Beaches occur in what seems to be an endless variety. Some beaches are so soft that your feet sink into the sand when you walk on them, while others are hard enough that you can drive and park your car on them without fear of getting bogged down. Beach colors range from black to yellow to brown to gray to white to green and even red. While sandy beaches change shape with every tide, some beaches are made of boulders so large that they hardly ever move. A beach may go on for miles and miles as an unbroken strip of sand (e.g., 90 Mile Beach, Australia) while others are only a few yards long in the midst of a rocky coastline. Some beaches are fronted by miles of tidal flats at low tide in locations where there is a large tidal range, such as along the Bay of St. Malo, France; Morecambe Bay, England; and the Bay of Fundy, Canada. In contrast, many of the world’s beaches are just a few yards wide. Some that exist at low tide are nonexistent at high tide, especially in front of seawalls, esplanades, or rock cliffs.
Trying to make sense of this endless variety of beaches is the task of the coastal geologist or geomorphologist, and chapters 3, 4, and 5 outline some of the reasons for this variability. The differences between beaches lie in the combinations of many different factors. There are many clear and predictable differences between a beach in the Indian Ocean’s Mauritius and another in Greenland; the most obvious difference is that it is pleasant to swim at one and not the other. In many instances, though, beaches separated by only a few hundred yards are vastly different. A passenger landing at Cardiff Airport in Wales will see a 250 yd (229 m)-wide fine sandy beach at the resort of Barry, right next to a 100 yd (91 m)-wide gray beach composed entirely of fist-size gray cobbles arranged into a series of descending steps like a staircase. The two are separated by only a 100 yd (91 m)-wide rocky headland. Some of the differences between beaches (such as the type of sand) are the result of local factors, while others depend on where a beach is located on the Earth. Beaches differ from one another because of climate (e.g., the tropics vs. the Arctic), geologic setting (e.g., a glaciated region vs. a volcanic island), and oceanographic setting (e.g., a beach facing the open ocean vs. one in a protected embayment). The differences that arise through variation in wave exposure can be seen by comparing the open-ocean beaches in the vicinity of Mar del Plata, Argentina, and the beaches of the nearby Rio de la Plata estuary, or the exposed east coast beaches of Barbados and those on the relatively sheltered west side of the island. Geologists sometimes differentiate leading-edge and trailing-edge coasts (see chapter 2). This categorization distinguishes beaches located on steep (usually rocky) coasts with a high degree of river influence (leading edge) from those on gently sloping coasts (usually with coastal plains), on which the continental shelf supplies sand and rivers play a much-reduced role in sediment supply.
This oblique aerial view near Barry, Wales, illustrates how different two adjacent beaches can be. Sediment transported from the left is trapped against the rocky headland, while the darker sand to the right of the headland comes from a different source in that direction. The beaches differ in both material and grain size, from the light-colored beach of limestone cobbles to the brown, quartz sand beach.
Sand and gravel have been sorted by waves into two distinct sections on the beach at Streedagh, County Sligo, Ireland. Wind continues the sorting process, concentrating fine sand into dunes at the back of the beach.
At its simplest level, a beach is a sedimentary pile of loose grains that are sorted and shaped by waves, tides, and currents that are characteristic of a particular oceanographic setting. The sand, gravel, or boulders on a beach come from a variety of sources, the main ones being adjacent beaches, rivers, glaciers, the seabed, cliff erosion, and human activity. There are, of course, many different types of grains, and they come in an endless variety of shapes, colors, weights, mineralogical compositions, and sizes. Such diverse sediment properties create much of the variation among beaches, but not all of it, because the waves, tides, and currents that sort out these grains are also infinitely variable.
A beach is made up of millions of individual grains, each of which has its own shape, size, and composition. When combined, the size and shape of all the grains on a beach give the beach a distinctive texture. The texture of sediment on a beach is important for many reasons. For example, grain size determines the slope of a beach between the high- and low-tide lines, with larger grains forming steeper slopes. The response of a beach to spilled oil also will vary with beach sand grain size: Very fine sand permits very little penetration of the oil into the beach, while pebbly beaches have lots of spaces between grains, and these rapidly fill with oil. The communities of microscopic organisms that live between the grains of sand similarly vary with the size of the grains that make up their homes.
Grain size gives us our first impression of a beach. Sand is our common expectation, but beaches often are coarser than sand, ranging from pea- and pebble-size gravel to cobble and even boulder beaches. These coarser sediments are held onshore by wave energy, while fine sediments (muds) are held in suspension and ultimately are carried to deeper water. As noted earlier, mud beaches are rare, almost a contradiction to the term beach.
There are exceptions to the no-mud-on-the-beach rule. When a migrating shoreline, often on a barrier island, uncovers a compacted mud or peat layer that once existed in a marsh or riverbank deposit behind the island, the layer appears on the beach. These hardened mud layers may last several years on a beach and are usually taken away by storms eventually. During storms, waves may rip off chunks of the mud and roll them about to form mud balls. The same is true for peat layers, which are usually muddy, the result being peat balls. When a mud ball rolls around on a beach, coarser grains of sand and gravel are plastered onto the clay surface of the mud ball, giving it an armored outer surface that helps to preserve the ball from further erosion (armored mud ball). Outcrops of clay-rich glacial tills, along shorelines in areas that were affected by continental glaciations, are a common source for clay-ball formation as the tills erode. The Gay Head Cliffs of Martha’s Vineyard, Massachusetts, are a well-known source of variegated clay balls that become armored as they roll on the beach. Tar balls are analogous features—residual blobs of tar from oil spills or natural seeps are rolled about and become rounded and armored, like those on the shores of the Gulf of Mexico.
This fine-sand beach on Xefina Island, Mozambique, has a gentle gradient. The building ruin is a World War II lookout bunker that was originally atop the rapidly retreating sandy bluff.
Upper left Mud layers exposed on the beach, Jekyll Island, Georgia, United States. The mud contains the roots of Spartina sp., indicating that it was deposited in a marsh. Marshes exist on the back side of barrier islands such as Jekyll, so in order for the mud layers to be exposed on the beach, the island had to migrate landward over the marsh. Such mud layers break off in chunks, which are then rounded into mud balls. Exposure of the mud layer is in part due to the seawall effect of reflecting wave energy and steepening the beach profile.
Upper right An armored mudball on the Sefton coast, England, that was eroded from Pleistocene deposits exposed on the beach. Small gravel pieces became embedded in the mud as the ball rolled over the beach and are the armor that retards the rate at which the ball is reduced in size. The British fifty-pence coin is shown for scale.
Lower The darker-colored sediment on this North Carolina beach is mud, deposited in shallow depressions. Note the shrinkage cracks in the thin mud deposits (foreground) and the lighter-colored tongues of sand carried by the last round of higher waves that are burying the mud patches (background). This site is adjacent to Ocracoke Inlet, which accounts for the source of the mud.
Tidal flats, the broad, flat platforms between the high- and low-tide lines, can be composed of sediments of varied sizes, but the largest deposits of mud at the shoreline usually are found in such flats or along the edges of lagoons. Examples of mud flats, with varying amounts of sand, include the Wash of eastern England and South America’s Suriname shoreline north of the Amazon River. In contrast, the tidal flats of the North Sea’s German Bight are very sandy. Mud also may be deposited directly on sandy beaches when water from adjacent muddy estuaries is ponded on the beach during high tide and the mud settles out.
Most of us have a good mental image of mud, sand, and gravel, but geologists use a common measurement scale to ensure that everyone knows precisely what is meant by the different terms that describe size. The Udden-Wentworth scale divides grains according to their diameter into several classes, including very fine, fine, medium, and coarse sand, all with precisely defined limits. Distinctions between sizes are made by sieving grains through known screen-mesh sizes, or by timing the fall of grains through a tall, water-filled settling tube, or simply by eyeball comparison of sands, with a card displaying known grain sizes used for reference. Material coarser than sand is divided into granules, pebbles, cobbles, and boulders. Pebble beaches are very common in high latitudes that have been affected by glaciers, which characteristically carry coarse material, and along mountainous coasts. Other examples of pebble beaches are composed of broken coral fragments, commonly found in the tropics.
On some shorelines, different-size grains accumulate in particular areas of the beach. In Ireland, for example, many low, sandy, intertidal beaches change to steep pebble beaches near the high-tide level because the pebbles roll across the sand, rather like bowling balls. On other beaches, grain size might vary laterally along the beach from fine to coarse sediment. On the 18 mi (29 km)-long Chesil Beach in England, the pebbles are much larger in the southeast (potato size cobbles) than in the northwest (pea size granules). Lateral variation like this may occur as longshore currents (discussed later in this chapter) pick up poorly sorted material from a bluff or river mouth and carry the finer-grained material for longer distances than the coarser sediment. Big grains also fragment during transport, leading to fining of grain size in the direction of transport. Some beaches show little sorting of sediments by grain size and simply consist of a poorly sorted mixture of sand and gravel throughout the beach.
To describe the range of grain sizes in a particular beach’s sediment, geologists use a measure called sorting. This measure of uniformity or variability of the grain sizes ranges from very well-sorted sediment, in which nearly all the grains are the same size, to very poorly sorted sediment, which contains a wide range of grain sizes with no particular size being dominant. Sorting is affected both by the size range of the original sand and by the ability of waves, currents, and wind to sort the grains by size, shape, and density. Often the windblown sand at the rear of a beach is particularly well sorted because the wind selects out only fine sand sizes from those available on the beach. This well-sorted windblown sand will usually be finer than the coarser grains left behind on the beach as a lag deposit. Accumulations of very coarse shells and shell fragments are found on the surface of some beaches. These shells often originate as a lag deposit as the finer sand is removed, but the shells then form an “armor” on the surface that prevents additional removal of fine sand by the wind (see chapter 8).
Upper left These basalt boulders making up the beach at Fingal Head, New South Wales, Australia, provide a good example of the boulder size category as well as material derived from a single source in terms of composition. These blocks were eroded from the adjacent headland.
Lower left This gravel beach on Oman’s shore is dominated by pebble-size grains, but a few coarser cobbles are present on the back beach. During times when smaller waves hit the shore, the swash sorts out the sand fraction (the light patches on the lower beach). The beach sediments are derived from material eroded from the adjacent bluffs. Photo courtesy of Miles Hayes.
Upper right Good zonation of size sorting by waves is seen on this beach in Alaska’s Pribilof Islands (St. George). Medium-size boulders are concentrated on the lower beach at the water’s edge, the man is standing on a concentration of pebbles, and the ridge and upper beach are composed of cobbles and some small boulders. An erosional scarp, visible at the back of the beach, is the immediate source for some of the beach sediment, but the ultimate source is the uplands seen in the distant background.
Lower right A beach composed of coral rubble eroded from a coral reef offshore and transported onshore during storms at Beef Island, British Virgin Islands. The skeletal fragments of corals and associated organisms range in size from sand to cobbles.
Table 3.1 Udden-Wentworth Sediment Grain Size Scale
Grains on a beach also vary according to shape. This shape variation in grains is particularly obvious on gravel beaches. Cobbles and pebbles, for example, are commonly divided into four main shape categories by measuring the lengths of their longest, shortest, and intermediate axes. This approach categorizes how closely pebbles resemble spheres, blades, discs, or rollers. Perhaps you have picked up one of these pebbles, flattened and disc-shaped, sometimes called a “worry stone,” the ideal shape for skipping. The different ways in which different-shaped grains interact with waves result in their sorting across the beach. Discs and blades are easily lifted by waves because of their large flat surfaces, so they are often found high on the beach, where they have been thrown by waves. Spheres and rollers are less likely to be lifted in the first place, but they also have the ability to roll back down the beach, so they are more common on the lower beach, close to the low-water mark.
Upper left Beach steepness is controlled by grain size, with coarser material producing steeper beaches. This steep boulder and cobble beach at Annalong, Northern Ireland, consists mainly of rounded boulders (approximately i ft [0.3 m] in diameter) of Mourne granite piled high by storm waves. Author Andrew Cooper is standing just beyond the high-tide line, where the steepness of the beach increases significantly. The dark-stained boulders are the intertidal zone, which is not as steep because the boulders are deposited on a wave-cut platform in the underlying bedrock. The smaller cobbles and pebbles are very well rounded, and some are spherical. The dark line through the middle of the photograph is the wrack line, consisting mostly of seaweed.
Lower left An Antarctic beach dominated by boulder- to cobble-size material, but pebbles and sand-size material are present as well. These fragments of pink granite were deposited by icebergs and have not been extensively reworked and abraded as a result of wave activity, so the rocks are angular and generally not rounded. Contrast this picture to those of gravel beaches where the grains have been rounded by wave action. Photo courtesy of Norma Longo.
Upper right A gravel beach in Patagonia, Argentina, shows a variety of pebble types in terms both of shape and of composition, as reflected by their varied colors. Photo courtesy of Allen Archer.
Lower right An Alaskan beach made up of boulders, cobbles, and sand derived from the Malispina Glacier. These various sizes have not been well sorted by waves; however, the rounding of the edges of the coarser sizes suggest water transport and abrasion.
Simplified Zingg diagram to show some common grain shapes. Pick up an irregularly shaped pebble and imagine that internally it has three perpendicular axes—a long, short, and intermediate-length axis. The shape diagram is based on the relationships between these axes (e.g., in a sphere all three axes are of equal length). The shape categories are discs, blades, rollers, and spheres. Drawing by Charles Pilkey.
Often, the shape of sand grains is initially inherited from the rock texture, the original mineral grain shapes, or the creatures (e.g., shells) from which the grains are derived. Mechanical abrasion then takes over as the shaping agent, rounding edges and corners of grains as they are transported by various stream and ocean currents and waves. Similarly, gravel-size material may inherit shapes from bedding, rock cleavage, or fracture patterns in the mother rock, which produce varied shapes. As they do with sand, the processes to which the rock fragments are subjected usually will impose the final shape. Slates, for example, tend to produce flattened rock fragments (shingles), while fractured granites are more likely to produce blocky cobble-size sediments. Abrasion by surf and waves will round the corners and edges of such granite cobbles to produce well-rounded and even spherical cannonball-size stones, well suited to be beach rollers. Shingles and other flat-shaped rocks (discs) often are deposited in an imbricate pattern, lying atop one another like the shingles on a roof. Pieces of animal skeletons that form beach grains also inherit shapes from the original part of the skeleton from which they are derived. For example, sea urchin spines and stick-shaped coral fragments become rollers, while some microscopic foraminifera tests (shells) create nearly spherical sand-size grains.
Besides occurring in various sizes and shapes, the grains on a beach can be made of many different rocks and minerals. The most common beach sediment components are terrigenous (land derived from the erosion of other rocks), volcanic (directly derived from volcanic activity and volcanic rocks, as on many islands), and biogenic (composed of the shells and skeletons of dead marine organisms).
Although volcanic sediments may be land-derived by erosion, their distinction from terrigenous sediments is useful because in many island settings the beach sediment comes directly from volcanic activity or preexisting volcanic rocks. The black sands of Pacific Islands often are sand-size fragments of volcanic glass or fine-grained basaltic rock, sometimes extruded below sea level, as off the big island of Hawaii, for example. Cobble-, pebble-, and sand-size fragments of explosive volcanic ash and rocks (pyroclastics) also are found on island beaches, as in the Caribbean, parts of the Mediterranean, the Pacific Islands, and some Alaskan shores. In the Mediterranean (Greece, for example), volcanic sediments may account for some red-colored beaches. Perhaps the most unusual volcanic beach sediment is pumice, a frothy glass with a fibrous texture that traps air in the rock. The result is a rock that floats, and pebble- to cobble-size pieces of pumice can be transported long distances to wash up on beaches far from the volcanic activity of origin.
The term shingle is applied to cobble beaches where the cobbles are flattened and rest on one another in a layered fashion, as do the cobbles on the upper part of this beach at Kettle Cove, Cape Elizabeth, Maine. The rock is a metamorphic phyllite, which is slaty in character, producing flattened slabs or disc-shaped cobbles when eroded from the outcrop (in the background). These cobbles were tossed onto the upper beach by storm waves.
The range of terrigenous grain types is as wide as the range of rock types on the Earth. By definition, a rock is an assemblage of mineral grains that are bound together as a result of crystallization (e.g., igneous and metamorphic rocks) or cementation (e.g., sedimentary rocks). A mineral is a natural compound with a characteristic chemical composition and crystal structure and characteristic physical properties; examples include the common beach minerals quartz and feldspar.
Most beach clasts comprise pieces of rock that have been rounded by transport to the beach or were derived from eroding sea cliffs or rock ledges on or near the beach. Most gravel-size particles on the beach are fragments of such preexisting rock. Occasionally even sand grains can be rock fragments rather than single mineral grains. Most sand grains, however, are composed of a single mineral.
Rock fragments provide clues as to the origin of the beach sediment. Some pebble and cobble beaches are made up of just one rock type, which is often an indication that the rock is locally derived (e.g., from erosion of a sea cliff or adjacent headland). Other coarse beaches have a wide variety of different rock types, suggesting mixed sources or perhaps a larger, more variable source area. On glaciated coasts, it is common to find pebbles that were carried long distances by glaciers or icebergs and mixed with many other rock types picked up along the way by the ice.
In Ireland, for example, pebbles of a unique rock, a granite containing a distinctive blue mineral known as riebeckite, can be found in beaches all along the east and north coasts. The source rock occurs only on the island of Ailsa Craig in western Scotland, so the occurrence of pebbles and cobbles of this unique riebeckite granite on the beaches of Ireland was important in helping early geologists conclude that glaciers transported the sediment from Scotland to Ireland. With the retreat of the ice sheets, these rocks, together with other, less distinctive rock fragments from all over northern Britain, were incorporated into the beaches.
As noted earlier, geologists categorize beach sediments into terrigenous, volcanic, and biogenic (carbonate) grains, depending on their origin. The composition of continental beaches is often expressed in terms of the ratio of the terrigenous and biogenic components to each other. On some islands such as those in the Caribbean Sea and the Pacific and Indian oceans, the ratio of volcanics to carbonates is useful in understanding the onshore and offshore sources of the beach sediment (e.g., volcanic activity versus reef erosion).
Terrigenous (land-derived) sand grains are the products of rocks that have broken down into individual mineral grains as a result of weathering and mechanical abrasion. Their mineral composition is diverse, but by far the most common type of sand grain in beaches is quartz, a light-colored to translucent mineral composed of silicon dioxide that occurs in igneous granitic rocks, metamorphic rocks such as gneisses and schists, and preexisting sedimentary rocks such as sandstones. Feldspars, another important light-mineral group, are the most abundant rock-forming minerals in the Earth’s crust, but unlike quartz, feldspars are much more variable in composition (they are made up of aluminum silicates with varying amounts of potassium, sodium, and calcium). The feldspars are much less resistant to both chemical weathering and mechanical abrasion than quartz is, so the feldspars are preferentially removed. The result is that feldspar, in spite of its global abundance, is usually much less abundant than quartz in beach sands, although a small feldspar fraction is almost always present in terrigenous sands. Those few beaches that are rich in feldspar tend to be close to the source rock from which the feldspar is derived. For example, glacial deposits derived directly from so-called shield areas (exposures of very ancient rocks, such as the Canadian Shield, composed mainly of feldspar-rich granites and gneisses) produce beaches rich in feldspar. Some of the beaches of Lake Superior in North America are rich in feldspar. The average feldspar content in beaches of the southeastern United States, which are a long way (200 mi [more than 320 km] or more) from their source rocks in the Appalachian Mountains, is only 3 to 4 percent.
Upper left Microscopic view of a few grains of Hawaiian black volcanic sand. The well-rounded, lighter-colored grain is a sand-size rock fragment of basalt, and the black, glossy grains are volcanic glass, which is formed when lava comes in contact with seawater. The red bar scale is 0.04 in (1 mm).
Lower left This very coarse, poorly sorted beach sand from Redondo Beach, California, is a good example of beach sand in which many of the grains are fragments of preexisting rocks rather than grains of individual minerals. Most of these sand-size rock fragments are from a variety of rocks that were fine grained, relatively hard, and resistant to breakdown (e.g., volcanics, sedimentary chert), but individual mineral grains can be seen in a few of the rock fragments. Such diverse assemblages of sand grains are typical of beaches on the leading (mountainous) edges of continents and are characterized as compositionally immature. Grains range from very well rounded to subangular, indicating their history of transport and abrasion, some having traveled a long distance to the beach while others were derived more locally. The polished appearance and roundness also reflect wave abrasion in the beach environment.
Upper right Pebble-size fragments of pumice on an Australian beach (Whitsunday Islands). Pumice is a natural glass produced by volcanic eruptions and has a low specific gravity because of its frothy texture, so much so that pieces of pumice will float on water, allowing currents to carry them far from their source. These pumice grains are somewhat dark in color, but pumice often ranges in color from a lighter gray to white.
Lower right Sand from a Batchawana Bay beach on Michigan’s Lake Superior in North America reveals a history of glacial origins. The sand is compositionally immature, having been derived from the glaciers’ eroding of crystalline igneous and metamorphic rocks (e.g., granites, basalts, and gneisses) of the Canadian Shield. Coarse sand-size rock fragments along with mineral grains of the dominant pink feldspar and white-to-clear quartz make up the sand. Note that the shape of the grains ranges from very angular to a few subrounded grains, and the lighter-colored quartz grains show some yellow-brown staining. This sand has not been subjected to the same degree of abrasion as has the sand in the Redondo Beach, California, example.
Any long time period and great travel distance will result in the quick breakdown of feldspar grains. When feldspars and other silicate minerals weather chemically, they produce clay minerals, which are also clay size—the source of mud. Of the common terrigenous rock-forming minerals, quartz is the most resistant to both chemical weathering and mechanical abrasion. So as other minerals weather to clays or are broken down into silt and clay-size particles, the sand-size fraction becomes richer in quartz. Thus, it is not surprising that quartz is the most common terrigenous beach material. The sparkling white beaches of the Pensacola, Florida, area have very high percentages of quartz sand.
Do not think, however, that all beaches are quartz rich. Most of the world’s leading-edge beaches tend to be dark in color and have a wide range of grain compositions. Pacific Coast beaches of the Americas, from Alaska to Chile, and beaches of the western Pacific, eastern Indian Ocean, and parts of the Mediterranean that are near mountain ranges are often dark gray to dark green because the sand grains are rock fragments of preexisting fine-grained rocks such as metamorphic slates or volcanic rocks such as andesite and basalt.
Sometimes a beach’s color is not the same as the inherent color of the beach’s grains but results from the staining of the grains. The most common color of stained beaches is the familiar light brown or tan: The light-colored grains, such as quartz, feldspar, and even shell fragments, are stained by iron oxide, which gives these grains, and the overall beach, their yellow-brown color. In contrast, the sparkling white beaches near Pensacola, Florida, reflect the true color, or lack thereof, of the dominant quartz grains. Often the iron that stains the sand grains is furnished by the weathering of heavy minerals within the sand. However, such staining may have several different origins: It may have been formed from pore water in the beach; it may have been inherited from the grains’ source (from oxidized soils, for example); or it may have originated while the sediment was on the inner shelf. Curiously, there are few studies of the character of such staining. One detailed study of stained quartz sands from beaches in southeastern Australia revealed (by examination using a scanning electron microscope) that the hydrated iron oxide (goethite) coatings were adhering not directly to the quartz grains, but to a submicroscopic clay coating. An abundance of microorganisms in the sand also can affect beach color (e.g., turning it shades of yellow and green). Perhaps the most extreme case of beach coloring originating from a source other than the sediment is in Brazil, where the water moving through the beach is so charged with iron in solution that it forms the red mineral hematite on exposure to air, forming areas of brilliant-red-colored beach where the water seeps out at the surface.
Upper left This fine-grained sand from a beach near Hiroshima, Japan, is an example of terrigenous sediment. Weathering of the island source rocks probably removed many of the unstable materials, so the resulting sand is dominated by quartz grains. Even the creamy white grains and brown-stained grains are probably quartz, although a few grains that may be feldspar, small rock fragments, and dark heavy minerals are present. Note that most of the grains are angular to subangular, suggesting that this sand has not undergone severe transport abrasion, either before or since arriving on the beach.
Upper right This 100 percent quartz sand in which the grains are glassy, colorless, and unstained is an endpoint in beach sand composition and is said to be supermature because all other mineral constituents have been removed. Typically such sands are of fine to very fine grain size, they are well sorted, and the grains are rounded. Quartz sand beaches are most common on trailing-edge shores and are prized for their white purity.
Lower This heavy-mineral-rich, dark sand from Focene Beach, Italy, southwest of Rome, has a diversity of grain types, including glassy and white (reflective) quartz grains, possible light-colored feldspar, volcanic rock fragments (black), and a variety of heavy minerals (green and black), which reflect its source. The sand was derived from the Tiber River, which drains volcanic and terrigenous terrain. The fine sand is fairly well sorted (it has a uniform grain size), and the sample contains a few shell fragments as well. Some of these Tiber-source beaches were mined during World War II for the heavy minerals.
Although light-colored minerals dominate, dark-colored minerals also occur on beaches. The most common are heavy minerals; the “heavy” definition derives from the fact that these minerals have considerably higher densities than both quartz and carbonate minerals. For example, quartz has a density of 2.7g/cm3 (water is 1), and heavy minerals range in density from 2.9 to 4.3g/cm3 for various types of garnet, to 3.5g/cm3 for diamonds, 4.6g/cm3 for zircon, 4.7g/cm3 for ilmenite, 5.ig/cm3 for magnetite, and 19.3 g/cm3 for native gold. It is no wonder that early miners used panning techniques to separate gold from junk minerals on the basis of density differences.
Groundwater discharging on this beach in Brazil has a high dissolved iron content, which forms red hematite on oxidation, giving the beach a vivid red color. Photo courtesy of Willard Moore.
Heavy-mineral sand grains (or “heavies”) tend to be smaller than the associated light minerals in the same handful of sand. The size difference is because the smaller but denser heavy-mineral grains will behave the same hydraulically (e.g., sink at the same rate; require the same amount of energy to erode or be transported) as the equivalent coarser sand grains of common light minerals such as quartz or feldspar. The heavy minerals are the less abundant breakdown products of various rock types. Usually heavy minerals are scattered through the beach sand, typically making up 1 to 5 percent. Regional trends in their concentrations often reflect the beach’s proximity to the source of the sands and the processes that delivered the sands to the beach. For example, along the Atlantic Coast of North America the highest concentration of heavies is found in Canadian and New England beaches (e.g., Block Island, Rhode Island, beaches have more than 40 percent heavies, mainly ilmenite and garnet), where the sands came off the crystalline rocks of the Canadian Shield and were delivered into what is now the coastal zone by the continental glaciers. Farther to the south, the beach concentrations of heavies decline as the coastal plain widens and the distance to the ultimate sand source increases. Here the mode of delivery was by rivers rather than glaciers and the result was elimination of some of the minerals by weathering and abrasion. The decreasing trend continues to the south, and the beaches of the southeastern United States typically contain only between 1 and 2 percent heavy minerals. In contrast, there are beaches in Alaska and on some volcanic islands in which heavy minerals make up more than 90 percent of the grains.
On a beach comprising a mixture of light and heavy minerals, the heavy minerals can become concentrated by the sorting action of waves and currents into placer deposits, just like the placer concentration of gold in stream sediments. Look in the swash zone for a weak placering effect, streaks of dark heavies being sorted out from the rest of the sand. Similar thin placers may be found in the dunes at the back of the beach, and sometimes heavies will be sorted differentially in both water ripple marks and wind ripple marks. The thickest heavy-mineral deposits form during storms at the back of the beach, where storm waves break and then run up with great energy; the less energetic backwash can transport only light minerals, leaving behind the dark sand concentrates. In fact, on some beaches, black sands appear after every storm. Ditches dug by backhoes at the back of Gold Coast, Australia, beaches have revealed numerous black sand layers, each layer representing an individual cyclone.
Another process that concentrates heavy minerals is the formation of small antidunes by backwash (see chapter 6). The process can be an effective sorting agent and leaves a characteristic striped pattern on the beach as the backwash zone migrates across the beach on a falling tide.
Beach strollers sometimes mistake these black sands for oil pollution. Signs have been erected on the recreational beaches of Durban, South Africa, to reassure beachgoers that the black sand on their feet is a natural heavy mineral and not oil.
Some beach surfaces are composed almost exclusively of heavy minerals, most commonly black sands that typically are dominated by magnetite (iron oxide) or ilmenite (titanium oxide). A simple test for magnetite is to pass a magnet over the (dry) black sand. If magnetite is present, the magnet will pick up sand grains. Other beaches are dominated by more colorful heavy-mineral sands, such as the red garnet sands of Labrador, Canada, or the green and red beaches of Lanzarote in the Canary Islands. Such colorful beach sand is formed when individual minerals are separated out from the rest of the heavy minerals by the sorting action of waves and wind. The red beaches of Labrador (Hutton Garnet Beaches), New England, and Bogue Banks, North Carolina, are the result of a concentration of garnet. Green beaches may be the result of the separation of epidote (Outer Banks of North Carolina) or olivine (Pu’u Mahana Beach, Hawaii) from other minerals. The color may occur in the entire beach, as in the case of the olivine beach in Hawaii, or in small thin patches, as in the case of the garnet on Bogue Banks beaches.
Careful study by mineralogists of such heavy-mineral sands shows that in addition to a dominant mineral such as magnetite or garnet, there can be twenty to thirty additional mineral species, although many are present only in trace amounts. Like rock fragments, these heavy minerals in beaches tell us much about the source rocks from which the beach sand came. For example, minerals such as kyanite, staurolite, and sillimanite are from metamorphic rocks. In addition to the black, opaque minerals magnetite and ilmenite, other common heavy minerals include various garnet species, various amphiboles and pyroxenes, olivine, rutile, tourmaline, and epidote.
One unusual heavy mineral, which is light rather than dark in color, is the diamond. On the beaches of Namibia, diamonds behave in much the same way as the more common dark heavy minerals. This characteristic helps geologists in their search for places where diamonds are likely to be concentrated by waves and currents; they look for the associated heavy-mineral deposits.
Another interesting mineral that occurs on beaches is mica. Mica characteristically breaks into flat flakes that range in color from black (biotite) to purple (lepidolite) to golden to colorless (muscovite). It occurs in a variety of rock types, typically granites and coarse-grained metamorphic rocks. On beaches, mica grains are less abundant than quartz because coarser mica flakes are easily broken down into smaller particles and also because the flat mica grains are easily moved and often swept away from the beach into deeper water. Where mica does occur, it is either close to its source rock or adjacent to a river mouth that is contributing sediment to the shore. Periodic calm conditions favor the deposition of the mica grains, so, for example, many beaches of northern Queensland, Australia, in the shelter of the barrier reef, are covered in a fine layer of mica at low tide. The reflection of sunlight by the platy mineral surfaces can give these beaches a beautiful golden sheen. Similarly, beaches in the U.S. state of Georgia often sparkle from the reflective mica flakes, even though they make up less than 1 percent of the beach sand.
Upper Heavy-mineral layers (black) exposed in a trench on Whiterocks Beach, Portrush, Northern Ireland, separated by lighter-colored quartz-rich sand beds. The heavy mineral concentrations form as a result of storms, whereas the lighter sands accumulate during quieter conditions.
Center Wind rippling at the back of the beach on Bogue Banks, North Carolina, has caused some sorting by mineral composition, with a concentration of the wine-colored garnet on the steep ripple face, black magnetite on the lee face of the ripple, and light-colored quartz in the troughs. The 35 mm film canister is shown for scale.
Lower Heavy minerals are often concentrated in antidunes as they form in the backwash zone. On a falling tide this zone migrates across the beach, leaving behind a characteristic set of stripes, as seen on this South Carolina beach. Photo courtesy of Miles Hayes.
The second common group of beach grain compositions is the carbonate minerals, which are discussed in greater detail in chapter 10. Carbonate grains are composed of the two common calcium carbonate minerals aragonite and calcite. Calcium carbonate is precipitated from seawater either in the form of sea creatures’ shells and skeletons or, less commonly, as inorganic carbonate grains. These grains are transported by waves and currents and deposited on beaches, most typically in the tropics, although there are exceptions.
Many other interesting natural and man-made materials form grains on beaches from time to time. Probably the most abundant human contributions to beaches are bricks. On beaches close to towns, it is common to find fragments of at least the occasional brick as well as other construction material such as tiles and cinder block. Often these have had the sharp edges knocked off as they rolled around in the surf, and some are completely rounded and barely recognizable as bricks except for their orange color. The flat, sandy beach at Crosby Point, in northeast England, has a very distinctive gravel beach at high tide made up of bricks eroded from an old dump site where debris from the World War II bombing of Liverpool was discarded. These bricks have been rounded and sorted by the waves to produce a cobble beach. In fact, waves can sort and form beaches from almost any material they encounter. At the fishing port of Portavogie, in Northern Ireland, decades of scallop fishing have seen the discarded shells mobilized by waves to form a beach almost entirely of scallop shells.
A listing of the terrigenous minerals found in beach sands around the world. Some of the minerals on the list, such as amphiboles, garnets, and pyroxenes, are groups of minerals that include several distinct mineral varieties.
Light Minerals
Quartz
Feldspars
Orthoclase
Plagioclase
Micas
Muscovite
Biotite
Lepidolite
Heavy Minerals
Opaque Minerals
Magnetite
Ilmenite and leucoxene
Pyrite
Hematite and limonite
Gold
Translucent to Transparent Minerals
Olivine
Pyroxenes
Amphiboles
Garnets
Epidote
Apatite
Staurolite
Kyanite
Sillimanite
Tourmaline
Zircon
Rutile
Monazite
Topaz
Diamond
The BBC News headline read, “Jamaica Puzzled by Theft of Beach.” In July 2008, sand thieves had removed five hundred truckloads of sand from its Coral Springs beach at the site of a planned resort. That this story made the international news was somewhat surprising, given that the destruction of beaches by both legal and illegal beach mining has been going on for more than a century in Jamaica and on other Caribbean islands. Most cases receive little attention beyond the local news. Perhaps the notoriety of the Jamaica story was due to the large amount of sand removed in a short time, and the economic impact of the beach loss at the construction site of a new resort. In fact, the authors themselves have witnessed illegal mining of beaches in the Caribbean region. While participating in an oceanographic cruise off the shore of Arecibo, Puerto Rico, one of this book’s authors (Orrin Pilkey) observed just such an operation; it went on for all twenty-one days of the cruise. For eight hours a day, a backhoe continuously loaded dump trucks with sand, which was hauled away to an unknown location. A later visitation to the mining site revealed a nearby sign proclaiming that sand mining from beaches is illegal!
Beach mining is universal, and the largest legal operations have been associated with mineral extraction. Beach placer deposits of valuable minerals such as zircon, rutile, monazite, ilmenite, leucoxene, garnet, gold, and diamonds have been exploited in numerous countries, including Australia, South Africa, Canada, China, Norway, the United States, India, Brazil, Mozambique, Madagascar, and Senegal. Sometimes the mining operations restore the beaches after mineral extraction, but too often this is not the case. For example, beaches in southern Namibia and northwest South Africa have been degraded or destroyed by diamond-mining activities. In some cases, however, although during mineral mining the beach is significantly disturbed, the volume of sand actually removed is small. The “waste” sand can be returned to rebuild the beach. For example, the beaches of Nome, Alaska, have been mined for decades (one can still see sluice boxes on the beach, the property of individuals who hope to find gold-bearing sand that was missed in the gold rush). The actual volume of gold removed from the beach was minute compared to the volume of sand on the beach.
Sand is also used for the manufacture of glass and abrasives, but beach sand is most commonly used for construction aggregate. Legal and illegal beach mining is especially common in third world and developing countries, where rapid coastal development generates a high demand for aggregate used in concrete production or simply for fill. In a time of rising sea level, island nations are particularly hard hit by beach mining. In the Caribbean, virtually every island nation is affected. A 1995 study of twenty Caribbean islands showed that beach mining was the second-most pressing problem for the islands, behind sewerage and solid waste disposal, and was closely associated with coastal erosion. Antigua has lost entire beaches to sand mining on its west coast. Anguilla’s beaches have experienced narrowing due to illegal mining. Sand is so scarce on St. Lucia’s coast that sand is now obtained by breaking up or crushing lava rock, a much less damaging source. Often beach mining is associated with low-income populations, but the systematic removal of dunes on Barbuda was carried out by a sand-mining company with which government officials were affiliated. When an injunction was issued to stop the sand mining, it was ignored. The minister of agriculture and mining-company officials were given short prison sentences, but the governor-general pardoned them. The mining of Barbuda beaches not only has resulted in beach erosion, but also has caused freshwater aquifers to become contaminated by salt water.
Upper Local mining of a gravel beach in northern Sumatra is typical of third world mining of beaches for construction aggregate. Photo courtesy of Marianne O’Connor.
Lower This example of beach mining in Morocco is on a scale that significantly impacts the beach, both in terms of sediment loss that will lead to beach erosion and in terms of lost habitat. Photo © Lana Wong.
In rural Scotland and Ireland, where rocky glacial soils are common, farmers have for centuries taken sand from beaches and ploughed it into the soil to “lighten” it. The practice was probably relatively benign when done manually with a horse and cart, but the advent of tractors made the process much more efficient and damaging. Traditional “pebble dashing” of houses in Ireland and Scotland still sustains the (often illegal) practice of beach mining for small pebbles, which are scattered into the outer plasterwork of houses to create a distinctive wall covering.
Inland mining of creek and river sands that feed sand to the coastal beach system also impacts beaches, as is the case for beaches of the Placencia Peninsula, Belize, and beaches associated with the Rio de la Plata estuary in Uruguay. Montserrat banned beach mining in recognition of the problem, but locals continue the practice. In KwaZulu-Natal Province, South Africa, the impact of sand mining from rivers on beach erosion caused a heated debate in the local press in 2009. On Grenada (e.g., in Mt. Pleasant, Sabazan, Tibeau, and Grand Bay) sand mining continues to devastate beaches, and the issue pits citizens who attempt to protect beaches against politicians who say one thing and do another, truck drivers who ignore regulations, and builders who say they do not want to harm beaches, but continue to buy the sand. The Caribbean list goes on, with problems in Guadeloupe, Martinique, Puerto Rico, Trinidad and Tobago, St. Lucia, St. Vincent, and the Grenadines. The latter have imported sand from Guyana in an effort to reduce beach mining and meet the needs for construction aggregate, but this only transfers the problem to another locale.
The impact of sand mining on beaches and dunes is well documented in the Azores, where mining began in the 1960s. The mining was stopped in 1995 by legal enforcement, but the destroyed beach and dune systems have not recovered. In São Tomé and Principe, beach mining compromised the integrity of beaches and destroyed turtle-nesting habitat. Mainland Africa countries experience identical problems. Possibly the largest-scale beach mining in the world is in northern Morocco, where huge coastal dune fields are being removed as you read this passage. In a country with almost no lumber resources, sand for concrete is critical, but it need not be obtained from beaches. Lines of dump trucks wait their turn at the dune face; the trucks are quickly filled by an army of workers wielding shovels. Where steep slopes make truck access to the beach difficult, trains of donkeys string back and forth, each loaded to the maximum with sacks of sand. As many as 200 dump-truck loads of sand per day were said to be hauled from some mining sites. These operations were witnessed by authors Joe Kelley and Orrin Pilkey.
Beach mining in northern Morocco is on a grand scale; relays of large dump trucks carry away tons of sand daily. Note the protective barrier of sand left between the sea (to the left) and the mining operation. Ultimately that barrier will be breached, and the shoreline will move a significant distance landward. Although no buildings are threatened, the damage in terms of land and habitat loss is beyond recovery.
In 2007, increased coastal flooding in Liberia was blamed on coastal changes caused by beach mining, both for construction aggregate and for filling sandbags to build defensive structures against coastal erosion. From Cape Mount to Cape Palmas, coastal erosion is impacting housing development. In the adjacent country of Sierra Leone, the village industry of coastal Lakka was beach mining to feed building construction in Freeport; now Lakka faces an erosional threat from the sea because it removed its protective beach. Not far away, Benin is a good example of a small country with a coastal erosion problem greatly exacerbated by beach mining that was not controlled or regulated; it was driven both by high construction demand for aggregate and by economics. For a truckload of sand, beach miners could earn more than the country’s average monthly salary, and beach communities collected a small fee for each truckload of sand mined, an income for the community. In 2008, the country was attempting to ban beach mining and shift mining to inland sand sources associated with rivers and lakes, but this created new problems.
On Africa’s east coast, the rim of beach rock around the sandy barrier islands of Mozambique is the only material available for construction. The practice of mining this beach rock, however, is tantamount to digging out the very foundations of the islands. Elsewhere in the Indian Ocean, even the remote Andaman Islands experienced beach loss due to mining for construction in Port Blair. In the Andaman Islands and nearby Nicobar Islands, at least twenty-one beaches were reported to have been destroyed between 1981 and 2000, with the associated loss of turtle-nesting habitat and the protective function of beaches, as demonstrated by the 2004 tsunami. In 2008, it was reported that sand mining could erase some islands from the map in the Maldives; mining the coral sands from beaches and lagoons there is a tradition, but it was accelerated after the 2004 tsunami, as aggregate was needed to rebuild and to fill sandbags for shore-hardening structures. In Sri Lanka, the impact of the 2004 tsunami was more destructive than it otherwise would have been because protective coral reefs from Akurela to Hikkaduwa had been mined for years to produce construction lime. Occasionally a local populace will make beach mining an issue, as in 2007, when people blocked a road in southwest India’s Kerala Province because officials had failed to check unauthorized beach mining that had left the beach pockmarked with dangerous pits.
These piles of beach rock are being mined from the beach on Bazaruto Island, Mozambique. This beach rock helps stabilize the island, but it is the only rock available for use in construction on this otherwise completely sandy island. In effect, this is the contradiction of most beach-mining operations; the natural beach protection is being destroyed in order to obtain construction material to build buildings in the high-hazard coastal zone.
Pacific islands large and small have lost beaches and dunes to mining. Up until the 1970s, large volumes of sand were mined from Maui, Hawaii, beaches to provide not only aggregate but also lime for sugarcane processing. Sand was taken from Molokai to nourish beaches on Oahu, including Waikiki. Reportedly, replenishment beach sand for Waikiki Beach also has been obtained from Australia, Los Angeles, and from beaches on Maui, as well. Sand mining has been reported on various islands in Micronesia. Sand has been removed from beaches in American Samoa for golf courses, and an examination of sand traps on Hawaiian golf courses suggests similar sources there. Low-lying islands threatened by the sea-level rise cannot afford to mine away beaches, one of the last natural lines of defense against the coming storm and the very basis of their tourism economies, though small-scale mining by locals often isn’t regarded as harmful. Sand is hauled away a horse-cart-load at a time, and the borrow holes fill with sand on the next high tide. It seems reasonable to conclude that no harm is done, but the net loss over time is real and damaging.
First world countries are not innocent of similar attitudes and mining impacts. Monterey Bay, California, and Jasper Beach, Maine, in the United States, the Kurnell Peninsula of Queensland, Australia, and Rodney, New Zealand, are just a few examples. Mining of North Stradbroke Island out of Brisbane, Australia, for rutile, zircon, ilmenite, and silica sands began in the 1960s and has destroyed frontal dunes. Although sand may be returned to the system after processing, the destroyed ancient Aboriginal middens and campsites as well as the ecosystems that were sheltered by the dunes are lost. In 2010, a mining company there proposed that an area of the island be added to a national park, but that company still retained mining leases on 45 percent of the island.
The attitude toward such environments in the 1950s and 1960s was that they were resources for the taking. For example, General Motors in the 1950s advertised new dump trucks, showing them being loaded with sand from North Carolina beaches. Although sand mining continues, there is a growing recognition that beaches and dunes are far too valuable in their natural state to be carried away in dump trucks or even in wheelbarrows and buckets.
A carbonate beach on the shore of Spencer Gulf, Australia, composed mainly of snail shells of various types. The Australian ten-cent coin is shown for scale. How many different genera of snails can you find in the photo?
Beach glass, or sea glass (the “mermaid’s tears” prized by beachcombers), is the broken-down and rounded remains of bottles and other broken glass that have been smoothed and sorted on beaches. These granule- to pebble-size glass fragments can be of various colors; the more colorful and unusual pieces are sought by beachcombers. (Beach-brick collectors are a much smaller group!) Another type of glass, reduced to sand size, has been produced from ground-up recycled bottles and used to nourish beaches in Florida and Hawaii; this approach has not met with public acceptance.
Rare human artifacts may also be found on beaches. Prehistoric stone tools are difficult to discern from other gravel- to cobble-size beach materials, but the lucky beachcomber may make such finds. Fragments of pottery and other ceramics that span human history make for exciting finds as well. Pottery fragments found in some Tunisian beaches can be identified as Carthaginian, Roman, and every age since.
A cobble beach at Crosby Point, England, composed of thousands of bricks, most of which have been rounded by wave action and deposited just like the cobbles on a natural cobble beach. The bricks are eroded from an old dump site dating from World War II; debris from the bombing of Liverpool was disposed of in this area.
Often, waves can erode material that underlies the beach; when this happens, blocks of the eroded material are transported around the beach, rounded and shaped on the way. Cemented beach materials (beach rock), too, can be eroded and reincorporated into the beach.
Beaches can also be the final resting place of seaweed and sea grass that is ripped up during storms and carried to the beach. In western Ireland and Scotland, so much kelp is deposited on some beaches that they are completely covered. Usually this seaweed either decays or is washed out to sea. On recreational beaches the seaweed is often removed, but this can be harmful to the beach since the seaweed acts as a fertilizer for beach and dune plants, and many tiny animals live among the seaweed. Similarly, the thin blades of sea grass can form thick accumulations on beaches. In some places, sea grass can be a major component of the beach. In the Spencer Gulf, in Australia near the town of Whyalla, is a beach composed almost entirely of sea grass more than several feet thick. Along parts of the Mediterranean coast, concentrations of marine vegetation are common; so much of the green plant material is in the surf zone that the breaking waves have an intriguing green color.
Beach rock is a special kind of rock that forms when beach grains are cemented together by minerals precipitated from the water in a porous beach. The minerals grow as tiny crystalline needles on the surface of the sand grains, and eventually the crystals join, filling the pores as a cement that binds the grains together as rock. The process often involves rainwater, which is slightly acidic, dissolving calcium carbonate as it seeps down through the beach. When it reaches the seawater level in the beach, the carbonate is precipitated because the seawater is saturated with carbonate and cannot hold any more. Because this chemical reaction is faster under higher temperatures and because carbonate-rich beaches are more common in warm climates, beach rock is also most commonly found in the tropics.
Beach rock is usually found near the mid- to low tidemark as a solid rock layer on beaches exposed to a tidal amplitude of 3 ft or less (less than 1 m). The rock is made of grains that have exactly the same composition and shape as those in the adjacent loose beach sand. Some beach rock, we know, is very young. For example, along the north coast of Puerto Rico there is beach rock that contains beer bottle caps, nails, glass fragments, and even a crescent wrench, all firmly cemented in the sand. Beach rock usually forms slablike layers that are up to l ft (30 cm) thick and several feet (meters) wide and that dip in a seaward direction. These rock outcrops can extend for miles (several km) along a beach, as a kind of pavement. Indeed, the submerged beach rock on the banks off Bimini, Bahamas, which was left behind as sea level rose, has been mistaken for an ancient paved road from the mythical Atlantis.
The raised area that looks like a roadway in the right of the photo is a beach-rock formation in Togo, West Africa. The rock is made of the same material as the beach except that the grains have been cemented into sandstone. In a sense, this formation forms a natural seawall, and like a seawall it reflects waves and lacks a beach on its seaward side. Photo courtesy of Miles Hayes.
The beach rock on this stretch of beach in the Whitsunday Islands, Queensland, Australia, virtually covers the beach and has changed the beach’s recreational use.
Beach rock along the lowermost part of the beach near Lake St. Lucia on the Zululand Coastal Plain, South Africa, gives the appearance of a roadway, but it is a completely natural formation. One can imagine how such a feature might be mistaken for an ancient road or ruin. Note that there is a beach on the landward side of the beach rock.
These beach-rock slabs on the beach in Fujairah, United Arab Emirates, were probably deposited by very strong storm waves, or perhaps even a tsunami.
The large piece of beach rock in the right of the photo has been interpreted as a deposit from the 1755 tsunami that struck this shore at Cape Trafalgar, Spain.
Once formed, beach rock can undergo several different fates. The loose sand underneath the solid beach rock can be eroded out, undermining the rock layer, which then breaks into large slabs. These slabs can be moved by storms and thrown to the rear of the beach. At Cape Trafalgar, Spain, for example, slabs of beach rock were ripped up and deposited landward of the shoreline during the 1755 tsunami. Similarly, along the shores of the Gulf of Oman, which is bordered by the Gulf Emirates, an unknown event has thrown slabs of beach rock up from the beach. In another instance, a beach rock-eolianite deposit on the coast of Tunisia has been reported to contain evidence of a tsunami.
Beach rock may provide important habitat, as it is often colonized by plants and animals that otherwise don’t occur on beaches. The very action of colonization can lead to the breakdown of the beach rock as organisms burrow and bore into it. Waves too can break pieces off the beach rock and transport these fragments as pebbles on the beach.
Undisturbed beach-rock layers often are found submerged offshore; or higher on the beach, above the existing low-tide line; or even landward of the beach. These layers mark fossil shoreline positions, so beach-rock lines are a good measure of sea-level changes. However, such nearshore ledges can also be a hazard to navigation. Such an underwater layer was responsible for the 2004 sinking of a marijuana-laden boat off Bazaruto Island, Mozambique.
A closely related rock type, eolianite, is similar to beach rock in origin, except the original sandy landform was a windblown dune. Again, natural cementation converted the dune sand to rock, in this instance most commonly in tropical areas. As sea level rose or fell, these fossil dunes formed resistant knobs, standing as small nearshore islands or small hills in back of the beach.
Beach rock can be the only solid rock on many tropical beaches. On the northeast coast of South Africa it forms seaward-protruding rocky headlands on an otherwise completely sandy coast. On Bazaruto Island, Mozambique, the authors have seen beach rock being mined for construction because it is the only solid rock on the island. When beach rock forms headlands and fixed points, the mobile parts of the beach can adjust to the new conditions imposed by the beach rock.
This eolianite outcrop at Black Rock, South Africa, forms a headland and is one of the few rock outcrops of cemented dune sands on this sandy coast. It provides a habitat for a small lizard that lives in the splash zone and is found nowhere else on the African continent. The lizard is confined to this single rock because it cannot compete with other lizards outside the harsh conditions of the splash zone.
The Black Rock outcrop of cemented dune sand in which the original dune bedding can be seen (above the boy’s head).
Upper left A beach in North Uist, Outer Hebrides, Scotland, covered in seaweed deposited during a recent storm.
Upper right The sea grass deposited in the wrack lines on this sand beach in the British Virgin Islands provides good evidence of the last series of high tides. The plant remains are an essential part of the food chain for beach and dune organisms.
Lower An incredible concentration of sea grass on a beach near Whyalla, South Australia. Such masses of decaying vegetation can generate gases that are unpleasant to beachgoers and can even be dangerous.
