, Gloucester, and York Counties, and the City of Dynamics Laboratory and NOAA (2013) concluded that Poquoson, will experience more frequent and severe storm because of climate warming, hurricanes during the next century flooding, and will be inundated as sea level rises. Even before are likely to be more intense globally and to have higher rainfall residential structures are attacked, critical infrastructure such as rates than present-day hurricanes. Whether the total number of water wells and septic systems will fail. In some of these areas, tropical storms per year will change is unknown, but it is likely the population density may not be sufficient to allow affordable that the annual number of category 4 and 5 storms will “increase community sewer and water systems. As houses become by a substantial fraction.” uninhabitable, real estate values, and local tax revenues, will It is clear that processes driven by climate change will plummet. impact Virginia, and the coastal regions may be at special risk. The City of Norfolk has been cited by the national press In July 2014, Virginia’s governor signed an Executive Order for its efforts to mitigate the effects of sea-level rise, and has convening the Governor’s Climate Change and Resiliency acknowledged that at some time it will be necessary to abandon Update Commission (McAuliffe, 2014). The Commission is some structures and areas. The U.S. Navy and the Newport charged to find creative ways to mitigate climate change and News shipyards are developing plans to address the threats of to adapt to the effects that are evident already. Another part of sea-level rise to their facilities in Tidewater. the charge is to review, update, and prioritize recommendations The range of management strategies and actions that can from a predecessor commission in 2008 and to assess the be taken to combat the threats posed by sea-level rise and implementation of that commission’s recommendations. more severe storms is limited: protect, accommodate, retreat, Understanding the interactions and impacts of a rapidly rising or abandon. The choice of strategy depends largely upon the sea level and more intense storms with basic coastal processes relative values of what is to be protected compared to the cost will be essential for the Committee. of the proposed action. Understanding the physical setting and Rising sea level and a greater number of severe storms pertinent geological processes is critical in making rational directly threaten the shore and coastal communities. The most decisions when selecting options for mitigating the consequences obvious threats are erosion and inundation. The inundation risk of climate change. is twofold: more frequent flooding by storm surge, and persistent inundation by the elevated water level. Also, as sea level rises, FUTURE WORK coastal habitats naturally migrate landward and up-slope. In places where the shore is backed by hard structures such as Studies Concerning Changes Related to sea walls, the habitat is trapped, compressed, and, eventually, Climate Change and Sea-Level Rise disappears. Methods such as beach nourishment, commonly used to mitigate the effects of shoreline erosion, will become Sea-level change is a major factor in coastal studies that ineffective. As the beach becomes trapped against a sea wall and consider phenomena with time scales of a decade or more. sea level continues to rise, the beach and immediate nearshore NOAA (2007) stated that from 1927 through 1999, the average zone will narrow and steepen. The increased slope results in a rate of sea-level rise at the Sewells Point tide station in Hampton permanently narrow beach and nearshore that are unable to retain Roads was 4.42 mm/yr (1.47 ft per century). There are many enough sand to create either a buffer against storms or a pleasing indications that the rate of rise is increasing and will increase recreational beach. This is the likely the fate of the recreational during the next century (Titus and Narayanan, 1995; Carton beach of Virginia Beach in the coming century. Marshes, with et al., 2005; among others). Miller et al. (2001) stated that their important natural functions, also will be squeezed against salt marshes “will be the first ecosystem to feel the effects of the backshore. If sea level rises more rapidly than the marsh can an increased rate of sea-level rise.” The interplay between the accumulate sediment, the marsh vegetation may fail completely. rates of vertical accretion of the sediment surface and of local The barrier islands of the Eastern Shore are at risk, especially sea level is a critical factor in determining the viability of the the thin, sediment-starved islands within Chincoteague Bight, marsh. When the rate of sea-level rise exceeds the ability of the 368 Atlantic Coast and Inner Shelf marsh surface to keep pace, the marsh begins to fail. Oertel et The response of the southeastern Virginia shoreline to sea- al. (1989) found that sediment was accumulating at 36 to 57 level rise also needs to be considered. Will the barrier beach percent of the rate of local sea-level rise in marshes on the lower extending south from Sandbridge drown in place as the Atlantic Virginia Eastern Shore. Erwin et al. (2006) stated that near Ocean and Back Bay rise? What would change if an inlet Wachapreague and Mockhorn Island “marsh surface elevations should open somewhere along the northern portion of the long do not seem to be keeping pace with local sea-level rise.” They Currituck barrier spit? Is the local rate of sea-level rise too great postulated that submergence and flooding of the marshes could to be mitigated by beach nourishment? have major negative consequences on the value of the area as habitat. The salt marshes of the coastal zone of Virginia are, at Surveying the Quaternary Sediments best, on the cusp between survival and failure. Studies designed of the Middle and Outer Shelf to monitor the situation would be beneficial as they could provide both a warning and the impetus to plan for the changes Although a significant amount of work has been conducted that would accompany massive loss of the marshes. Monitoring on the inner shelf, within about 20 km (12 mi) of shore, the studies also would guide site selection for more detailed studies Quaternary deposits of the middle to outer shelf off Virginia are of the marsh processes that respond to sea-level change. largely unknown. Wright (1995) discussed the late Pleistocene The increased area of open water could affect the wave energy and Holocene history of the Mid-Atlantic shelf in terms of that reaches the mainland shore of the back-barrier lagoons. The cumulative duration of exposure to potential bottom-disturbing loss of marsh area decreases the mitigation of storm surge, as processes. The areas shallower than 20 m have had relatively experienced in Louisiana with Hurricanes Katrina and Rita. The short exposure to the higher-energy environment of shoaling and increased volume of intertidal open space accompanying marsh breaking waves and strong longshore currents. In contrast, areas loss would result in an increased tidal prism flowing through of deeper water have experienced resuspension and transport the inlets, which would yield changes in the cross-sectional area of sediment for a longer time. During the interval between the of the inlets and affect sediment dynamics of the inner shelf. previous interglacial sea-level highstand (MIS 5e at 125 ka) and Future studies could monitor, assess, and predict these changes the present, the shelf has experienced at least 50,000 total years and provide additional understanding of the ways in which the of resuspension events (Wright, 1995). The cumulative time of marshes, especially their seaward edges, buffer wave energy and exposure increases seaward across the shelf with depth, which storm surge. suggests that the preserved record offshore (middle to outer The Virginia barrier islands are essentially pure, untouched shelf) may have a more complex stratigraphy than the nearshore. in any significant way by man. As such, the islands area The geologic record of the intermediate stands of sea level natural laboratory for determining how barrier islands respond (between the high and low extremes), which actually account to physical forcings. Two of the major consequences of global for most of the time during the Quaternary, are preserved in the warming are sea-level rise and an increase in the severity stratigraphy of the shelf. Further, the rest of the story associated of tropical storms. These are two of the four phenomena that with the major paleochannels of the Susquehanna-Potomac determine the impact of storms on the coast (Hobbs, 1970, River system and James River, and their potential connection 2012). Barrier islands can respond in any of several ways to these with the submarine canyons, lies beneath the shelf. mechanisms. They can remain in place and drown as the sea rises around them; they can roll landward in a quasi-equilibrium as storm-driven overwash carries sand across the island and Environmental Studies Concerning sea-level rise translates the lagoonal shoreline farther inland; Potential Production of Offshore Energy or they can be overtopped by storms and essentially disappear if there is not enough time between destructive events for the Bayer and Milici (1987) studied the deeper sedimentary shoreface to recover. The third case appears to be situation in the structure underlying the continental shelf off Virginia with northern group of barrier islands on the Virginia Eastern Shore an emphasis on the potential for petroleum exploration. They as the sediment-starved Wallops, Assawoman, Metompkin, and reported a set of diapirs in the strata underlying the 200 m (660 Cedar Islands diminish in elevation and width above sea level. ft) isobath, roughly along what they considered the “paleoshelf It also appears that the rate of erosion along Parramore Island, edge” and noted a discontinuous, Mesozoic reef beneath the immediately south of Cedar, is increasing. Is this in response shelf edge, roughly in line with the diapirs, and two synrift basins to a progressive, downdrift exhaustion of mobile sediment in about 40 km (25 mi) offshore (Milici, 2016 [this volume]). the nearshore or to a change in the rate of sea-level rise? New They concluded that “the probability of the Virginia continental studies could both monitor the ongoing changes to the barrier margin containing commercially recoverable quantities of oil islands and address the processes driving those changes. and/or gas [ranges] from fair (moderate) to poor (low).” As noted earlier, the barrier islands of the lower Delmarva Exploration of the deep stratigraphy underlying the outer Peninsula can be separated into three morphological groups and continental shelf and slope during the 1980s yielded tremendously the boundaries between the groups are coincident with major valuable information related to the early development of the changes in the morphology of the underlying Chesapeake North American Atlantic continental margin (Sheridan and Bay Impact Crater. Determining whether there is a causative Grow, 1988), but few economically viable targets for petroleum relationship or if it is coincidence would be beneficial in extraction. However, improvements in drilling technology, assessing other potential effects of the crater on the regional including hydraulic fracturing and the ability to drill in deep geology. water, have led to renewed interest and recent Federal approval 369 The Geology of Virginia of geophysical exploration of the Atlantic margin (BOEM, with high-resolution seismic profiling to map the distribution 2014). Should that interest develop to the level of any bottom- of economic mineral sands and characterize the sand bodies disturbing activities, the generally pessimistic outlook by Bayer in which the minerals occur. The earlier studies did not have and Milici (1987) notwithstanding, site-specific studies of the sufficient scope to develop models for the occurrence ofthe near surface geology and sea-floor processes would be prudent. concentrated heavy minerals. With proof of the mineral resource Similar studies would be required for potential drilling sites and and commercial interest, additional studies should be undertaken pipeline routes. to evaluate the environmental impacts of extraction. These Rooney-Char and Ayers (1978) concluded that aside from studies would be similar to the environmental assessments and any actual well locations, the site of the landfall would be the monitoring of sand mining for beach nourishment that have been most significant element in determining the offshore routing of conducted for the federal Minerals Management Service (Hobbs, any pipeline. Their assessments of possible pipeline landfalls 1998, 2000, 2006; among others). Hennigar and Siapno (1990) broadly considered sites in three areas: the Eastern Shore stated that further government-funded studies should focus on south of Assateague Island, the Chesapeake Bay mouth, and defining the heavy mineral deposits to improve assessments of the southeastern shore. Although they had general conclusions the economic potential of these deposits. about various sites, the study should be updated as nearly three decades have passed since the work was performed. Additionally, Updating Sand Inventories the dynamics of the near-bottom sediments should be studied with regards to the stability of the pipelines that likely would run Since the 1980s, numerous studies have addressed offshore from the production platforms to shore. sand resources on the southeastern Virginia shelf that serve the The popular press has reported on the proposals to ongoing beach-nourishment projects for Virginia Beach, the construct wind farms for the generation of electricity in the U.S. Navy facility at Dam Neck, and Sandbridge (Williams, offshore regions from Virginia to Maine. One scenario considers 1987a, 1987b; Kimball and Dame, 1989; Dame, 1990; Kimball construction of large windmills in an area roughly 20 km (12 et al., 1991; Hardaway et al., 1995; Hobbs, 1998, 2006; among mi) offshore of the Chesapeake Bay mouth. The geological and others). The material used at Sandbridge and Dam Neck has been coastal process concerns with constructing such a facility would extracted from Sandbridge Shoal about 5 km (3 mi) offshore. parallel those for developing the infrastructure for offshore The volume of the shoal has been estimated as 20 x 106 m3 production of petroleum. It would be necessary to consider the (26 x 106 yd3), from which about 3.5 x 106 m3 (4.6 x 106 yd3) characteristics of the subsurface sediments for their ability to had been removed through 2003 (Diaz et al., 2006). Even though support the windmill structure. Sediment movement could affect this is slightly more than 15percent of the shoal’s volume, there the stability of structures as could currents and storm waves. has been no assessment of how much of the shoal could be As with petroleum pipelines, studies would be necessary for removed without causing unacceptable environmental change. siting the landfalls and offshore routes of the major electrical As it has been two decades since these volumetric assessments supply cables. These studies would be in addition to the studies were made, and as it is likely that there will be a continuing of potential impacts on fisheries, birds, and navigation. need to access the offshore sand resources, it soon may be appropriate to update and expand the resource inventories. Studies Concerning Potential Economic In addition to revisiting Sandbridge Shoal, the shore-oblique Deposits of Heavy Mineral Sands ridges off False Cape should be assessed. The filled channels that are offshore are another potential resource. If cores indicate Work in the 1980s by Grosz and Eskowitz (1983), Berquist that the channel fill is sand suitable for beach nourishment or and Hobbs (1986, 1988a, 1988b, 1989), and Berquist (1990) construction aggregate, careful mapping with sub-bottom considered the economic potential of deposits of heavy- profiling could allow economical mining. More broadly, mineral sands on the continental shelf. These studies basically renewed and detailed mapping of the inner shelf would provide were “academic” in nature and there was little interest and no valuable data for development of a regional sediment budget in participation by industry. However, there are occasional hints of support of resource management by the Bureau of Ocean Energy renewed commercial interest. Should that possible interest grow, Management (formerly the Minerals Management Service) and a two or three stage sequence of studies would be necessary. applied projects by the U.S. Army Corps of Engineers. The first stage would involve shallow coring (5 to 10 m) along

370 Atlantic Coast and Inner Shelf

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