Four Coastal Geomorphic Types
The New England and Mid-Atlantic region is composed of four general coastal geomorphologic types: 1) rocky coasts;
2) bluffs with narrow fronting beaches; 3) mainland beaches, which are considered to be beaches connected to the mainland,
whether fronting bluffs, dunes, or extensive marshes; and 4) barrier beaches (which for this report include both islands
and spits).
Figures 1 and 2 shows the regional distribution of these features and highlights the diversity of landform type
along the New England and Mid-Atlantic coast. Long, linear barrier beaches, with associated lagoons, inlets, and spits,
are most common in the Mid-Atlantic, whereas the rocky coasts and the majority of the bluffed coasts are in New England.
|
Shoreline types along the New England and Mid-Atlantic coasts.
|
Shoreline Type
|
Length (km) |
Percent |
| Rocky coast |
211 |
15 |
| Bluffs with linear beaches |
104 |
7 |
| Mainland beaches |
340 |
23 |
| Barrier beaches |
805 |
55 |
| Total length |
1460 |
100 |
|
The table at left provides the relative distribution of shore type in New England and the Mid-Atlantic, which is dominated by
linear or mainland beaches (78 percent), followed by rocky coasts (15 percent) and bluffs with fronting beaches
(7 percent). A schematic
diagram of different coastal features found along the New England and Mid-Atlantic coasts is shown in Figure 3 and photographic examples of each type of coastal geomorphology are shown in Figures 4a-4d.
Coastal Landforms of New England
Coastal landforms of New England (for this report, defined as extending from central Maine through Rhode Island) are
paraglacial (Forbes and Syvitski, 1994) and include fjords, rocky headlands, periglacial deposits such as moraines
and drumlins, and sand and gravel beaches derived from outwash and other glacial deposits. Glaciogenic sediment,
which can range widely in size, is dominant in paraglacial coasts. As a result, mixed-sediment beaches derived
from these materials are composed of a wide range of sediment sizes, and are commonly composed of sand and cobble-to-boulder-sized material.
Barrier beaches in New England are typically small and discontinuous as a result of localized and limited sediment
sources and partitioning of the coast by headlands (FitzGerald and others, 1994; van Heteren and others, 1998). Small
pocket beaches, bound by rocky headlands, occur in areas that lack a substantial sediment source and sufficient lowland
area to migrate or grow as a barrier spit. Beaches in New England are primarily transgressive, with barrier-spitbeaches
exhibiting evidence of overwash and peat exposure in the intertidal zone. With no room to transgress, pocket beaches tend
to be composed of lag sediments dominated by larger sizes (gravel and larger), as sand tends to be lost from the system during large storms.
Rocky and bluffed portions of the coast comprise outcroppings of crystalline bedrock and glacial deposits, respectively.
Rocky areas of the coastline form headlands that protect and isolate pocket beaches and are relatively stable coastal
features. Coastal bluffs in New England and northeastern Long Island are the result of the erosion of higher relief
landforms, such as drumlins and moraines, during thousands of years of sea-level rise. Eroded bluff sediment forms
narrow beaches in front of eroding bluffs. Coarser material may form a lag deposit that remains after finer sediments
are transported away, marking the approximate former position of the bluff (Kelley, 2004).
Coastal Landforms of the Mid-Atlantic
The most common coastal landforms in the Mid-Atlantic are barrier islands and barrier spits, with fewer occurrences of
mainland beaches. A small stretch of high glacial bluffs with fronting beaches occurs at the eastern terminus
of Long Island. The shape and morphology of barrier beaches are a function of the wave energy and direction, and tidal
range (Hayes, 1979). Long, linear barrier beaches form in microtidal environments where storm processes, such as overwash
and breaching, dominate. Inlets are widely spaced along coast and have small ebb-tide deltas compared to flood-tide deltas.
The relatively low tidal flow in microtidal settings is not sufficiently strong to maintain an opening if an island breaches
during a storm. Flood currents through established inlets tend to be much stronger than return (ebb) flows.
Short, wide barrier islands are also common in the Mid-Atlantic region and are associated with a mixed-energy environment in
which both storm processes and a relatively large tidal range work together to shape the islands and maintain the inlets
(Hayes, 1979). Stronger tidal currents develop within island breaches, allowing storm-created channels to remain open. Large
ebb deltas commonly form on the seaward side of mixed-energy barriers. The ebb shoals can influence patterns of wave approach,
leading to higher rates of erosion adjacent to the inlets and transportation and deposition of sediment along the downcoast
portion of the island. As a result, the islands narrow on one end and widen on the other, forming what is referred to as a
"drumstick" barrier (Hayes, 1979; Davis, 1994).
Estuaries and Lagoons in the New England and Mid-Atlantic Areas
According to a classification by Healy and Kirk (1982), there are four types of estuaries, including (1) fiords, which are
flooded valleys of glacial origin; (2) drowned river valleys; (3) barrier-enclosed estuarine lagoons composed of barrier
islands and spits that enclose or partially enclose a shallow lagoon; and (4) structurally induced estuaries. These categories
are not independent or mutually exclusive; rivers and glaciers may follow structurally weak zones that are subsequently flooded
as sea level rises. Most of the large estuaries in New England and the Mid-Atlantic are fiords and drowned river valleys,
respectively, and are perpendicular or at a high angle to the shoreline. Some of the larger estuaries include Buzzards Bay,
Narragansett Bay, Delaware Bay, and Chesapeake Bay. Narragansett Bay is the largest estuary in New England and Chesapeake Bay is
the largest estuary in the United States.
Lagoon systems occur throughout New England and the Mid-Atlantic coast. Some of the larger systems include Great South Bay,
Barnegat Bay, and Chincoteague Bay. Many of the narrow coastal lagoons, especially in the Mid-Atlantic, are formed from flooded
smaller rivers and streams that flowed parallel to shore.
Estuary and lagoon environments are important coastal ecosystems and occur along much of the Atlantic Seaboard. They serve as
sediment sinks, preventing material from reaching the Atlantic littoral system. The accumulation of continental sediments in
estuaries provides the substrate upon which extensive marsh systems develop. Estuary and lagoon shorelines are not evaluated
in this report because of the focus on open-ocean coasts.
References
Davis, R.A., Jr., 1994, Barrier island systems - A geologic overview, in Davis, R.A., Jr., ed., Geology of Holocene barrier island systems: New York, Springer-Verlag, p. 435-456.
FitzGerald, D.M., Rosen, P.S., and van Heteren, S., 1994, New England barriers, in Davis, R.A., Jr., ed., Geology of Holocene barrier island systems: Berlin, Springer-Verlag,
p. 305-394.
Forbes, D.L., and Syvitski, J.P.M., 1994, Paraglacial coasts, in Carter, R.W.G., and Woodroffe, C.D., eds., Coastal evolution - Late Quaternary shoreline morphodynamics: Cambridge, University Press, p. 373-424.
Hayes, M.O., 1979, Barrier island morphology as a function of tidal and wave regime, in Leatherman, S.P., ed., Barrier islands from the Gulf of St. Lawrence to the Gulf of Mexico: New York, Academic Press, p. 211-236.
Healy, T.R., and Kirk, R.M., 1982, Coasts, in Soons, J., and Selby, M., eds., Landforms of New Zealand: Auckland, Longman-Paul, p. 80-104.
Kelley, J.T., 2004, Coastal bluffs of New England, in Hampton, M.A., and Griggs, G.B., eds., Formation, evolution, and stability of coastal cliffs - Status and trends: U.S. Geological Survey Professional Paper 1693, p. 95-105.
van Heteren, S., Fitzgerald, D.M., McKinlay, P.A., and Buynevich, I.V., 1998, Radar facies of paraglacial barrier systems: Coastal New England, U.S.A.: Sedimentology,
v. 45, p. 181-200.
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| Figure 1: Map of the New England coast showing the geomorphic provinces and the general distribution of coastal landforms. USGS Image. |
| Figure 2: Map of the Mid-Atlantic coast showing the geomorphic provinces and the general distribution of coastal landforms. USGS Image. |
| Figure 3: Schematic diagram showing common coastal landforms of New England and the Mid-Atlantic. USGS Image. |
| Figures 4a - 4d: Oblique aerial photographs showing the various geomorphic shore types along the New England and Mid-Atlantic coasts: (A) pocket beach, Cape Neddick, Maine; (B) bluff with linear beach, Colony Beach, Massachusetts; (C) mainland beach, Hither Hills Beach, New York; and (D) barrier island beach, Assateague Island, Maryland. |
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