OCEAN CITY

resort, city, Cape May county, southeastern New Jersey, U.S., on a barrier island between Great Egg Harbor (bridged to Somers Point and Longport) and the Atlantic Ocean, 12 miles (19 km) southwest of Atlantic City. Laid out in 1879 by Methodist ministers as a Christian seaside resort, it has remained faithful (by public vote) to its founders' stipulation that no alcoholic beverages be sold; this has made it a popular place for religious conventions and family vacations. In the 1950s the evangelist Billy Graham broadcast his radio program from Ocean City. It has developed as a fishing and summer resort with marinas and boatyards, 8.5 miles (14 km) of public beaches, and a 2.5-mile (4-km) boardwalk, built in 1928. A number of large mansions, built by wealthy Philadelphians at the beginning of the 20th century, has left a Victorian imprint on the city. The nearby Richard Somers House (1726), home of the U.S. naval hero of Barbary Coast fame, is a state historic site. The Ocean City Historical Museum contains valuable relics salvaged from industrialist John D. Rockefeller's ship Sindia, which was wrecked offshore in 1901. To the south lie the Marmora Coastal Wetlands, and across the harbour to the east is the TuckahoeCorbin City Fish and Wildlife Management Area. Aside from tourism, boatbuilding is the city's economic mainstay. Inc. 1897. Pop. (1990) 15,512; (1996 est.) 15,661. resort town, Worcester county, southeastern Maryland, U.S. Ocean City lies along a 10-mile (16-km) barrier beach between a chain of bays (Sinepuxent, Isle of Wight, and Assawoman) and the Atlantic Ocean, 29 miles (47 km) east of Salisbury. It is the state's largest seaside resort, with a summer population of 300,000. The Ocean City Inlet (which had long been silted up until it was opened by a great storm in 1933) connects the bays with the ocean. The bays offer safe harbour for boats and amenities for deep-sea fishing, bathing, and surfing. The site, with its sandy beaches, began to attract tourists after the building of the Atlantic Hotel in 1875. The arrival of the railroad in 1878 facilitated the development of this resort town; later highways from the BaltimoreWashington, D.C., area provided access to the city. Ocean Downs Raceway is noted for harness racing. Assateague Island, a narrow barrier island and national seashore, is across the ocean inlet at the southern end of Ocean City's barrier beach. Stephen Decatur, the naval hero, was born (1779) on a farm near Berlin, a few miles west on the mainland. Inc. 1880. Pop. (1990) 5,146; (1996 est.) 6,766. Coastal and nearshore features Coral reefs, coral islands, and atolls Coral reefs are masses of carbonate of lime built up from the seafloor by the accumulation of the skeletons of a profusion of animals and algae; eventually they rise to the surface of the water. Reef-building corals, chiefly the stony corals or Scleractinia, grow best in shallow, sunlit water, between the low-water mark and a depth of 11 metres, but they can still construct reefs in water as deep as 40 metres, and they may have a sparse existence between 40 and 55 metres. These corals prefer water of normal salinity and with an annual maximum temperature above 22 C but below 28 C. Their reef-building activities, however, may be carried on in waters whose minimum temperature in winter is not less than 15 C. A second group of corals in present-day seas grows in thickets and coppices that develop banks rather than reefs on the outer, deeper, colder, and darker parts of continental shelves and platforms. These organisms flourish in water with a winter minimum temperature ranging between about 4 and 15 C at depths of about 60 to 200 metres. In any one thicket there are commonly only two genera of delicately branching corals involved. Such coral banks are known along the eastern Atlantic shelf edge (or continental slope) from Norway to the Cape Verde islands and again off the Niger River delta and in the west Atlantic around the Gulf of Mexico, The Bahamas, and the Orinoco River delta. Off New Zealand such banks have been recognized on the Campbell Plateau and the Chatham Rise; they also occur in the northwest Pacific near Japan. The third coral assemblage of the modern seas is associated with even colder or deeper seas; it consists of small, solitary corals of relatively few genera, known from the abyssal floors of the oceans and from the shelves around Antarctica, Patagonia, and the Falkland Islands in waters 2 to 6 C in temperature. Buried fossil reefs on ancient continental shelves are targets for petroleum exploration. The porosity of reefs and the characteristic curvature of nonporous enclosing sediments cause them to be prospective reservoirs for oil and gas. The rich oil fields of Alberta, for example, are associated with Devonian reefs (about 408 to 360 million years old). Fossil reefs recently have become targets for metal prospecting because some corals contain small percentages of metals, such as zinc and copper, selectively incorporated from seawater. A living coral reef may also have economic potential in that it constitutes a major tourist attraction. Reef accumulation Tropical water conditions Water conditions favourable to the growth of reefs exist in tropical or near-tropical surface waters. Regional differences may result from the presence or absence of upwelling currents of colder waters or from the varying relation of precipitation to evaporation. Tropical seas are well lit, the hours of daylight varying with the latitude. Light intensity and radiant energy also vary with the depth. Thus, at latitude 3244 N (the Madeira Islands) the day in March has a length of 11 hours at a depth of 20 metres, 5 hours at 30 metres, and only about a quarter of an hour at 40 metres. Nearer the pole these figures decrease further. Light intensity has a profound effect on the growth of the individual reef-coral skeleton because of the symbiont zooxanthellae of reef corals (see below Biological factors). The number of species present on a reef also may be related to light intensity and radiant energy. Turbidity may be high in lagoons, where shallow water lies over a silt-covered seafloor and where storms and windy periods cause considerable disturbance of the bottom silt. The average transparency may be low (about 12 metres), and light penetration is reduced. Inside the Great Barrier Reef, on the shallow continental shelf of Queensland, the oxygen content of the water is high, exceeding 90 percent saturation most of the time; in deeper water, during the calm periods of the rainy season, the saturation may fall to about 80 percent. Plant nutrients such as phosphate and nitrate show no seasonal change in quantity; both are present in very small quantities throughout the year. Constant mixing of the shallow sea prevents any stratification of the nutrients. As a result, growth of phytoplankton is possible and almost uniform throughout the year, providing a constant supply of food for the zooplankton, which in turn form the chief food supply of the corals. Some nutrients enter the lagoonal waters with the oceanic water that flows through the reef openings, but the dissolved phosphates in the lagoons are probably derived chiefly from bacterial decomposition of the organic matter on the sea bottom, as well as from detritus swept in from the reef surfaces. This environmental pattern is typical of many atoll lagoons. Continental margins General considerations Figure 11: Physiographic divisions of the continental margin. Continental margins are the submarine edges of the continental crust, which is, as noted above, relatively light and isostatically high-floating in comparison with the adjacent oceanic crust. Figure 11 is a block diagram that schematically shows the physiographic divisions of a continental margin. These divisions are the continental shelf, the continental slope, and the continental rise. Characteristics of the various continental margins are shaped by a number of factors. Chief among these are tectonics, fluctuations of sea level, the size of the rivers that empty onto a margin as determined by the amount of sediment they carry, and the energy conditions or strength of the ocean waves and currents along the margin. Margin types Figure 11: Physiographic divisions of the continental margin. Continental margins on the leading edges of tectonic plates, like those around the rim of the Pacific Ocean, are usually narrow and have steep continental slopes and either poorly developed continental rises or none at all. The continental slope is often steep and falls away directly into a deep-sea trench. In many cases, the leading-edge margins are backed by mountain ranges. Continental margins on the trailing side of tectonic plates, like those around the Atlantic Ocean, are broad, with gentle continental slopes and well-developed continental rises. The adjacent land area is commonly a broad coastal plain (see Figure 11) that, depending on the state of sea level, may become submerged from time to time and hence part of the continental margin. Since continental margins are the shallowest parts of the world's oceans, they are most affected by changes in sea level. Worldwide changes in sea level, called eustatic sea-level changes, have occurred throughout geologic history. The most common causes of such sea-level changes are global climatic fluctuations that lead to major glacial advances and retreatsi.e., ice ages and interglacial periods. Other causes that are not as well understood may include major mountain-building events and isostatic changes in crustal plates. When continental glaciers advance, as they did several times during the Pleistocene Epoch (which extended from about 1,600,000 to 10,000 years ago), water that would normally be in the oceans is locked up as ice on land, resulting in a drop in sea level. As the glaciers retreat, more water is fed to the ocean basins and the sea level rises. Fluctuations from highstand to lowstand have totaled 250 metres or more during Cenozoic time (roughly the last 66.4 million years), with concomitant fluctuations in exposure and flooding of the continental margins. (During a highstand the sea level is above the edge of the continental shelf, while during a lowstand it is below the shelf edge.) Rivers bring a variety of sediments to the coast. These are classified by their mineralogy and by particle size and include sand, silt, and clay. Sand to sedimentologists is a grain of any composition from 63 to 2,000 micrometres (0.0063 to 0.2 centimetres) in its largest diameter. Silt is 4 to 62 micrometres, and clay is any particle less than 4 micrometres. Most of the detrital minerals brought to the continental margins by rivers in sand and silt sizes are quartz, feldspars, and mica; those of clay size are a suite of clay minerals that most commonly include smectite, kaolinite, and illite. (Clay can, in other words, refer either to particle size or to a group of minerals.) These, then, are the mineral constituents, which, together with calcium carbonates produced in the oceans by biogenic activity as shells and the hard parts of plants and animals, go to make up the sedimentary packages that are deposited on and constitute a fundamental part of continental margins. A constant battle is being waged between the rivers that bring sediments eroded from the land to the sea and the waves and currents of the receiving body of water. This dynamic struggle goes on year after year, century after century, sometimes for millions of years. Take, for example, the north coast of the Gulf of Mexico into which the Mississippi River flows. The continental margin at this site is subject to relatively low wave and current energy, and so the river has filled up most of the adjacent continental shelf with a delta and dumps roughly 256 million tons of sediment each year directly at the top of the continental slope. By contrast, the Columbia River in the Pacific Northwest of the United States carries 131 million tons to the coast, where the sediments are attacked by the large waves and currents normal for that margin. As a result, sediments are widely dispersed, and the shelf is not filled with a large subaerial delta. The effects of this battle are easily seen where human activities have interfered with the transport of sediments to the sea by major rivers. For example, the Nile River delta is retreating rapidly, widening the submerged portion of the continental margin, because the Aswan High Dam has trapped much of the sediment normally fed to the delta front. The lower Mississippi River has been artificially maintained in a channel by high man-made levees. These have stopped the floods that fed much of the western delta margin. Because of this, coupled with a slow rise in sea level and the effects of canals dug in the delta wetlands, the coast has begun to retreat significantly. When rivers carrying sediment from the interiors of continents reach the sea, several things happen. Velocity in the river jet decreases rapidly, and the sand particles drop out to be picked up by the waves and currents along the coast, where they feed beaches or barrier island systems (see structural landform: Landforms produced by coastal processes: Landforms of depositional coasts). If the river has a large enough discharge, the finer-than-sand-sized materials may be carried for kilometres onto the margin in a fresh- or brackish-water plume. The surf system then acts as a wave filter, trapping the sand in the coastal zone but allowing the finer materials to be carried out onto the margin. When estuaries are the receiving bodies of water on the coastal boundaries of continental margins, as in the case of the east coast of North America, virtually all the sediments brought down by the rivers are trapped within the confines of the estuaries. In addition to the two primary types of continental margin, there also are special types that do not readily fit either category. One of the most intensely studied margins of the world is the Borderland, the continental margin of southern California and northern Baja California. It consists of a series of offshore basins and ridges, some of which are exposed as islands. This system of basins and ridges formed as the result of faulting associated with the movement of the Pacific Plate past the North American Plate. It remains tectonically active today and is related to the San Andreas Fault system of California. A second special type is the marginal plateau. The Blake Plateau off the east coast of Florida is a good example. Such a plateau constitutes a portion of a continental margin that has many of the features of a normal system but is found at much greater depth1,000 metres in the case of the Blake Plateau. Continental margins can be either constructional or erosional over varying periods of geologic time, depending on the combination of factors discussed above. When deposition exceeds erosion, the margin grows seaward, a process of progradation that builds out as well as up. When the erosive forces are predominant, the margin remains static or actually retreats over time. Some geologists think that the continental margin of the eastern United States has retreated as much as 530 kilometres since Cretaceous timei.e., during the last 66 million years or so.