Coastal Environments

Our national parks contain diverse coastal environments: high-energy rocky shorelines of Acadia National Park in Maine, quiet lagoons within War in the Pacific National Historic Park in Guam, and the white sandy beaches of Gulf Islands National Seashore in Mississippi and Florida. In general, the coastal environment can be defined as that area lying at the interface between land and sea (or other large body of water). It includes both the zone of shallow water within which waves are able to move sediment, and the area landward of this zone, including beaches, cliffs, and coastal dunes, which is affected to some degree by the direct or indirect effects of waves, tides, and currents. While the terms shoreline and beach refer to the area of frequent, or at least occasional, wave action along the edge of the sea or a lake, the coastal environment itself may extend inland for many miles (kilometers).

A variety of factors—including wave energy, tidal range, sediment supply, sediment type, continental-shelf slope and width, and past geologic history (e.g., glaciation, volcanism, and plate movement)—characterize coastal environments.

 

Rocky Coasts

The pounding surf and breaking waves found on rocky coasts have inspired ocean lovers for generations. Erosion characterizes these high-energy environments, which are typically located on active margins with narrow continental shelves (on account of subduction). Rocky coasts may be composed of any rock type (i.e., sedimentary, igneous, or metamorphic) and are usually the site of complex tectonic landforms such as faults, folds, and igneous intrusions and extrusions. Bedrock composition, climate, and wave patterns dictate the profile of rocky coasts. Resistant bedrock combined with high-energy wind and wave activity will create a steep profile, whereas easily erodible rocks in low-energy environments will create a more gradual profile, for example, the high cliffs of Kalaupapa National Historical Park (Hawaii) versus the gently sloping rocky coastline of Dry Tortugas National Park (Florida). In addition, glacial activity may produce steep, rocky coasts through the production of fjords and talus slopes, and carbonate coasts, dominated by skeletal and shelly materials may form eolianite dunes—calcium carbonate beach deposits that are lithified quickly into steep cliffs and bluffs by extreme wind action.

Acadia National Park
Channel Islands National Park
Olympic National Park
Point Reyes National Seashore

 

Geologic Features of Rocky Coasts

Rocky coastlines have many spectacular features. Waves cut arches and sea stacks that jut into the water. Bluffs, cliffs, and terraces form as rock is eroded. Fjords are made when glacial valleys are filled with water when sea level rises.

 

Fjords

Between 1.8 million to 10,000 years ago, Pleistocene glaciers carved steep valleys that were eventually drowned by rising sea levels. The inundated valleys created by glacier movement are called fjords. Kenai Fjords National Park in Alaska provides excellent examples of rocky coasts along fjords.

 

Headlands, Pocket Beaches, and Wave Refraction

Headlands and pocket beaches of Channel Islands National Park in California are distinctively shown in aerial photographs. Waves are refracted around the headlands, increasing erosion at seaward positions on the islands in the park. The protected embayments, where wave action is subdued between headlands, are often transformed into sandy pocket beaches. In lower-energy pocket beaches, longshore transport is not able to carry sediment downshore except during increased wave, wind, and storm activity. Pocket beaches may be ephemeral and change seasonally, or even disappear, on account of increased energy events.

 

Sea Caves

The headlands on rocky coasts are exposed to intense wave, wind, and storm action. Eventually sea caves may form in less resistant, easily erodible bedrock located on promontories. These caves are distinctive environments that are particularly suited for bryozoans, sponges, barnacles, tubeworms, and some species of shade-tolerant red algae. Sea caves are popular with recreational boaters and divers.

 

Sea Arches and Sea Stacks

The constant erosion of rocky headlands may produce a variety of particular geomorphic structures, including sea arches and sea stacks. These isolated remnants of the headland have been detached from the mainland. With prolonged erosion, sea arches may collapse to form sea stacks—steep pillars of rock a short distance from the mainland. Both sea stacks and sea arches are impermanent features that will eventually disappear with continued erosion.

 

Sea Cliffs

The shape and appearance of sea cliffs depends on the stratigraphy, geologic structures, angle of deposition, and lithology of the bedrock material in which the cliffs are formed. Massive rocks, such as granite, will normally erode in a uniform manner, whereas layered sedimentary rocks may erode in a step-wise fashion. Because of increased wave activity found in the midlatitudes, numerous steep cliff slopes exist there, as compared to the high and low latitudes. Na Pali—“sea cliffs”—rise thousands of feet above the peninsula and ocean within Kalaupapa National Historical Park in Hawaii. Recognized as a significant remaining example of sea cliffs in our nation’s natural heritage, this area was designated as the North Shore Cliffs National Natural Landmark in 1972.

 

Tidewater Glaciers

Tidewater or tidal glaciers terminate at the sea or within a fjord. These glaciers are some of the best-studied glaciers because of easy access to their termini or snouts. Depending on climate, topography, and amount of snowfall over time, tidewater glaciers may periodically experience rapid retreat, creating many large icebergs. If icebergs drift into nearby shipping lanes—especially those containing oil tankers, for example, in Alaska—such recession can be catastrophic. Tidewater glaciers are usually bright whitish-blue due to ice density and the tendency of calving (breaking into the sea). Glacier Bay National Park and Preserve in Alaska hosts many tidewater glaciers.

 

Sandy Coasts

Sandy beaches are highly dynamic environments subject to rapid, extreme changes. Typically located on passive margins, in areas characterized by low-wave energy, a wide continental shelf, and high offshore sediment influence, sandy beaches are found in wave-dominated, depositional settings such as the Atlantic Ocean and Gulf of Mexico coasts. Many of our favorite beaches are found on sandy coasts: Cape Cod National Seashore in Massachusetts, Cumberland Island National Seashore in Georgia, Assateague Island National Seashore in Maryland and Virginia, Padre Island National Seashore in Texas, and Gulf Islands National Seashore in Florida and Mississippi. Sandy beaches may be found in any tectonic setting or depositional environment, such as pocket beaches at Channel Islands National Park in California.

Depositional settings along sandy coasts produce barrier structures such as bay barriers, barrier spits, and barrier islands. Coastal barriers are highly complex and dynamic landforms that experience constant change and movement. These narrow strips of sand serve as obstacles to wave activity, protecting fragile environments that lay further inland, for example, marshes, tidal flats, and lagoons.

Assateague Island National Seashore
Cape Cod National Seashore
Gulf Islands National Seashore
Padre Island National Seashore

 

Geologic Features of Sandy Coasts

Depositional processes along coastlines, such as longshore sediment transport, form sandy beaches and create highly complex landforms that experience constant change and movement. Features such as spits, barrier islands, tombolos, and dunes are classic forms in sandy beach environments.

 

Barrier Islands

Barrier islands are one of the most common and distinguishable features of the Atlantic coast. These important environments protect the mainland from storm events and wave action, while providing a vital ecosystem for many species. Features such as sand dunes, maritime forests, inlets, lagoons, back-barrier marshes, and vegetation constitute these fragile coastal systems. Without intervention, barrier islands maintain a state of dynamic equilibrium between sediment exchange, wave energy, and sea-level rise. Human activities may interfere with this balance, causing costly damage, both economically and environmentally.

 

Barrier Spits

Barrier spits are made up of sediments that have been suspended by waves and transported by currents. A barrier spit is the landform resulting from the deposition of sediments in long ridges extending out from coasts. Barrier spits may partially block the mouths of bays. If a spit grows long enough, it can completely cut off a bay from the ocean, forming a lagoon. The spit is then called a bay barrier or a bay-mouth bar. Cape Cod (Massachusetts), which looks like a flexed arm, is a famous example of a spit.

 

Beaches

Although varying in length, width, sand composition, and permanence, beaches are the most well-known coastal feature. A beach is defined as the location along a shoreline where the sediment is in motion, being moved by waves, tides, and currents. The beach is often bounded on the upland side by a cliff, dune, or vegetation. Beaches are very dynamic environments, with coastlines that can change daily, making them challenging places to manage.

 

Dunes

Dunes are critical to the health and sustainability of sandy beaches. The primary dune ridge (foredunes) lies adjacent to the shoreline. Secondary dune fields may lie further inland. Dunes may form anywhere that eolian processes (wind transportation) occur. Dunes provide much-needed protection to back-barrier environments (including human development) against severe wave, wind, and storm events. In addition, these geomorphic features provide critical habitat to a variety of migratory birds and mammals. Dune vegetation is very important for the formation and stabilization of dune complexes on barrier islands. Both the root system and exposed vegetation restrict sand movement around plants, helping to secure the dune.

 

Tombolos

A tombolo is created when sediment connects an offshore landform—such as an island or a sea stack—with the mainland. The tombolo forms because the island or sea stack refracts the waves, causing a zone of slow moving water behind the island. Whenever water carrying sediment slows down, sediment is deposited. The sediments that form tombolos often form in ridges along an underwater wave-cut terrace.

 

Estuaries

The term “estuary” is derived from the Latin word “aestuarium,” which means tidal. In a geomorphic sense, a typical estuary is a semi-enclosed, elongated coastal basin that receives an inflow of both freshwater and saltwater. From a chemical and physical standpoint, estuaries are buffer zones between river (freshwater) and ocean (saltwater) environments that are affected by tidal oscillations. Geologically speaking, most estuaries are young basins, established by the flooding of fluvial (river-eroded) or glacially-scoured valleys during the Holocene rise of sea level. Estuaries are generally short-lived: they are quickly destroyed by rapid sediment infilling that is fostered by the high influx of river sediment. Circulation in estuaries not only traps large amounts of river sediment but also imports sand and mud from offshore areas.

Not all semi-enclosed coastal bodies of water are estuaries. For example, lagoons are protected bodies of water that are little affected by tides. Lagoons may receive inputs of seawater and freshwater but are typically dominated by one or the other, making their water motions less complex than the mixing and circulation patterns associated with true estuaries.

Everglades National Park
Golden Gate National Recreation Area
Glacier Bay National Park and Preserve

 

Deltas

Where rivers provide large quantities of sediment to the shore, estuaries are filled and river sediments are discharged directly into the ocean. If the rate of sediment supply exceeds the rate of sediment removal by waves and tidal currents, a buildup of sediment occurs at river mouths. These deposits, which commonly assume triangular shapes in planar view, are termed deltas because they resemble the Greek capital letter delta (Δ).

In actuality, not all deltas display the classic “delta” form. This characteristic shape develops typically at river mouths, where waves and tides do not influence the amount of sediment supplied by the river. Such systems, exemplified by the Mississippi River delta, are called river-dominated deltas. Waves dominate riverine deposits in coastal areas where wave energy is high. Wave erosion and strong longshore currents disperse the sediment away from the river mouth, producing a relatively straight coast with only slight seaward bulges of the shoreline. In some regions, a large tidal range overshadows river and wave effects, creating tidal-dominated deltas. The strong flood and ebb tidal currents rearrange the river-supplied sediment into long, linear submarine ridges and islands that tend to fan out from the river mouth, creating funnel-shaped basin geometries.

Jean Lafitte National Historical Park and Preserve

 

Tropical Coasts

The same physical processes act on tropical coastlines—to produce either rocky headlands or long, flat sandy shorelines—as in other latitudes. However, in warm tropical waters, colonies of corals form, mostly between 30° north and 30° south latitude. Corals are animals, but they are stationary; their food source is washed to them through continuous water motions. Corals have an associated diverse array of reef fish, which attract snorkelers and scuba divers. As live corals grow atop the skeletons of dead corals, a coral reef is formed.

Because living corals require a minimum sea temperature for growth, they are concentrated in the tropics, as are various calcareous algae that form carbonate encrustations along many tropical shores. They also favor clear waters (they cannot live in deltas or muddy environments) and, in general, depths shallower than 75 feet (23 m), although some species tolerate depths of up to 500 feet (152 m) (Wyckoff 1999).

The most well-known type of reef is a barrier reef, which is built in shallow waters that may deepen through time. These reefs may reach enormous proportions. The Pacific island, Tahiti, is encircled by a barrier reef. The Great Barrier Reef of Australia (actually a composite of some 2,500 small reefs) is about 1,200 miles (1,931 km) long, with a lagoon tens of miles (kilometers) wide.

Buck Island Reef National Monument
Dry Tortugas National Park

 

Geologic Features of Tropical Coasts

Reefs that are exposed above sea level are among the most massive and impressive landforms. Today, such reefs exist as huge, colonies; they also exist as “fossil” relics making up much of the world’s limestone. Among reef formations in the United States is the one that makes up Guadalupe Mountains and Carlsbad Caverns National Parks in Texas and New Mexico, respectively.

Erosion of coastal landforms, especially cliffs, is not generally a significant source of sediment in the tropics where environments with low wave energies are common. This observation is supported by the relative lack of coasts formed of bedrock in the tropics. Even where present, cliffs formed of well-consolidated strata recede slowly and supply little sediment.

 

Lakeshores

The designated lakeshores in the national park system have shorelines and are, therefore, considered coastal and are managed accordingly. Nevertheless, because even the largest lakes are very small compared to oceans, some distinct differences exist, geomorphically speaking. Many lakeshores have much smaller waves and currents. Furthermore, lakes are naturally short-lived: they tend to fill with sediments and to be emptied by streams downcutting their edges. Thus, some lake waves and currents have insufficient time for creating large landforms. Also, lake tides, where they do exist, are generally too slight to provide waves and currents with a wide vertical range to work in (Wyckoff 1999).

Although astronomical lake tides (those caused by Moon–Sun gravitational attraction) are relatively insignificant, other movements of lake water can be substantial. In the event known as a seiche, wind pushes water up against one shore and the water then flows back to the opposite shore, like water in a washtub being rocked. Seiches can be important in the transportation of sediments. Investigators have measured seiches in Lake Erie up to 8 feet (2.4 m) in height. Seiches also can be produced by earthquakes, sudden changes in atmospheric pressure, heavy rains, surges in glacial meltwater from nearby mountains, and variations in water density (Wyckoff 1999).

Waves on large bodies of water such as the Great Lakes shape shores of loose material, building sand barriers (including spits), as well as beaches with scarps, berms, and beach ridges. Storm waves can attack weak bedrock that fringes lakeshores, undercutting them and creating cliffs. In cold regions in winter, expanding lake ice pushes sand and rock fragments up the beach, creating ridges. Waves on the largest lakes, such as Lake Bonneville during the pluvial period, have cut terraces on adjacent highland slopes (Wyckoff 1999).

Lakeshores also exhibit rill marks, swash marks, and ripple marks. On the landward side of beaches, enough sand may be deposited for dunes to form; for example, the spectacular Indiana Dunes, of glacial drift, at the southeastern edge of Lake Michigan.

Sleeping Bear Dunes National Lakeshore
Indiana Dunes National Lakeshore
Apostle Islands National Lakeshore

 

Links

Introduction to Coastal Geology

Coastal National Parks

Coastal Processes

Coastal Materials

Coastal Environments

Challenge Your Understanding

 

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