CAPE LOOKOUT Vegetation and Ecological Processes on Shackleford Bank, North Carolina NPS Scientific Monograph No. 6

Geology

The geological origin of Shackleford Bank is a relatively recent one. Milliman and Emery (1968) stated that the sea level was approximately at the present level about 30,000 to 35,000 years ago during the last interglacial stage. Due to the subsequent growth of the glaciers, the mean sea level was lowered to about -130 m at the climax of the Wisconsin regression which occurred about 16,000 years ago. According to Richards (1950), the shoreline was then some 90 miles east of the present beach. Milliman and Emery (1968) suggested that Holocene transgression probably began about 14,000 years ago. For the next 7000 years the sea level rose rapidly as the ice sheets melted. Thereafter, the rise was more gradual and the shoreline advanced to its present position. From the evidence of old sod and stumps along the coast, Richards (1950) believed that the sea level is probably still rising but at an almost imperceptible rate.

Johnson (1919) proposed a classical theory concerning the formation of the Outer Banks in terms of two stages. In the early stage, sea-water encroachment eroded the sea bottom by means of onshore waves. These eroded materials were deposited locally to form offshore submarine sand bars. Subsequently, through deposition of materials by longshore currents, a chain of barrier islands was built, largely of loose sand and, of course, totally barren on emergence. The theory has been questioned by recent coastal geologists. Fisher (1967) discussed in detail the arguments against the submarine origin of the Outer Banks. However, Johnson (1956) and Shepard (1963) indicated the possibility of an offshore supply of beach sand due to wave action.

The beach erosion study (U.S. Congress 1948) reported that 6 miles of the shoreline of Shackleford Bank has retrograded an average of 1.55 ft (45 cm) per year and another mile has prograded 0.24 ft (7 cm) per year. Engels (1952), from the comparison of the U.S. Coast and Geodetic Survey maps of 1851-53 and 1949, discerned that the ocean beach of Shackleford Bank has moved northward (landward) from 40 to 150 yards (37-137 m) along virtually its entire length. However, the inner beach did not have a comparable landward movement as Johnson (1919) had proposed. Similarly, Pierce (1964), from a comparison of the U.S. Coast and Geodetic Survey charts of Core Banks from 1856 to 1957, found a general shoreward movement of the coastline and he confirmed such long-term coastal changes with data obtained from borings. He also discovered that most of the sediment supplied to the barrier islands is coming from the north as longshore drift.

Recently, a new hypothesis concerning the origin of the Outer Banks was proposed by Fisher (1967). The Outer Banks are considered to have developed primarily as distal prograding spits beginning 5000-7000 years ago when the sea level was stillstand. Since that time, the sea level has fluctuated no more than 6 m around mean high tide. The fluctuation even diminished to less than 1 m about 3750 years ago. The migration and growth of these coast-parallel spits were the result of a dominant longshore movement from north to south and the reworking of the near-shore bottom material. No new offshore material is now available and barriers are undergoing worldwide erosion. Fisher (1967) further suggested that the age of Cape Hatteras is about 1200 years and the age of the Outer Banks is probably double that, i.e., about 2400 years. Since this recent hypothesis is based on intensive field studies, it probably represents a more accurate account of the formation of the Outer Banks.

Recent History

During the past century, Shackleford Bank has received more than its share of both biological and physical abuse. The island was almost completely covered by forest until at least 1853, according to the U.S. Coast and Geodetic Survey map. It was quite suitable for human habitation and hence was one of the earliest settled areas on the Outer Banks. According to Lewis (1917), former inhabitants remembered that it was "possible to sit in a tree and cast a fishing line into the water," demonstrating the closeness of the forest to the ocean. According to Stick (1958), the first permanent house was built in the 1760s. By 1885, about 600 people lived on the island. There were 500 residents of Diamond City, which is situated at the eastern end of the Bank near Cape Lookout on Core Banks. The population made its livelihood primarily from whaling and fishing. However, due to the intrinsic nature of human inhabitancy, such as cutting trees for homes and ship building, burning for cultivation, and raising livestock, the once luxuriant woodland vegetation gradually degenerated and dwindled away.

In August 1899, a severe hurricane struck the island. The sea level rose so high that the sea covered most of the island and killed most of the remaining forest. Having found the island to be unsafe, the islanders emigrated and most of them settled on Harkers Island across Back Sound. Within 3 years, Shackleford Bank was completely deserted. Because of the lack of vegetational cover, successive storms moved the loose sand from the ocean side across the island. As the sand moved, it covered dead and live trees in its path, and only dead trees remained when the sand dunes moved on. Lewis (1917) estimated that the sand wall was advancing at a rate of 4-12 ft (1.2-4.0 m) per year. Uncovered tree skeletons make up a unique scenery—"ghost trees" or "graveyard forest"—on the Outer Banks. Today, there are no permanent residents and less than 5% of the island is covered by forest.

Physiography

Although Shackleford Bank scarcely exceeds 2000 acres (810 ha), considerable diversity in plant habitats can be found—from the driest dunes to the wettest fresh marshes and ponds—depending on the closeness of ground surface to the water table. Nevertheless, only three major types of vegetation are here recognized: the woodland, the grassland, and the marshland.

Plate I. (1) Outer beach: wide and barren, with abundant shell fragments; (2) Embryonic dunes developed on newly deposited land at the western end of the island; (3) Heavy salt spray moistening the leaves of sea oats at the windward side of fore dunes (visible as reflective white area on leaves); (4) Massive root system of sea oats is an excellent sand binder; (5) Sorting effect of wind on sand particles. Notice darker colored (coarser) particles in the trough and white (finer) particles on the slopes of the fore dunes; (6) Uncovered formerly buried forest, south of Mullet Pond; (7) Grassy dunes on the western half of the island; (8) "Ghost trees"—remaining skeletons of red cedar.

Plate II. (9) A dune marsh (formed by "blow-outs") during dry period; (10) Same dune marsh with a temporary freshwater pond, formed after prolonged rain; (11) Sand wall, a contact zone of the grassy dunes and maritime forest, is actually the lee slopes of the rear dunes; (12) Another view of the sand wall; (13) Goats, the most destructive mammal on the island, roaming along the sand wall; (14) Sheep grazing on the back of the rear dunes; (15) Animal path extending from the sand wall to the forest. The activities of goats and sheep apparently accelerate sand invasion to the forest; (16) Inner beach at low tide.

Plate III. (17) Maritime forest, with Spanish moss hanging on the branches of live oak; (18) Another view of maritime forest (the picture is orientated sideways); (19) A flag-form red cedar, caused by wind-borne salt spray as well as sand blasting; (20) A wet thicket behind rear dunes, with abundant climbing vines; (21) Juniperus-Myrica scrub flat, a successional stage leading to maritime forest; (22) Inner beach. At this particular location, 10 ft of maritime forest have been undercut by wave erosion in the winter of 1967-1968. The standing dead tree in the distance was killed by sea water; (23) A stand of Juncus roemerianus on the grassy sand flat; (24) The winter aspect of grassy sand flat with Spartina patens as dominant. Cows graze on the grasses in the distance.

Plate IV. (25) Mullet Pond, the largest freshwater pond on the island, with Typha marsh on the far side and Hydrocotyle bonariensis covering the foreground; (26) Dune marsh in close-up; (27) Cladium stand of a freshwater marsh; (28) A wet depression on grassy sand flat with many hydrophytic species; (29) Horses grazing on salt marsh (Spartina alterniflora) at the sound side of the island; (30) Creek marsh, with Distichlis spicata at the right foreground, Spartina alterniflora on left midground, and Juncus roemerianus in front of maritime forest in the distance.

The general features of the vegetation and topography of the island are illustrated by two transects in Fig. 2.

Fig. 2. Transects showing general physiography of Shackleford Bank.

At the western third of the island, as depicted in the upper transect, the sequence of physiographic pictures from ocean to sound is: a wide and bare outer beach (Pl. I-1); fore dunes; occasional patches of grassy flat; dunes proper (Pl. I-7); rear dunes; a narrow belt of marsh-like depressions; maritime forest (Pl. III-17, 18) with intra- or inter-forest marshes; and a narrow inner beach (Pl. II-16). There is generally a fairly sharp transition from one type of vegetation to another.

The outer beach and possibly the fore dunes are subject to occasional inundation during storms. Behind the fore dunes, occasionally one can find small depression areas called "slacks" or "swales" where the ground water approaches the surface of the sand and often forms a temporary shallow pond after heavy rains. Storm waves also find their way between the fore dunes to the rear "slacks," which are flooded in times of heavy storms.

The dunes proper are a series of irregular grassy ridges running soundward (northward) with increasing elevation, primarily colonized by sea oats. Here and there appear "ghost trees" (Pl. I-8)—the uncovered skeletons of red cedar. The dunes, under the constant impact of wind, are continually changing their shape, particularly in areas lacking vegetational cover. Hollows or "blow-outs" are produced by wind erosion and some of these depressions become dune marshes (Pl. IV-26) while others are just exposed, bare sand flats. As a result of heavy rainfall, the hollows become temporary ponds (Pl. II-9, 10). The topography becomes more stable as one approaches the rear dunes.

The highest land on the island is represented by the rear dunes, usually about 30-40 ft (10-13 m) above sea level. The soundward slope of the rear dunes is called the "sand wall" (Pl. II-11, 12), occasionally possessing a 20-30° inclination on some steeper slopes. The sand wall is slowly advancing in a few places, mainly along the larger animal trails (Pl. II-15) where tongues of sand advance from the sand wall to the forest. Behind the rear dunes in this portion of the island, there is a marshy belt about 10-30 ft (3-10 m) wide.

The rest of this section is covered partly by woodland vegetation, varying from very dense maritime forest to dwarf dry thickets, and partly by fresh marshes (Pl. IV-25), brackish marshes, and creek marshes (Pl. IV-30). Though the original forest cover of the island has been largely destroyed, the forest remnant confined here represents a good example of the distinctive maritime forest of the southern Atlantic coast. Marshes are distributed between thickets or on the soundward side of the forest. Also, wherever the ground surface approaches or is lower than the water table inside the forest, wet thicket (Pl. III-20) or fresh marsh (Pl. IV-27) is formed. As a result, maritime forest, thickets, and marshes compose an intricate vegetational pattern.

The inner beach is much narrower than the outer beach. Although the water of Back Sound is generally calm during the summer, the north to northeastern winds in winter create forceful waves which undercut the beach at places and wash out dead trees killed by salt water (Pl. III-22). This erosion process apparently occurs every winter and consequently produces an unstable inner beach in certain locations. During the winter of 1967-68, I observed that 10 ft (3 m) of maritime forest at one location were lost through wave erosion.

Another area deserving specific mention is the western end of the island to which 150 acres (60 ha) of land have been added in the past 10 years (1956-66). The newly added portion is basically a loose, sandy waste but small dunes have recently appeared (Pl. I-2).

The physiographic appearance of the eastern two-thirds of the island is much simpler, as shown in the lower transect of Fig. 2. Here, salt marsh (Pl. IV-29) substitutes for inner beach bordering the sound. A slightly elevated, grassy sand flat (Pl. III-23, 24) or meadow occupies most of the area instead of woodland or dunes, and low grassy dunes less than 10 ft (3 m) in height rise along the ocean front. Scattered on the grassy flat behind the low fore dunes are a few shallow depressed areas (Pl. IV-28) on which numerous hydrophytic species thrive. Except for several stumps in one small area, all traces of the previous forest have vanished from the eastern two-thirds of the island.