Department of Geography, University of Maryland, College Park, MD 20742 (U.S.A.) (Accepted for Publication May 17, 1984)

0043573

Marine Geology, 63 (1985) 173--195 173 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

GEOMORPHIC AND STRATIGRAPHIC ANALYSIS OF FIRE ISLAND, NEW YORK

STEPHEN P. LEATHERMAN Department of Geography, University of Maryland, College Park, MD 20742 (U.S.A.) (Accepted for publication May 17, 1984)

ABSTRACT

Leatherman, S.P., 1985. Geomorphic and stratigraphic analysis of Fire Island, New York. In: G.F. Oertel and S.P. Leatherman (Editors), Barrier Islands. Mar. Geol., 63 : 173--195.

Barrier islands along the East Coast of the United States are believed to be migrating landward in response to sea-level rise. Modes and rates of displacement of Fire Island along the south shore of Long Island, N.Y., were investigated through geomorphic and stratigraphic analyses. Inlet processes are principally responsible for bayshore sediment accretion and hence landward displacement, while overwash has contributed to increasing the island's elevation. Portions of marsh areas covered by washover deposits were often raised above the tidal level permitting colonization by barrier-flat vegetation. Photographic analysis showed that only the 1938 hurricane resulted in bayshore accretion by washover (a small amount at one locality). Most of the bayside marshes formed on relict flood-tidal delta shoals. Former inlet sites are characterized by typical geomorphic features, including relict inlet ridges, indentation of the bay shoreline, wide marsh plains, and relict flood-tidal delta islands in the bay. Historical records and relict inlet features indicate that as much as 85% of the area has been affected by inlet activity. The geomorphic and vegetative data supported by the stratigraphic interpretations showed that the western section of Fire Island, N.Y., is migrating landward more slowly than the eastern section of the barrier chain ; this trend is probably due to hurricane tracks and to an increased sediment supply to the western part. Barrier migration is occurring continuously over geologic time, but considered on a short-term basis, displacement is sporadic and related to inlet processes. Over the past 1000 years, Fire Island has experienced shoreface erosion as well as bayshore erosion and submergence.

INTRODUCTION

Barrier islands along the Atlantic and Gulf Coasts of the United States are migrating landward over geologic time in response to eustatic sea-level rise. Since barriers are affected by inlet dynamics, overwash processes, and aeolian transport to varying degrees, they exhibit different characteristics on a geo- graphic basis. Variables, such as rate of sea-level rise, tidal range, storm tracks, wind and wave regime, and sediment supply, determine the relative magnitude of the transport processes, making each barrier setting unique. These sedimentary processes are important in understanding the evolution of the different landform and associated vegetative features. This study was directed

174

towards delineating the processes principally responsible for bayshore accre, tion and landward migration. Fire Island is a 48 km-long barrier extending from Fire Island Inlet (Dem- ocrat Point) eastward to Moriches Inlet; it is part of the barrier system which runs parallel to the south shore of Long Island, New York (Fig.l). These barriers are known to be migrating westward as sediment eroded from Montauk Point is carried by longshore currents and deposited at Democrat Point (Taney, 1961). in 1825, the inlet existed within 150 m of Fire Island Lighthouse; by 1940, it had migrated approximately 8 km westward to its present stabilized position. As a result of the westward migratory tendencies of the barriers, many of the inlets along the chain exhibit an updrift, seaward off-set. A jetty was built at Democrat Point in 1940 in an effort to contain these westward-migrating sediments; it filled to capacity by 1960. The U.S. Army Corps of Engineers currently dredges the inlet annually to maintain its navigability. Sediments that nourish Fire Island are believed to come primarily from several sources. The eroding headland section (Montauk Point area) is a major source of sediment for the barrier islands with offshore, reworked shoreface and inlet scouring contributing materials as well (Taney, 1961; McCormick and Toscano, 1981). Tides in the study area range from 1.3 m at Fire Island Inlet to 1.0 mat Moriches Inlet. The tidal range in Great South Bay is generally less than 0.3 m. This bay, which separates Fire Island from the mainland, is 40 km long and 3.2--8 km wide. With the exception of some shallow flats, it ranges in depth from 1.2 to 3 m. The prevailing winds are from northwest to southwest (Fig.l). Winds from the east are less frequent, but are generally stronger

"~ Long Is/and Sound i-z7

Annual { . ~\: ~ \c j~ .-" Southamplon

• ~d,." 'ooo~ " s,k e / ;? .~-.

Atlantic Oceat)

i Summe~" o 5 lo o 5 )o 15 I km - i • , [.

...... "~" :" iJ Winter

Fig. I. Location map of study area with wind roses (from McCormick and Associates, 1975). 0043575

175 and associated with storms (McCormick and Associates, 1975). Both hurricanes and extratropical storms (northeasters) affect the study area. Over 70% of the deep-water waves approach the shore from the directions ENE through SSE; these waves are responsible for producing the net westward littoral drift (U.S. Army Corps of Engineers, 1977). Rates and modes of landward migration of Fire Island, New York, were determined through morphological analysis of charts and historical aerial photographs as well as stratigraphic correlations of core data. Comparison of historical maps and photos indicate general barrier changes in shoreline and geomorphic features, including shoreline recession or progradation, marsh erosion or bayward growth, and inlet openings and closures. Since washover deposits are generally not recorded on maps, historical aerial photographs are invaluable for their determination. Unfortunately, the photographic record is limited to the past 53 years. Volumetric calculations of backbarrier facies are necessary to determine the amount of sediment transported across the island by overwash and inlet processes (and thereby evaluate their relative contribution to landward barrier migration). Stratigraphic correlation of cores taken along strategically chosen transects (areas of known overwash, former inlets, or presumed stability) may allow such comparisons to be made.

METHODOLOGY

Historical maps and vertical aerial photographs were used to determine morphological changes along Fire Island during the last 150 years. Physio- graphic features evident on the air photos and in the field were used to make geomorphic interpretations regarding past inlet activity. Landform/vegetation units were mapped and their development and relationship to barrier island processes were traced through time (1938--1979). Cores were taken along seven north--south transects across the island (Fig.2). The transects were chosen either in areas thought to have been stable for some time: Sunken Forest (S.F.), Water Island (W.I.), and Watch Hill East (W.H.E.); or in areas historically documented as being former inlet sites: Long Cove (L.C.) and Smith's Inlet (S.I.); or finally in areas known to have experienced overwash between 1938 and 1962 as determined from post-storm air photos: Robbin's Rest (R.R.) and Smith Point (S.P.) (Joneja, 1981). The number of cores taken per transect was dependent upon island width at that location. To attain greater depth and to avoid coring through the most recent sediments, cores were taken in low areas in the backbarrier, while in the dunes they were taken either in deflated areas or in pre-dug holes, usually about 0.5 m deep. The cores varied in length from 1.4 to 3.0 m, this variability was due to the nature of the sediment at the coring site and the coring technique employed. The cores were taken by two methods: pile-driving (Leatherman, 1979) and vibracoring (Lanesky et al., 1979). Each transect was surveyed by transit and rod to determine elevations of the cores with respect to each other. Benchmarks were not available so it was only possible to estimate elevations relative to bay still mean water level. <~ / ?reatSouthBay" " 8~aPOrtco4~ ~~--

Point I-.I. '~ T-Davis East e 0043576 L,ghthouse ~onotd0s Park

938 954 960 962 J

~elict Intet Ridges ~ -~'

t 2 3 456 __~__ i t i ~ I

kilometers

Fig. 2. Distribution of physiographic features and washovers resulting from four hurricanes along Fire Island, N.Y. (from Leatherman and Ioneja, 1980; and Johnson, 1982). Coring transect lines are also indicated. 0043577

177

The laboratory analysis of split cores followed standard techniques. Organic materials recovered at depth were age dated by radiocarbon methods. Sediment-size analysis by sieving was undertaken to characterize depositional environments since texture is a function of source and mode of deposition. Sediments deposited by various aeolian or hydraulic processes exhibit certain grain-size characteristics which may be expressed as statistical parameters: mean, standard deviation and skewness. While the source area plays the principal role in determining the type and availability of material, the various transport processes should produce characteristic grain-size distributions which can be recognized in a statistical sense (Friedman, 1967). Visher (1969) curves and a discriminant analysis program (Nie et al., 1970) were used to differentiate sedimentary environments. Grain-size characteristics established for six "known" environments (dune, washover, beach, shallow inlet channel, spit, and tidal flat) were compared to grain-size characteristics of Fire Island core samples from "unknown" environments (Joneja, 1981).

GEOMORPHIC ANALYSIS

Barrier physiography

Physiographic features can be used to provide an interpretation of barrier dynamics. During the landward and lateral migration of barrier islands by inlet and overwash processes, new morphological features are continually being created as earlier ones are destroyed. Overwash generally results in a loss of physiographic features as low areas are filled and high areas are planed off (Leatherman et al., 1977). Therefore, washover deposits often result in the formation of wide, gently landward-sloping barrier flats that can be colonized by terrestrial vegetation, such as grasses and shrubs. Aeolian reworking of unvegetated washovers prior to plant colonization can aid in the development of barrier dunes. Inlets have a much more pronounced effect on barrier physiography, and the presence of inlet-related features provides good evidence of migration history (Fisher, 1967). Three apparent geomorphic features in the study area indicate previous inlet sites: (1) relict inlet ridges; (2) relict inlet channels; and (3) relict flood- tidal deltas. Inlet ridges are recurved beach/dune ridges that form along the updrift and/or downdrift sides of an inlet. If the inlet migrates, a sequence of recurved ridges develop on the updrift side of the inlet while any downdrift inlet ridges are eroded. However, not all previous inlet sites are associated with inlet ridges. Recurved ridges never formed at certain (usually ephemeral) inlet sites, or were covered by washover deposits, or removed by human disturbance and development. A large number of arcuate backdunes (associated with inlets) are found along Fire Island, particularly west of Watch Hill (Fig.3). Recurved ridges believed to be associated with relict inlets were found at Democrat Point, just east of the abandoned Fire Island Inlet lighthouse, and from Point O'Woods to Davis Park (Fig.2). Arcuate dune ridges were also mapped at 0043578

178

• x x "•• •L.'- .

Democrat Point

Sunken Forest Fig.3. Patterns of arcuate backdunes along western Fire Island (from Johnson, 1982).

Watch Hill and Old Inlet (Fig.4), and between Old Inlet and Smith's Inlet (Johnson, 1982). Previous inlet locations are also indicated by relict inlet channel and flood- tidal delta features. These physiographic features are present at Watch Hill, Long Cove, west of Bellport Beach to Old Inlet, Smith's Inlet, and Pattersquash Island to Halletts Inlet (Fig.2). Historical information on inlet dynamics along the barrier chain indicated that relict flood tidal deltas (salt marsh islands) were associated with inlets that remained open for approximately 60 years or longer. Ephemeral inlets, such as many of those created by the 1938 hurricane, did not result in promi- nent, long-lasting physiographic features. Inlets that remained open for a considerable length of time may also leave discernable sedimentary records on the shoreface and inner shelf. Williams (1976) mapped areas of fine-grained sediments off the southern coast of Long Island. John (1977) noted that fine-grained sediments were winnowed out of the Cape Henlopen spit complex and were deposited in the ebb-tidal delta. Fine sediments were found offshore of Moriches and Shinnecock Inlets as well as the Old Inlet and Davis Park/Watch Hill/Long Cove areas, McKinney and Friedman {1970) made a detailed offshore surficial sediment map for a section of Fire Island that showed fine sediments just seaward of the Sunken Forest area (Fig.2). Relict inlet features, combined with historical data, were used to determine the percentage of Fire Island that had been affected by inlet ac~vity. Based on known historical inlet sites alone,: approximately 12 km or 2~% of the study area once contained an inlet. If possible, but not proven, relict inlet sites (e.g., large marshy areas, ebb tidal deltas) are included in the total, then inlets may have existed along 85% of Fire Island. 0043579

179

Fig.4. Old Inlet area illustrates the features typically associated with inlet formation. The northeast--southwest indentation of the bay shoreline is an abandoned inlet channel remnant, while the marsh protruding into the bay and marsh islands developed on the sandy substrate of the flood tidal delta. The bluntly arcuate-shaped dune is a relict inlet ridge.

Inlet-related morphological features also result in the topographical conditions prerequisite for forest development. In each of the four locations on Fire Island, climax forest vegetation is found on and/or behind relict inlet dune ridges (Johnson, 1982). These dune ridges provide the relief to keep the vegetation above the level of bayside flooding and serve as a physical barrier between oceanic overwash surges and the forest. Foredune and/or secondary dune lines also protect the forests from burning salt seaspray. The Sunken Forest is especially large and well developed because it is built on a whole sequence of relict inlet ridges situated behind a high secondary dune. Radiocarbon dates by Sirkin (1972) for the Sunken Forest sediments indicate deposition about 250 +- 80 yrs B.P. (B.P. = 1950 A.D.). Tree-ring dates suggest that the climax forest (American holly, white sassafras and shadbush, dominant tree species; Art, 1976)has been there for approximately 170 years. Thus, the climax forest developed within 80--160 years of the postulated deposition of these sediments. The forests at Watch Hill and Smith' s Inlet are much smaller because they are built on or behind individual relict inlet ridges which are unprotected by 0043580

180

other secondary dune lines. However, both locations have high foredune lines, which provide some protection against overwash and salt spray. The forests at Watch Hill and Smith's Inlet must have formed since the closing of those inlets. Inlets present in the Watch Hill area probably closed in the 1830's, and Smith's Inlet closed in 1834 (Leatherman and Joneja, 1980). Climax vegetation was established on the inlet sites within 150 years after inlet closure.

Overwash sedimentation

Overwash was not recorded on maps in previous centuries, and is only rarely done so today. Therefore, the distribution of washover deposits can only be evaluated since the first (1938) good aerial photographs of Fire Island. Four major storms (1938, 1954, 1960 and 1962) resulted in overwash at various locations along Fire Island (Fig.2). During the interval of photographic record overwash was not observed between Water Island and Watch Hill, nor between Long Cove and Old Inlet. The former stretch of island is quite narrow (114 m near Water Island), while the latter is comparatively wide (770 m) with high dunes. Long Cove, site of a former inlet, is the only area to have experienced overwash during all four of the documented storms, except for the western spit section. There are presently no active inlets through Fire Island, but a number of former inlet sites has been reported (Joneja, 1981; Fig.2). These inlet areas remained susceptible to overwash for decades after closure. Overwash sedimentation increased island elevation, and vegetative recovery was initiated by barrier flat grasses. Aeolian reworking of the barren washover deposits aided in closing the foredune breach and precluded most subsequent overwashes. Shrubs eventually became established with protection from salt spray and salt-water flooding. Along Fire Island, overwash sediments rarely reach the bay, and little new land has been added to the barrier bayshore during the past half century (0.017 km ~ at one locality resulting from the 1938 hurricane). However, bay areas may have become shallower where influenced by washover deposition. Where washover deposits buried existing marshes, the vegetation usually changed from marsh to barrier-flat vegetation. The backbarrier elevation was raised above the intertidal range, and barrier flat grasses and shrubs colonized the washover surface (Johnson, 1982). Therefore, overwash (even during the 1938 hurricane) has not been effective in building the Fire Island barrier system landward. Instead, it has been a major process in increasing backbarrier island elevation, often at the expense of salt marshes.

Inferences for barrier migration

Fire Island Inlet to Southampton constitutes a shoreline segment along the south shore of Long Island, New York (Fig, l). Geomorphic features indicate that the eastern part of the barrier chain is more dynamic than the 0043581

181 western section. Examination of inlet locations showed that 28 inlets have occurred during the past 300 years, of which only five were cut through Fire Island to Great South Bay (Fig.2). The destructive hurricane of September 21, 1938, opened five inlets into Moriches Bay, two through Westhampton Beach, one into Quantuck Bay, and four into Shinnecock Bay, and the Hampton barrier beaches were severely overwashed. Part of this pattern may be explained by the hurricane's track (and that of most hurricanes since 1815) which has passed over the barrier islands immediately west of Moriches Inlet. Since a hurricane's strongest onshore winds are to the east of the storm center, greatest damage should be expected around Westhampton Beach (between Moriches and Shinnecock Inlets, Fig.l). This climatic factor may contribute to the more dynamic nature of the eastern section of the barrier island chain. The general trend of the barrier chain is straight and parallel to Long Island's south shore. However, the section from Fire Island Inlet to the Old Inlet area (Fig.2) bulges seaward. This seaward bulge corresponds with a lobe of sediment on the shelf (Fig.5) and may be the remnant of a river delta formed at the mouth of the ancient Huntington Channel (Williams, 1976). The shelf sediment lobe is speculated to contribute sediment to the bulging section of Fire Island because of the large increase in longshore sediment transport westward along the barrier chain. Panuzio (1969) estimated that 230,000 m 3 yr -1 (300,000 yd 3 yr -~) of littoral drift pass by Shinnecock Inlet. The amount increases to 270,000 m 3 yr -1 (350,000 yd 3 yr -1) at Moriches Inlet, which is further to the west (downdrift) but updrift of the sediment lobe. Downdrift of the "ancient delta" the amount of net littoral drift increases dramatically to 460,000 m 3 yr -1 (600,000 yd 3 yr -~) at Fire Island Inlet. The exact transport mechanisms

Parabolic I[ Dunes Arcuate [i Backdunes I Parallel _~ ] Backdunes [ Patchy Shrubs ~ Narrow Strip of Shrubs ,, Sandy Bayshore I ~ Marshy Bayshore ! Island Seaward Bulge _ 1 _ Straight Island Chain

Sediment

90it ~. o 5 k~

Fig.5. East--west changes in barrier island features (from Johnson, 1982). 0043582

182 responsible for moving the sediment landward and into the littoral system are not known, but these large deltaic shoals are speculated to be the source of nearshore sediment (McCormick and Toscano, 1981). The seaward bulge is associated with east--west differences in barrier island features (Fig.5; Johnson, 1982). Large expanses of bayside marsh are found east of Watch Hill, where the barrier islands parallel the mainland and bay fetch is small. West of Watch Hill the island's seaward bulge increases the already large fetch (exceeding 20 km in some directions), thus either restrict- ing bayside marsh formation and/or eroding former marshes. There has been no historically documented inlet activity between Davis Park and Fire Island Lighthouse, indicating that any prehistoric marshes have had ample time to erode and no new inlet-related marshes have had the opportunity to form. West of Watch Hill (Fig.2), shrubs grow in large patches and clumps. East of Watch Hill, the shrubs are restricted to a narrow, relatively continuous zone between the foredunes and the marshes (Fig.6; Johnson, 1982). This line of shrubs along the island exhibits a "zig-zag" pattern, apparently following the edges of relict washover. Shrub growth is dependent upon a ground surface elevated above the tidal range, and the shrubline roughly follows the 3--4 ft contour line. Backdune patterns are also different east and west of the Davis Park/Watch Hill area (Fig.2). West of Davis Park the topography is dominated by either parallel or arcuate backdunes. From the Sunken Forest to Davis Park, large parallel dune ridges are major physiographic features, whereas well developed sequences of arcuate backdunes are present only at Democrat Point and in the Sunken Forest area. These arcuate dunes formed patterns similar to those created by a migrating inlet {Johnson, 1982). Further evidence for the stability of the western part of the south shore barriers is Fire Island's Sunken Forest, a true maritime forest, which could only have developed under

Fig.6. Aerial photograph shows the shrub pattern in the Long Cove Area, 1973. 0043583

183 conditions of prolonged protection from the environmental stresses of salt spray and salt-water flooding.

STRATIGRAPHIC INTERPRETATION

Introduction

A principal means of defining depositional facies and interpretating sedimentary layers as superimposed through time and space is stratigraphic correlation. This technique has been widely used by geologists in the oil industry to differentiate environments of deposition through geologic time on the basis of deep drilling and coring. Kraft (1971) has applied these prin- ciples to the study of modern (Holocene) sediments along the U.S. Atlantic Coast. General lithologies (e.g., barrier sands vs. backbarrier fine-grained sediments) from the transgressive Delaware barriers were delineated from auger data. The purpose of this investigation was to determine the relative importance of the three processes (inlet, overwash and aeolian) in sediment transport and thereby evaluate their relative contribution to landward barrier migration. Since predominantly sand-size particles are deposited by all three processes and the deposits can be directly superimposed, differentiation between these sandy facies can sometimes be quite difficult when detailed resolution is required. The sedimentary record revealed by cores was interpreted from visual des- criptions, sediment-size analysis, and various dating techniques. Table I indicates the criteria utilized to distinguish overwash, inlet and aeolian deposits. Since no single characteristic was able to provide unquestionable identification of a strata, a matrix of available evidence gave credence to a particular interpretation. Complex sedimentary sequences, comprising a variety of sediment colors and textures, displaying multiple internal structures (mainly horizontal layering), and often containing heavy minerals (of which garnet was a major component), were generally interpreted as overwash and/or aeolian sequences. By comparison, the inlet deposits were more uniform in character although some complexity certainly existed. Along the south shore of Long Island, the normal evolution for an inlet after formation results in bi-directional displacement, both northward (landward) and westward (downdrift). Inlet migration in two directions gives rise to a suite of sedimentary deposits involving a variety of different inlet-related environments: bay bottom, deep to shallow inlet channel, flood-tidal deltas, spit platform, and spit (Kumar and Sanders, 1974). Only the surficial inlet deposits were encountered in this study due to core length limitations. Overwash processes result in uni-directional movement of beach and shoreface sediments (Leatherman et al., 1977). Variations in sedimentary structures and textures occurring within a washover deposit are related to two factors: backbarrier topography and storm intensity. Initially, bayward steeply dipping layers are deposited as delta foreset beds into ponded water 0043584

TABLE I

Identification of barrier depositional sequences, Fire Island, New York

Characteristic Overwash Inlet (upper unit) Aeolian

Color (not always Light brown, whitish, often with Light grey, often continuous Sugar white, orange-golden, some diagnostic) heavy-mineral layers; sometimes color tone throughout sequence heavy minerals heavy-mineral bands Appearance Many distinct horizontal laminae Faint parallel-bedding to massive "Blotchy" to "swirled" with in most cases (no bedding apparent) ;biotur- bedding to no apparent bedding bation occurs where deposited with dune grasses (often with woody fragments) Sand size Range of sizes -- fine to coarse Flood tidal delta generally fine- No shells or pebbles; generally sand with some hydraulic lags; grained; occurrence of granules fine-grained sand with little pebbles possible; shell fragments or pebbles; shell fragments; inter- textural change through sequence occur bedded clays and silts;fining- except for occurrence of aeolian upward sequence lag deposit Dips of beds Horizontal to slightly bayward, Little detectable bedding, horiz., Any direction- primarily NW-- except for fan terminus- delta north or south dip possible SE due to prevailing wind direc- foresets are steeply landward tions- small-scale cross-beds dipping may be evident in core Presence of organics Woody-grassy materials may be Occurrence of sponges, shells; few Woody--grassy layers tend to be present in some layers plant materials; generally clean found throughout some sections sandy sequence with gradual deposition Nature of deposi- Episodic Continuous Intermittently continuous tional process 0043585

185 at the accreting edge of the fan terminus, which levels off as the washover elevation increases. The sand size becomes coarser as the storm nears its peak, then fines again as storm surge decreases (Leatherman and Williams, 1977). Aeolian reworking can result in a truncated sequence, and successive storms can result in vertically stacked washover deposits. Sequences of medium to fairly thick, clean, and generally homogeneous sand units were therefore interpreted as being indicative of overwash events. During spit growth, washover deposits capped by aeolian material, comprise the bulk of the subaerial sedimentary unit. Sedimentary sequences from three transects have been chosen for discussion: Robbin's Rest, Long Cove, and Watch Hill East (Fig.2). These transects illustrate the range of historical activity (areas of former inlets, known overwash, or presumed stability) and geomorphic characteristics (narrow, duned section to wide, marshy plain). Salt marsh peat was encountered along each of these coring transects, permitting age dating by the radiocarbon method.

Robbin's Rest transect

The Robbin's Rest (R.R.) transect was chosen in an area known to have experienced overwash in 1938 and 1962 (Fig.2). The island is narrow at this location with primary and secondary dunes having approximately 3.6 and 2.4 m of relief, respectively. The four Robbin's Rest cores indicate a basal unit of inlet sediments (Fig.7), although there are no known historic inlets. Assuming Taney's ( 1961) rate of westward migration for Fire Island Inlet (64 m yr-1), Fire Island Inlet should have existed in the Robbin's Rest area around 1770. A 1798 map by Wheeler shows that Fire Island Inlet was complex at that time with a "Slew", "Middle Channel", "South Channel" and "South Breakers", but Fire Island Inlet cannot be located precisely from this map (Joneja, 1981). Two radiocarbon dates were obtained for sediments found in core R.R.2: an organic-rich marsh deposit at 0.9 m from ground surface was dated at less than 200 yrs B.P. and an olive-black mud containing shell fragments at a depth of 2.55 m from ground surface was dated at 1100 + 80 yrs B.P. This basal unit is a lagoonal deposit, and a layer of medium sand containing heavy minerals separates the two deposits. Neither of the organic-rich deposits has a stratigraphic equivalent in cores R.R.1 or R.R.3, indicating much spatial variation at this well-resolved scale. Sand-size analysis was undertaken to aid in the determination of sedimentary environments. The results show that the lower core samples belong to environments related to the presence of an inlet: beach, spit, tidal flat, and shallow inlet channel (Joneja, 1981). Stratigraphic sequences in several cores from Robbin's Rest suggest that it was the site of one or several former inlets before the occurrence of more recent (~200 yrs B.P.) overwash events. ._BBeach ,, . , O / ~Foredune Robbins Rest F. ,.. . ..~ ~ ~eo,,an /,, \ Over.ash , \ fx, I Lagoona, / " : " \ / "~ Secondary ~ inlet / .: • "' Dune Peat ,. \ / \ i Marsh J ,' -- \ I ~ i

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~ig.7. Stratigraphic cross-section at Robbin'sRest indicates that the barrier platform is comprised of inlet sediments overlain by overwash [eposits tapering off landward and capped along the seaward edge with two well-developedbarrier dunes. 0043587

187

Long Cove transect

Historical reports and characteristic geomorphic features (salt marsh islands and northeast--southwest bayshore indentation) indicate that Long Cove is the site of at least one former inlet. Cores taken along this transect corro- borate this evidence; they contain stratigraphic sequences believed to have been deposited by overwash processes interlayered with salt marsh peats which in turn overlie basal lagoonal clays and sandy inlet deposits (Fig.8). Although the Long Cove area has experienced overwash during all four of the major storms of photographic record, two contiguous dune ridges presently separate a wide expanse of low backbarrier flats and marsh from the ocean beach. These dunes were not sufficiently developed to prevent overwash along the seaward portion of the Long Cove transect until recent times. Dunes are presently large and quite high, approaching 8 m above MSL. Four radiocarbon dates were obtained for the Long Cove transect. A peat layer from core L.C.1 at 2.65 m from ground surface was dated at 410 + 90 yrs B.P. The salt marsh material indicates that possible later inlets (1770 and 1827) were not located precisely along the coring transect, if indeed they actually existed. A single peat bed at 1.00 m depth in core 2 was radiocarbon dated at 180 + 60 yrs B.P. ; this salt marsh was both established on a washover deposit and then buried by a subsequent event. Core L.C.3 contained a 1.13 m deep peat dated at 350 + 70 yrs B.P., and a 1.95 m deep lagoonal deposit dated at 1350 + 90 yrs B.P. At this site on Fire Island, 0.8 m of sediment was deposited between approximately 1350 and 350 yrs B.P., a period of about 1000 years, while 1.06 m of sediment accu- mulated over the past 350 years. This occurrence demonstrates the sporadic nature of barrier island sedimentation. The sedimentary sequence in core 3 clearly shows two peat deposits interlayered with washover sands, which rest on a lagoonal base. Therefore, it appears that a major overwash event resulted in direct sedimentation into the bay (previous inlet channel), which formed the substrate for marsh colonization. This is one of the few cases where peats do not lie directly on inlet deposits. The barrier is very narrow at this location due to the position of the relict inlet channel, which is being in-filled with washover sands and salt marsh deposits with barrier transgression and sea-level rise. It was not possible to assign any given units to specific events known from the historic record. However, the complexity of the stratigraphic sequences overlying the peat layers attests to the dynamic nature of the past 420 years. No typical inlet sequence was found to confirm the occurrence of the hypo- thetical 1770 or 1827 inlet (Leatherman and Joneja, 1980), and it is clear that no inlet existed at the transect location during this time period. Known overwash events were not defined by distinctive sedimentary units. These sandy materials are likely to have been in part lost to erosion by wind deflation, particularly in an area such as Long Cove where overwash was frequent enough to preclude long periods of stability. ~O .ongCove )0 ~ Aeolian

~ Overwash ~econdary .agoonal 5~ Primary Dune nlet ~eat I"~::~ D U~

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:::::::::::::::::::::::::::::::.:.:.:,:.:-:,:.:.:*:-:*:-:-:,!:~:i:!:!::~:!:i:i:i:~:?:i:[:i:i:i:!:[:?:i:?;i:i:[:!:!:!:i:i:!:i:i:!~::~i~ : Core iiii!i!ii!iii!iii!ii!!~i!i!i!!i~iii!i~i~iiiiiiiiiiiiiiiiii!!iiii~!iii!]ili~ili~iii~ili~i~ii Core i , | '::::::::'::':':':':': ~..... " ...... ~~: .~-'.~cr-- 80 190 200t

14 : 4?0 ~90x ~ C14=500 t 60

Distancealong transect (meters) c ~ :~80 ~60 -'~ :~4:350~ ?0

~C14= 1350~90

7ig.8. The Long Cove stratigraphic sequence indicatesthat overwash has welded the barrier core to a previous marsh island (relict inlet ]ood tidal delta) approximately 200 yrs B.P. More recently, extensive salt marshes have become established on the overwash substrate md continued to built upward with sea-level rise. Also, very high relief dunes have developed at this low point ( throat area) of an old inlet, )ossibly with the encouragementof snow fencing. 0043589

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Watch Hill East

The Watch Hill East (W.H.E.) transect is located in a very wide part of Fire Island (approximately 770 m) and characterized by a high primary dune ridge, commonly 9 m above MSL (Fig.9). This region has not experienced overwash during the period of photographic record (Fig.2). The geomorphic features of the island in this area do not exhibit characteristics indicative of former inlet activity. The stratigraphic section clearly shows a basal inlet deposit. Massive washover deposits have been emplaced on this inlet basal unit, and a salt marsh has more recently developed along the intertidal bayward barrier fringe. Two organic layers were radiometrically analysed. In core W.H.E.4 a peat at 1.10 m depth was dated at 410 + 60 yrs B.P. In core W.H.E.6, a dark mud believed to be a lagoonal deposit at 1.5 m depth was deposited 1140 + 70 yrs B.P. The age of the peat is similar to that obtained for the peat layers in the Long Cove transects. However, the layer was not encountered in any other W.H.E. cores and may therefore merely represent an isolated marsh developed in a former low-lying area. Similarly, the age obtained for the lagoonal deposit in this transect (1140 + 70 yrs B.P.) is almost the same as that of the lagoonal deposit encountered in core R.R.2 from the Robbin's Rest transect {1180 + 80 yrs B.P.) at a greater depth, but also in a more shoreward (south- ward) position. Both of these units are slightly younger than the lagoonal deposit from core L.C.3 (1350 ± 90 yrs B.P.) at a depth of 1.95 m. A coarse sand unit can be followed through the lower sections of cores W.H.E.2 to mid-section of core W.H.E.4 (Fig.9), and this material is believed to have been deposited by overwash processes. The sequence of horizontally stratified sands is characteristic of a typical overwash deposit (Fig.10). One other overwash event is recognized in cores W.H.E.1 and W.H.E.2 (at 2 and 0.75 m depth, respectively), where shell fragments were found in medium brown sand units. This washover deposit did not extend far enough to reach the area where core W.H.E.3 was taken. Heavy minerals were conspicuously absent, and there was also surprisingly little organic matter present (when compared to other transects). This is an unexpected finding for an area which appeared to have been protected by high dunes and stable (at least during the period of photographic record).

DISCUSSION

Three mechanisms are responsible for landward barrier migration -- aeolian transport, overwash processes, and inlet dynamics. Aeolian transport is not a major factor in landward migration of Fire Island, New York because the prevailing winds blow offshore. However, aeolian transport aids in barrier accretion through dune construction. Overwash plays an important role in building the barriers upward, but this process has contributed little to actual landward migration. Most of the washover deposits mapped from the four individual storm events, including the WatchHill East ~x~ °redune ~ A=~a~ O 6 SecondaryDune [] o..... h

Inlet Peat

4 I

:':"~:-:-:"A~-:.:.:.:-:~ :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::.~ r

1:¸ . Core

~:::::~ 4 5 Core

0 - 20 i!Ii:::!2ooi:i:i:i:i:i: 2~0i:!:i:i:!2~ ...... 0043590

Fig.9. Stratigraphiccross-section at Watch Hill East is quite similar to thatencountered at Robbin'sRest (seeFig. 7). The basal inlet deposits are overlain by overwashsands and capped with dunes along the seaward edge. Salt marsh peat depositsare quite thin and are probably less than 200 years old. 0043591

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Fig.10. The 2 m core No. 4 taken at Watch Hill East graphically illustrates the pattern of deposition. The basal unit is inlet sediments upon which a salt marsh developed over 400 years ago, which was killed and buried by an overwash event. Note the horizontal stratifi- cation characteristic of overwash sands. The surface unit is a salt marsh which has more recently colonized the washover surface.

1938 hurricane, did not reach the bay. At only one locality was new land (approximately 0.017 km 2) added onto the barrier bayside (Johnson, 1982}. Where marshes were buried by washover deposits, raising the ground elevation above the tidal range, barrier-flat vegetation {shrubs and dune grasses} recolon- ized the area. The result was a net landward movement of the barrier-flat vegetation, while the bayshore remained stable or eroded. Inlet dynamics are largely responsible for landward migration along this barrier chain. A study of historical maps, charts and photos showed that the barriers are widened at inlet sites (Joneja, 1981). Geomorphic evidence also shows that the marsh islands in the bay and most of the bayshore marshes formed above flood-tidal delta sediments. Approximately 85% of Fire Island can be associated with inlet activity. This estimate includes both areas known to have been inlet sites historically 0043592

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(e.g., Old and Smith's lnlets), and those inferred to have been former inlet sites on the basis of core analysis and geomorphic features. Based on physiographic features and historical evidence, Fisher (1967) showed that over 35% of the Outer Banks of North Carolina had been affected by inlets. These U.S. southeast microtidal barrier islands have been subject to much overwash activity in recent times, which tends to obliterate relict geomorphic features, so that the total percentage of barrier affected would be found to be much higher if stratigraphic analyses were undertaken. It was initially envisioned that specific events recorded in sedimentary sequences could be distinguished, and barrier migration rates could be determined by obtaining dates for selected sedimentary units. Lack of sufficient datable material in the cores limited the application of this strategy. How- ever, radiocarbon dates did provide critical information on migrational rates where encountered (Robbin's Rest, Long Cove and Watch Hill East; Fig.2). Lagoonal deposits were dated at greater than 1000 yrs B.P. at relatively shallow depths (1.5--2.5 m below ground surface); these data indicate a slow rate of migration for at least these sections of Fire Island. This was particularly well illustrated by Watch Hill East Core No. 6, taken on the island's bayshore. A bay-bottom mud encountered at 1.5 m depth was dated at 1140-+ 70 yrs B.P. The depth of Great South Bay ranges from 1.2 to 3 m, therefore, essen- tially no landward island migration had taken place in this section of Fire Island over the past 1000 years. If Fire Island is indeed migrating landward relatively slowly, the effects of sea-level rise (erosion and submergence) should be readily observable. Quan- titative measurement of historical shoreline change (1830s--1979) confirms that both shorelines (ocean and bay) are characterized by a recessional trend (Leatherman, 1983). Wave activity in Great South Bay is causing marsh scarping, followed by submergence of the gently sloping barrier flats. At the Sunken Forest (Fig.2), tree stumps observed 10 m offshore in the bay provide evidence for drowning of the forest by submergence of the backbarrier environment. The 200 year old age of some trees reported by Sirkin (1972) indicates that the Sunken Forest area must have been stable and protected for at least that length of time. A rough time line could be drawn through stratigraphically equivalent peats in the Long Cove transect (Cores 2, 3 and 4; Fig.8). Radiometric analysis of peat deposits showed a general older to younger trend from bay to ocean, which could be interpreted as a submergence sequence. If the barrier were experiencing little or no landward migration, then sea-level rise would result in rapid submergence of backbarrier flats. Therefore, the salt marsh would be displaced seaward, accounting for younger peat dates being found toward the barrier center. Jarrett (1982), using C&GS maps, documented and quantified this occurrence along the Outer Banks of North Carolina. The stratigraphic information indicated that over at least the past 1000 years, western Fire Island has not experienced landward barrier migration in a continuous fashion. Displacement has been a sporadic and site-specific phenomenon. 0043593

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Physiographic features and associated vegetative communities clearly indicate east--west differences in barrier behavior along this chain. The barrier sections are being affected differently due to sediment sources and bay characteristics. It appears that as the deltaic inner-shelf shoals began supply- ing additional sediment to the littoral drift system, the island section west of Old Inlet/Smith's Inlet started to migrate more slowly than the barriers further to the east. Hurricanes also tend to cross over the eastern barriers (especially around Moriches Bay) so that they are subject to much more severe storm damage. As a result, more inlets (which are chiefly responsible for landward migration) have historically formed across the eastern half of this barrier section. Therefore, the eastern barriers have migrated landward more rapidly than western Fire Island.

CONCLUSIONS

This geomorphic and stratigraphic analysis of Fire Island, New York, has led to the following conclusions: (1) There are marked east--west differences in physiographic and vegetative features along this barrier chain. These differences appear to be related to relative barrier stability and hence to the rate of landward barrier migration. (2) Overwash events along Fire Island seldom result in sediment transport as far as the island's bayshore. Thus, washover deposits appear to contribute primarily to island migration by increasing the barrier's elevation in associa- tion with dune-building processes. (3) Inlet processes are principally responsible for providing sediment to Fire Island's bayshore, causing a widening of the island at former inlet locations and thereby promoting landward barrier migration. Historical records and relict inlet features indicate that as much as 85% of the area has been affected by inlet activity. (4) Western Fire Island has experienced shoreface erosion and bayshore erosion and submergence over the past 1000 years. Landward migration of the island during this period appears to have been a very slow and sporadic phenomenon, occurring in a quantum fashion at inlet sites. (5) The eastern section of the barrier chain appears to have been more dynamic -rid to have experienced a more rapid rate of migration than western Fire Island, resulting in a seaward bulge of the stretch of island between Democrat Point and Long Cove. (6) The trend of long-term landward barrier migration, common to other U.S. East Coast barriers due to eustatic sea-level rise, is applicable to Long Island's south shore barriers.

ACKNOWLEDGEMENTS

This research was supported by contract from the National Park Service, Boston, Mass. Much of the data presented here was derived from two unpub- lished masters theses in geology by Ms. Danielle Joneja and Ms. Cheryl 0043594

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Johnson, University of Massachusetts-Amherst, under the direction of the author. Critical reviews of this manuscript by Dr. James Allen and Dr. George F. Oertel are gratefully acknowledged.

REFERENCES

Art, H.W., 1976. Ecological studies of the Sunken Forest, Fire Island National Seashore, New York. National Park Service Scientific Monograph No. 7, 237 pp. Fisher, J.J., 1967. Development pattern of relict beach ridges, Outer Banks barrier chain. North Carolina. Ph.D. Diss., University of North Carolina, 250 pp. Friedman, G.M., 1967. Dynamic processes and statistical parameters compared for size- frequency distributions of beach and river sands. J. Sediment. Petrol., 37: 327--354. Jarrett, J.T., 1982. Changes in Hatteras Island, North Carolina, since the mid-19th century. Shore Beach, 50: 4--10. John, C.J., 1977. Internal sedimentary structures, vertical stratigraphic sequences, and grain size parameter variations in a transgressive coastal barrier complex: the Atlantic Coast of Delaware. Ph.D. Diss., University of Delaware, Newark, Del., 287 pp. Johnson, C.E., 1982. Historic and geomorphic evidence of barrier dynamics and the origin of the Sunken Forest, south shore of Long Island, New York. M.S. Thesis, University of Massachusetts, Amherst, Mass., 190 pp. Joneja, D.C., 1981. Sedimentary dynamics of Fire Island, New York. M.S. Thesis, Univer- sity of Massachusetts, Amherst, Mass., 174 pp. Kraft, J.C., 1971. Sedimentary facies patterns and geologic history of a Holocene marine transgression. Geol. Soc. Am. Bull., 82: 2131--2158. Kumar, N. and Sanders, J.E., 1974. Inlet sequence: a vertical succession of sedimentary structures and textures created by the lateral migration of tidal inlets. Sedimentology, 21: 491--532. Lanesky, D.E., Logan, B.W., Brown, R.G. and Hine, A.C., 1979. A new approach to portable vibracoring under water and on land. J. Sediment. Petrol., 49: 654--657. Leatherman, S.P., 1979. Barrier Dynamics: Nauset Spit, Massachusetts. In: S.P. Leatherman (Editor), Environmental Geologic Guide to Cape Cod National Seashore. SEPM-ES publ., pp.155--169. Leatherman, S.P., 1983. Barrier dynamics and landward migration with Holocene sea-level rise. Nature, 301: 415--417. Leatherman, S.P. and Joneja, D.C., 1980. Geomorphic analysis of south shore barriers, Long Island, New York, phase I. National Park Service Cooperative Research Unit, University of Massachusetts at Amherst, Amherst, Mass., 163 pp. Leatherman, S.P. and Williams, A.T., 1977. Lateral textural grading in overwash sediments. Earth Surface Proc., 2: 333--341. Leatherman, S.P., Williams, A.T. and Fisher, J.S., 1977. Overwash sedimentation associated with a large-scale northeaster. Mar. Geol., 24: 109--121. McCormick, C.L. and Toscano, M.A., 1981. Origin of the barrier island system of Long Island, N.Y. Northeast. Geol., 3: 230--234. McCormick, J. and Associates, Inc., 1975. Environmental inventory of the Fire Island National Seashore and the William Floyd Estate, Suffolk County, New York. National Park Service, Denver Service Center, Denver, Colo., 461 pp. McKinney, T.F. and Friedman, G.M., 1970. Continental shelf sediments of Long Island, New York. J. Sediment. Petrol., 40: 213--248. Nie, N., Hull, C.H., Jennings, J.G., Steinbrenner, K. and Bent, D.E., 1970. Statistical Package for the Social Sciences. McGraw-Hill, New York, N.Y., 2nd ed., 467 pp. Panuzio, F.L., 1969. The Atlantic Coast of Long Island. Proc. 1 lth Conference on Coastal Engineering, ASCE, pp. 1222--1241. Sirkin, L.A., 1972. Origin and history of maple bog in the Sunken Forest, Fire Island, New York. Bull. Torrey Bot. Club, 99: 131--135. 0043595

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Taney, N.E., 1961. Geomorphology of the south shore of Long Island, New York. Tech. Mem. No. 128, Beach Erosion Board, Corps of Engineers, 50 pp. U.S. Army Corps of Engineers, 1977. Final environmental impact statement for Fire Island Inlet to Montauk Point, New York, Beach Erosion Control and Hurricane Protection Project. New York District, various numbered pages. Visher, G.S., 1969. Grain size distributions and depositional processes. J. Sediment. Petrol., 39: 1074--1106. Williams, S.J., 1976. Geomorphology, shallow subbottom structure, and sediments of the Atlantic inner continental shelf off Long Island, New York. Tech. Pap. No. 76-2, U.S. Army Corps of Engineers, Coastal Engineering Research Center, 123 pp.