ORIGINS AND MAINTENANCE OF BARRIER ISLANDS AND CAPES
Most of the National Seashores have been established on barrier islands and outer capes, except for Cape Cod, which is mostly an eroding headland with barrier spits attached. Barrier islands are low strips of sand parallel to the shore, usually with broad salt marshes and estuaries behind them. Some distance back from the beach are lines of dunes, which may form irregularly or in specific patterns. Shrublands and grassland are the typical vegetation, although woodlands may grow up where the land is high enough or back far enough from the beach. It is natural for high storm waves to break across certain islands, sometimes flooding most of their area.
Geologists have argued heatedly about the origin of the barrier beaches. Modern stratigraphic evidence has recently repudiated the theories of Johnson (1919), which held that the islands were formed as the ocean pushed up ridges of sand off the sea bottom, with new islands continually forming offshore. Two other basic theories are presently being debated. Fisher (1962), and others, proposed that the barrier islands began as spits downdrift from eroding headlands. As the rising sea battered glacial deposits and sedimentary headlands, the littoral currents laid out the eroded sand in ever-lengthening spits. Then storms drove inlets through the spits at narrow places and cut them up into barrier islands (Fig. 4). The best evidence for this theory is that one can see all these processes going on today. Chapman (1960) and Redfield (1965) showed that marsh deposits progress in age from one end of a barrier spit to the other, with the youngest material behind the downdrift end. Flying over barrier islands, one often sees old dune lines, standing out because of their dark cover of trees, in parallel curves that follow spit growth (Fig. 77).
A second theory, proposed by Hoyt (1967) and others, maintains that the present barrier system formed during approximately the last 5000 years when Holocene sea-level rise slowed down somewhat. During this period, dune ridges had a chance to build up along a shoreline that was some distance seaward of the present coast, depending on the slope of the coastal plain. The rising sea then isolated the dune ridges from the mainland and lagoons formed behind them (Figs. 5 and 6). Continuing sea-level rise resulted in a general retreat of these islands and their associated marshes. During periods of no change or drop in sea-level, the beaches would build seaward and dune lines would form behind the beaches as they prograded. Another rise of sea level would result in retreat. There is ample evidence to support this theory. Mainland sediments deposited before the last glaciation extend under the lagoons to the barrier beach; stumps of trees have been found in the lagoons as well. On the seaward side, salt-marsh peat and the stumps of old forests may occasionally be seen on the beach at low tide. The shells of typically estuarine mollusks are buried deep in the sediments behind the barrier system; one would expect to find such shells only near the surface if the lagoons had been sea bottom as proposed by Johnson (1919).
It appears that the spit theory is probably the major means by which barrier islands formed north of the glacial boundary, where plenty of easily eroded gravel and sand must have been left in the glacial moraines and headlands. The submergence of dune ridges probably was the major way in which the southern coastal islands, most notably the Sea Islands, were formed. The Outer Banks probably represent a combination of the two, with submergence being the primary process. The formation of spits is also readily seen on the Outer Banks, and one whole island, Shackleford Banks, seems to have been built mainly by spit growth.
The origins of major capes are tied to barrier-island formation. Dolan and Ferm (1968) observed that the capes along the East Coast fit nicely into the eddy patterns of ocean currents; sand carried by littoral drift is dropped wherever the eddies border one another. Hoyt and Henry (1971), however, believed that the capes represent eroded fluvial deposits and that their locations correspond to the mouths of major river systems that flowed across the Pleistocene coastal plain. These deposits were then pushed back by the rising sea, and joined dune ridges which formed along the shoreline (Figs. 7 and 8). Pierce and Colquhoun (1970) proposed that the Outer Banks have changed position since their original formation, with certain portions once further seaward and others behind the present islands. They hypothesize that Cape Hatteras and Cape Lookout were originally both further north of their present positions and migrated south. Schwartz (1971) took the middle ground and proposed that the differing hypotheses reflect a "multiple causality" of barrier-island formation; he presents a classification scheme that incorporates these various ideas: 1. Primary I. Engulfed beach ridges; II. Secondary 1. Breached spits, 2. Emergent offshore barsa. sea-level rise, b. sea-level fall; III. Composite.
Discussion of these formation patterns will no doubt go on for some time. Of primary importance to coastal management, however, is the fact that the shorelines have changed dramatically during the last several thousand years, that they are changing today, and that they will continue to do so. Sea-level rise has resulted in a worldwide pattern of shoreline recession, or erosion. Such a general recession is likely to continue as long as the sea continues to rise. Indeed, it is absurd to expect the "natural ecology" of these islands to be the same today as it was sometime in the past. Barrier islands are not like the much more stable lands of the interior, such as the Appalachian highlands or Piedmont, where ecosystems have changed little for thousands, even millions, of years. The whole barrier-island system is less than 5000 years old, and any particular surface may be only a few hundred years old. Indeed, some alterations can be measured in decades. This frequent rearrangement itself is a major part of the "natural ecology." Even so, there have been some sort of coastal ecosystems in one place or another for countless eons throughout the rise of higher plants; otherwise, the vegetation could not have adapted to this difficult environment. The ability of these ecosystems to survive the constant physical alterations of their environment is testimony to the long-term, dynamic stability of barrier islands.
Short-term changes in beach widths and profiles have been clearly demonstrated. The fact that beaches grow seaward during the low wave energy regime of the summer and retreat when the waves strengthen in winter and during storms is well known. Dolan (in press) has also shown dramatic short-term alterations in beach widths superimposed upon the major cycles. He describes the movement of "sand waves" along the beach, noting how the beach will build seaward as the crest of a wave goes by, and retreat at that same point when the trough of the wave passes. The beach is thus not a simple straight-line system, but a complex series of undulations.
Shorelines retreat by two basic methods. Where the beach rises to an erodable cliff or very high dunes, the sea will cut an erosion scarp and the land will retreat, with sand being carried away by littoral currents. In these cases where there is a barrier to the movement of high water, there will be a very narrow berm and the high water will often reach the foot of the scarp (Dolan in press). This is the typical pattern of erosion on many shorelines that have been "stabilized" by engineering structures that are now too close to the sea, or those that are geologically high, such as the sea cliffs on Cape Cod and certain natural, retreating dune areas.
Along most of the low barrier islands of the eastern shoreline, and particularly the Outer Banks, there is a second method of retreat where it is not blocked by man-made dikes. In this system, storm winds and high water move sand (overwash) back across the berm, through dune lines, and toward the rear of the barrier island, and often into the lagoon behind. On some low islands this is a yearly event. When the high water retreats, the wind blows more sand back from the beach, if the island lies more or less at right angles to the prevailing winds. When islands lie parallel to the winds, as do Core Banks, sand is moved up and down, or off, the beach. Sand pushed into the interior of the island by overwash supplies material for later dune growth (Fig. 6). In most cases, overwash has been considered destructive because sand is removed from the beach and often appears to damage the surface over which it flows. In actuality, however, overwash is a constructive process which permits the low barrier islands to retreat as a complete system, as long as grassland vegetation is present to interact with the overwash.
Last Updated: 21-Oct-2005