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some of the most picturesque landscapes on earth owe their existence to glaciers. there are many examples-- spectacular mountain ranges such as the alps, the himalayas, and the rockies were sculpted by repeated glaciation. yosemite valley, here in the sierra nevada mountains, would have been another nondescript river valley if glaciers hadn't carved it to its present shape. many of the world's most beautiful lakes were gouged out of hard rock by glaciers, including north america's great lakes and the famous lochs of scotland. even the great expanses of rich agricultural soils that blanket china and the soviet union, canada and the united states owe their existence to glaciers. moving glacial ice pulverizes the underlying rock into silt-sized fragments. this silt was eventually transported and concentrated by the wind into the vast fertile soils of today.

early scientists didn't really appreciate the important geological role of glaciers. even geologists were convinced that glaciers had never existed outside of their present locations over the last one million years. a breakthrough came in 1836 when swiss scientist louis agassiz reported evidence that the inhabitants of medieval villages in europe had moved their towns to keep pace with advancing glaciers. further study revealed that glaciers leave behind a distinctive deposit of sediment, like these boulders, as they melt back and retreat. geologically-recent examples of these sedimentary deposits, found hundreds of kilometers from the nearest glacier, demonstrated to agassiz that vast portions of the continents of the northern hemisphere had been recently covered with glacial ice. observations like these led to the realization that glaciers are active and powerful agents of landscape evolution. glaciers are large, long-lasting masses of ice

which slowly flow across the land. while most places periodically have temperatures cold enough for snow and ice to form, only in a few places do conditions permit the growth of glaciers. glaciers come from the accumulation of snow, either in polar regions or in high altitudes, or even at the equator at very high elevations at the tops of mountains. wherever you have precipitation in the form of snow, you can get a glacier. but there's one other requirement-- more snow has to fall in the winter than melts in the summer, so that in every 12-month period, some of the previous winter's snowfall is left over. so as snow accumulates year after year, a glacier begins to form. the actual manner

in which snow is converted into glacial ice involves compaction and rearrangement of snow crystals. there's a process that goes on as each layer of snow is added year by year-- that's residual snow from the previous winter-- it compacts under the weight of new snowfall. and as that compaction takes place, the snow crystals are pressed closer together. the air in the original snowpack is generally expelled by this compaction, and the snow crystals join together to form an intermediate substance between snow and ice which we call firn. it's an old swiss term that's still used today. so eventually the firn itself gets more compact, more recrystallized, and it becomes glacier ice. subject to extreme, instantaneous stress, ice shatters like glass.

but if stress such as gravity is applied gradually over a long period of time, the ice bends. this process, called plastic deformation, explains how glaciers move. generally, ice must accumulate to a thickness of approximately 20 meters before movement starts. pulled by gravity, ice in a glacier typically shifts down slope a few millimeters per day. to study glacial flow, louis agassiz and his students built a hut on the ice itself. they observed that the center of the glacier moved most quickly, while friction slowed down movement along its sides. a similar phenomenon is observed in rivers and streams. scientists like agassiz also wanted to understand how glaciers flow internally. but it wasn't until early in the post-world war ii era

that glaciologists were able to drill a hole through a swiss glacier. and this was a hole several hundred meters deep, maybe a couple of inches in diameter or smaller. but they put an aluminum tube in that hole right down to the bottom of the glacier. the scientists discovered that the tube bent as it shifted down slope. just as friction slows movement of ice at the sides of glaciers, it slows movement at the base as well. scientists also discovered that glaciers not only creep over the bedrock, but in places break free to glide across it. such basal slip is lubricated by water melting from the ice. streams of this sub-glacial melt water

commonly pour from the snouts of glaciers. there are two common types of glaciers. in mountainous regions at high altitudes, glaciers fill stream valleys. their movement down slope is confined by the paths of the valley, and so these rivers of ice are known as valley glaciers. on land masses near the poles, such as greenland and antarctica, single giant glaciers cover vast regions. these glaciers are called continental glaciers, or ice sheets. ice sheets are actually spread across the continent like broad domes. the thickest part of the dome is at the center of the glacier, where the greatest snowfall takes place, and causes ice to build up to its greatest thickness. the weight of this thicker central region forces the glacier to radiate in all directions, as opposed to the relatively straight,

downhill movement of a valley glacier. regardless of type, all glaciers move ice from its point of origin to areas where it melts. in a typical valley glacier, ice builds up year after year at the head of the glacier, in the so-called "zone of accumulation." down slope, the ice melts away faster than it can build up at that lower, warmer altitude. this is the so-called "zone of wastage." these two zones are divided by the snow line, which can actually be seen o some glaciers during the summer months. down slope of the snow line, melting snow exposes old, silty firn in the zone of wastage. up slope, the glacier is permanently covered with fresh, white snow. the snow line shifts up or down

the surface of the glacier from year to year. during cool years, the snow line lies at lower elevations than in warm years. if ice moves into the zone of wastage faster than it melts away, the snout of the glacier will advance farther down slope. but if not enough ice moves into the zone of wastage to compensate for melting, the snout of the glacier will retreat up slope. whether the snout of the glacier advances or retreats, ice within the glacier itself is continuously flowing down slope to melt away. as it does so, the ice carries tons of rock, silt and other debris. some of this material comes from the mechanical weathering and toppling of rocks onto the surface of the glacier. other material is plucked and scraped out but the ice as it flows across the bedrock.

glaciers erode in a very characteristic style. they polish, they grind, they gouge, they pluck away at the rocks that are there, totally destroying any topography that was already present. any streams in its path are obliterated by the oncoming ice. the ice itself is hardly pure. it's dirty. it's filled with everything the ice previously scratched and ground away at. so a glacier is moving like ice-filled sandpaper and uses the rocks and sand debris within itself to further gouge and scrape the earth. the tremendous erosive power of moving ice is evident in areas where glaciers have melted away, exposing the bedrock. these surfaces were polished smooth by glacial ice. grit, caught beneath the glacier, carved long scratch marks, called striations, in the polished rock as the ice flowed across it.

on a more impressive scale, the valleys once occupied by glaciers are carved out, acquiring steep walls and broad, gently sloping floors. this u-shaped valley profile contrasts greatly with the v-shaped profiles typical of unglaciated stream valleys. at the edges of the glacier, eroded material is dumped out by the melting ice. this unsorted rock debris is known as "till." if the snout of the glacier remains in about the same position for a long time, a very great mound of till, called an end moraine, may form. moraines can also accumulate at other places next to glaciers. lateral moraines, for example, grow where till is deposited along the sides of glaciers.

where two glaciers combine, lateral moraines merge, forming medial moraines. understanding moraine deposition is important for geologists because it can help reconstruct the history of glacial movement. for example, overlapping moraines not only show the position of a glacier over time, but also indicate that glaciers typically advance and retreat over and over. evidence like this helped geologists recognize that the earth experienced repeated cycles of glaciation, or ice ages, over the last two million years. an ice age consists of a gradual cooling of the climate and growth of glaciers worldwide, terminated by a warm, interglacial climate, during which glaciers melt back and retreat. by reconstructing the glacial history of the earth, we know understand that changes in sea level and the evolution of life are also linked with glacial cycles. what's less clear, however, is the cause of ice ages.

a number of theories attempt to explain the processes responsible for glacial cycles. there have been about 10 ice age cycles over the last one million years. any theory to explain these cycles must take into account the regular repetition of glacial activity and the link between glacial cycles and global climate. the repetitive nature of ice ages during the past two million years suggests that the world may again be moving toward a period of deep freeze. this is not likely to begin within our lifetimes, but may commence during the next few thousand years. most scientists believe ice ages are tied to changes in the position of earth as it orbits the sun. as the earth rotates, if occasionally it tilts a bit, a good portion of the earth undergoes sudden cooling. we know that happens. another change occurs-- and we know this happens as well-- with a not-perfect revolutionary period

of the earth's rotation around the sun. it's more elliptical. and at certain stages, it becomes more elliptical than at other times. we know that happens. the theory for the global cooling of the earth in the formation of an ice age is that on occasion both the wobble and the non-circular rotation or revolution of the earth's passage around the sun occur together. and we get global cooling that is unique to that time. recent data support this orbital explanation for ice ages. drill cores from deep-sea sediments contain fossils of microscopic plants that were sensitive to ocean temperatures, indicators of past ice ages and the warm times in between. using these fossils, scientists have been able to chart the temperature changes in the world's oceans

going back nearly a half million years. scientists have also shown that these changes can be correlated to variations in the distance between earth and sun, which occur with regularity. but these cyclic changes in earth-sun geometry have been going on for the entire history of earth, and glacial epochs are uncommon events. perhaps the positions of the continents themselves play a role in triggering ice ages. throughout earth's history, plate tectonics have shifted continents, changed their shapes, and altered the pattern of ocean currents around them. all of these are critical factors in regulating earth's climate. and if land masses are near the poles, the can support ice sheets that could not grow otherwise. so, ice ages could be triggered

by a combination of earth's orbital pattern and the movement of land masses into polar latitudes. the last ice age left striking marks still visible on the landscape. coastlines the world over shifted because much of the water that had been available to the oceans was frozen instead. we see that sea level dropped as much as 300, 400 feet below present-day positions. and with that dropping of sea level, all the streams of all the continents of the world, being that much higher above present-day seas and needing to move down toward the seas, started eroding, digging down, running faster. so all the rivers begin to run faster throughout the world. there was more erosion due to streams. as the last ice age ended, melting ice caps discharged vast quantities of water into the ocean,

flooding the lower valleys of these streams. many have become inlets, harbors, or estuaries. some have filled with sediment, becoming flat coastal valleys. the retreating glaciers have had another though less obvious effect on the landscape. the other big effect happened at the end of the ice age. as the ice began to melt, a tremendous weight was taken off the continents of the world, and the land masses began to rise. we've already seen how sea level has dropped during the ice age, and now at the end, the lands begin to rise. the hudson bay region has risen as much as 400 feet, and it's still rising today. across the interiors of continents, the ice sheets left moraines hundreds of kilometers long. streamlined mounds of till, known as drumlins, were sculpted by flowing and melting ice.

thousands of lake-filled basins called kettles were caused by the melting of ice buried in till. sinuous ridges of sediment left by subglacial streams, known as eskers, wind across the land. most important to our modern civilization, however, are the windblown deposits of glacial silt, or loess, which have weathered to form rich farmland soil. among the most profound effects of the ice ages on our world was the formation of land bridges. the dropping sea level exposed low-lying coastal areas, in some cases linking land masses that are presently separate. the most famous land bridge between asia and north america allowed animals, including humans, to migrate between the two continents.

evidence of ice ages is not only recorded by the landscape. scientists have also discovered important clues to earth's past within glaciers themselves. glaciers contain within their stratographic layers all of the material that fell with the snowfall that it incorporated within it. one could liken glaciers as a depositional environment, a glacial environment. in the deposits are contained events of earth history that occur simultaneously during the depositional process. you see, ice sheets are essentially atmospheric processes. practically any material passing through the atmosphere may be trapped in glaciers as they form. pollen, volcanic ash, and meteorites have been discovered in the ice. even bubbles of ancient air have been found,

representing a valuable record of earth's past atmosphere and climate. since the early 1970s, an international project has been underway to drill into greenland's vast continental glacier and obtain ice core samples. called the greenland ice core program, this effort involves scientists from countries around the world. the prime difficulties in a field operation such as drilling in ice relate primarily to the climate, the weather. it's not the best place to work, but it's overcome somewhat by the technique used today to excavate trenches and cover the tops of the trenches over with some form of a material and to work, if you will, underground, with power sources available and so forth. so we can eliminate the inclement weather problems

and work on a 24-hour basis. it does take up to three years to drill through bedrock at any location, whether greenland or antarctica. that length of time requires a lot of stamina from the crew involved. the cores from greenland date back over 100,000 years. the age of the ice is determined by counting bands which form seasonally. high density ice bands are typical of winter months, low density of summer. in deep ice, the bands are compressed together, making them impossible to see, so chemical methods, including carbon-14 dating, are used instead to determine the age of the ice. not all of the ice core analysis is done in the field. ultimately the samples are carefully packed and transported to several research institutions.

one of the foremost centers for the study of ice cores is the state university of new york at buffalo. an important goal of this project is to determine the composition of gases trapped in the ice. in general, the content of nitrogen and oxygen in bubbles shows little change relative to our modern atmosphere. but the levels of such important greenhouse gases as methane and carbon dioxide rise significantly in bubbles trapped after the last ice age. scientists aren't sure whether the rise in greenhouse gases ended the ice age or the end of the ice age simply allowed these gases to increase because of greater biological activity. another indicator of past climate is stable oxygen isotopes, which are atomic variations of the element oxygen that do not decay.

these isotopes are frozen within the ice itself at the time of its formation. we have a very interesting specimen here. yes. [langway] the 0-18/0-16 ratio of a snow sample or ice sample in a core reflects the temperature at which the precipitation formed in a precipitating cloud above. this is preserved in the ice cores, and by measuring continuously the 0-18/0-16 ratios using mass spectrometers, one determines summer and winter layers. by providing data about past temperatures, stable isotope analysis has helped answer a longstanding question about the ice ages-- whether or not they occur simultaneously in both the northern and southern hemispheres. [langway] i think the major contribution of stable isotope analyses in ice cores,

if you look at it from a bipolar point of view-- the antarctic and the greenland ice cores-- the results from both records show us that approximately 10,700 years ago, the ice age ended in both north and southern hemispheres. it wasn't too many years ago that climatologists and meteorologists were even sure that wind systems cross the equator. glaciers also preserve evidence of human impact on the atmosphere. since the start of the industrial revolution, the levels of carbon dioxide, sulfate, and other pollutants have sharply increased in glacial ice. [langway] the industrial revolution, which is attributed to occurring in the early 1800s-- 1820, 1830-- produced a major change in the gaseous composition

of the atmosphere, and this was brought about primarily by fossil fuel, high sulfur-content fossil-fuel consumption, coal, soft coal, and the productivity of the factories and what have you. the degree to which industrial activity is affecting our global climate remains controversial. understanding natural climate cycles through ice-core research will help put human impact in perspective. it will also help us make the adjustments needed to sustain our civilization in a world of continual, dramatic change. glaciers and glaciation are among the first subjects that geologists attempted to study, and understanding them is more important than ever. global climate is linked with glaciation, and changes in glaciers are used to make long-term predictions of rainfall patterns and of extreme weather,

such as hurricanes and drought. understanding the cause of glacial cycles is also critical to the current debate over global warning and the greenhouse effect. our interglacial climate is currently undergoing a warming phase, which is, at least in part, natural. some of this warming, however, may be due to the 25% increase in atmospheric carbon dioxide that has occurred since 1850. only by understanding the natural process of interglacial warming can we accurately monitor and predict any human impact on the climate. glacial ice volume is the principal factor controlling sea level. sea level rose about 12 centimeters over the last century, again due to the warming of the interglacial climate and retreat of the glaciers worldwide. but the current rate of rise is three times as fast. estimates of sea level rise over the next century vary from 30 centimeters to over 2 meters. this may not sound threatening, but 30 centimeters of sea level rise would correspond to 500 meters of coastal flooding

in some areas of the world. so glaciation is tied to the future of the world's great coastal cities. glaciers are responsible for fascinating land forms and for cycles of great change on the earth's surface. the regular cycles of glacial advance and retreat are in a sense the pulse of the earth, or a clock for measuring portions of geologic time. but even more important, glacial cycles contain vital clues, clues to the conditions that future inhabitants of the earth will someday inherit. captioning performed by the national captioning institute, inc. captions copyright 1991, the corporation for community college television

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when we look at a sunset, we see waves of light energy that have traveled an immense distance to reach our eyes. when we look at an ocean, we see waves of water energy that may have journeyed thousands of kilometers to reach our shores. most waves derive their energy from the wind. as the wind blows over the ocean, some of its energy is transferred to the surface, forming waves that move through the water. and it is in large part the power of these waves that makes the coastal environment such a dynamic place. coastal areas are among the most beautiful and desirable places anywhere on earth.

the coast and coastal land forms like this beach are the result of a dynamic interaction between two competing geologic agents-- the rocky land masses and the energy of the ocean. people tend to think of these as separate and independent from one another, but by ignoring the intimate connection between land and sea, they fail to realize that this delicately balanced system is subject to continual change. building walls and boardwalks and homes on a shifting coastline is a gamble with nature that sometimes pays off with disastrous consequences. clearly, then, the coast is a part of our world that needs to be observed and understood. consider the waves, for example. their rhythmic motion and sound has made watching them a popular pastime, yet few people have a real understanding of what a wave is and how it works. understanding ocean waves is vital to predicting their impact on not only the beach environment but on coastal development. when a wave approaches the beach,

it's not the water itself that's advancing, but a surge of energy which is moving through the water. it's like the ripple that runs across a field of grain when the wind blows. the individual stalks don't run across the field. they simply bend as the wind strikes them. or take the wave at a football game, which creates the illusion that the spectators are rippling around the stadium, when all they're actually doing is standing up and sitting down. the same principle applies to water waves. consider what happens to a floating object as a wave of energy passes through the water. that object tends to stay in the same place, tracing a circular motion as it bobs up and down. the individual particles composing the wave behave in a similar way.

as the crest of the wave arrives, it lifts the particle up and forward, and then, when the trough of the wave follows, the particle falls down and backward. like the stalk of grain or the football fan, the particle returns to its original position after the disturbance has passed. at the water's surface, the circular orbit of the water particle has a diameter that is roughly equal to the height of the wave. as one looks below the surface, however, the orbit gets smaller and smaller until there is virtually no motion of water at all. the downward limit of wave motion in the water is called the wave base, and it's directly related to how far apart the waves are at the surface.

the depth of the wave base is equal to about half the wavelength, which is the distance between the crests of two waves. as the wave approaches the shore and the water becomes shallower, the sea floor intersects the wave base, confining the wave energy. the wave now starts to slow down as the sea floor begins to interfere with the orbital motions. this forces the wave up and shortens its length, because waves behind it, still in deeper water, are advancing faster and begin to overtake it. as this happens to a succession of waves, they bunch up like cars in a traffic jam. as the bottom of each wave is slowed by the frictional drag of the sea bed, the top continues to surge forward,

making the waves steeper and steeper. eventually, this steep front can no longer support the wave, and it breaks into surf. perhaps the ultimate ocean wave is the seismic sea wave, otherwise known as a tsunami. tsunamis can strike coasts without warning. with wave heights sometimes exceeding 30 meters, these waves have a potential for death and destruction that makes them the subject of legend throughout the world. unlike ordinary wind-generated waves, tsunamis are caused by a much more powerful force-- earphquakes. undersea and coastal earthquakes can cause the ocean floor to shift suddenly. this movement of the ocean floor displaces a vast volume of the overlying water, creating these unusual waves. tsunamis are tremendously fast-moving, some traveling in excess of 800 kilometers per hour. the wavelength of a tsunami may be 150 kilometers, and so the movement

of the water particles within the wave will stir up deep-sea sediments even in the mid-ocean. remarkably, however, such a tsunami may measure only a meter or so high in the open ocean, but as tsunamis approach the coast, they bunch up and rise, monsterlike, from the sea. in a few minutes, tsunamis can completely devastate a coastal community. one coastal community that experienced the crushing power of a tsunami firsthand was hilo, hawaii. on april 1, 1946, following an earthquake off the coast of alaska, one of the most destructive tsunamis of modern times sped across the pacific and obliterated the entire shore zone at hilo. the death toll that day was 159. fortunately, tsunamis are not everyday events, but even ordinary waves have some impact on the shoreline.

one very important process at work here is refraction, the bending of wave fronts as they approach the shore. when a wave approaches the shore at an angle, the near shore stretch of wave front reaches the shallow water first and is therefore slowed down first. this local decrease in velocity causes the wave front to bend or refract because the deeper water portion of the wave continues to move at its original speed. as a consequence of this refraction, the waves near shore tend to approach the coast nearly head-on, while those in deeper water continue along their original course. wave refraction has its greatest effect on irregular shorelines with deep bays and projecting headlands. waves are refracted towards headlands,

smashing into them from both sides. at the same time, they are spread out in bays. in other words, wave energy is concentrated on headlands and dispersed along the shoreline of bays. the net effect of refraction on irregular coastlines is to straighten them out. as the waves crash against the headlands, they erode sediment, then deposit it as sand in the bays. so the waves perform a double action-- simultaneously wearing away the headlands and filling up the bays. the erosion of coastal headlands is by no means the only source of sand. most beach sand comes from sediment that is brought down to the ocean by rivers and streams. once the sand reaches the ocean, the waves distribute it along the coastline.

this occurs as a result of wave movement up onto the sloping part of the beach, then back down again. each cycle of wave movement carries particles up and down the beach slope. because waves usually break at a slight angle to the shore, the grains of sand in this cycle are gradually worked along the shoreline in a zigzag path. sand gets moved along the beach face by waves approaching the coastline at an angle, and when the waves break, they have the momentum from their falling forward at that angle, so the waves rush up the beach face in the swash zone at an angle, but then gravity's going to pull that water straight back down the beach face. what you and i see is kind of an arc shape of water swashing up and then going straight back down, and the result is that as this occurs thousands of times a day, the sand moves in a zigzag motion

up and down the beach face. the yellow dye shows this movement. this flow of water along the shoreline is known as the longshore current. sand spits and bay-mouth bars are common products of longshore currents. what happens is that the sand is being carried along the coastline, the beach sand, and when the coastline reaches, say, a right-angle turn, an abrupt bend, the beach will tend to be carried still by that longshore current, straight along the coastline, so that the beach will start building out, creating a extension of the beach that will not necessarily follow the bend in the coastline. in this case, a sand spit has formed off the end of this breakwater. this wave tank shows how the sand spit built up. the waves strike the breakwater at an angle

and bend around it into the harbor. when sand is added, the waves carry it into the harbor, where it builds into a spit behind the breakwater. to prevent the harbor from being sealed off and the beach beyond from being deprived of sand, engineers installed a dredge to pump the sand back into the longshore current by picking it up in the harbor and dumping it further down the coast. not only do beaches change continuously as sand is moved through them by the longshore current, but seasonal changes occur as well. the beaches change from season to season. by summertime, the waves are fairly low and gentle, and that has a tendency to drag sand towards the beach and build up the beach and make it broader, wider, and as it piles up,

the sand has a fairly gentle slope. in the wintertime, though, the larger waves, more energetic waves, pick up that sand, tend to move it offshore, and store it in large sand waves, almost like underwater sand dunes. and so the beach becomes very narrow. what sand is there is very, very steep in slope, and most of the beach is really located offshore, finding a more stable position under the bigger storm waves. the beach is just one part of a much larger system that regulates the formation, supply, and deposition of sediment along the shore. this system includes the mountains, where weathering processes turn rock into sediment, the rivers, which transport that sediment to the coast, and coastal processes, like the longshore current, that redistribute the sediment along the shore. as we've seen with breakwaters, however, people can easily disrupt

the natural balance of this system and alter its ability to operate normally. dams are another example of our attempts to control natural processes. these structures serve a variety of valuable functions-- the generation of hydroelectric power, the establishment of lakes for recreational purposes, and in this case, flood control and the storage of water for drinking and irrigation. despite their value, dams are not without significant drawbacks. sediment that's normally carried down-river to the beaches, is trapped in the reservoir instead. beaches that don'n't receive a steady supply of river sediment will soon disappear. it is tempting to cast people as the villains in this apparent conflict with nature, but the issue's not that simple. what would happen if we didn't dam rivers? would we be willing to risk the exposure to catastrophic floods and to give up the electrical power and the fresh water dams provide? if not, is the damage they cause to coastal property and to the beach environment too high a price to pay?

these are difficult choices, and there are no perfect solutions. problems often arise as a result of special circumstances. during severe storms, for example, crashing waves can batter coastlines. such storms occur only once every few decades, but in the quiet periods in between, people tend to ignore the historical record of erosion and build along the edges of the shore. to protect the ocean-view homes and hotels that are perched atop sea cliffs and along beaches, sea walls have been erected that reflect the energy of the waves away from the coast and slow down erosion. however, what may have sounded fairly straightforward in theory has become quite controversial in practice. those in favor of sea walls argue that the cliffs must be protected to safeguard the real estate above them.

those who are opposed maintain that in the long run, sea walls do more harm than good because they represent a threat to the beach itself. coastal erosion is a natural process, and as we begin to put houses on the edges of coastlines, we're concerned about losing some of those homes, so you want to slow the erosion. well, you're trying to slow something that's quite natural. when you do that, you upset the balance of things. sea walls, to limit erosion, are also cutting sand supply, so putting in a sea wall will, for a short time, lessen the amount of erosion, but what the result is is that sediment is no longer there to be taken to the beaches. the beaches receive part of their sand supply from cliff sides. as you slow down the erosion of cliff sides, then the beaches are losing a source, an important source of their sediments.

another problem is that the flat surface of a sea wall reflects much of the wave energy directly back toward the beach. unfortunately, this can erode the sand at the foot of the wall, eventually undermining it. at the scripps institution of oceanography, scientists deal with this controversial issue on a continuing basis. scott jenkins of scripps center for coastal studies is one of those involved in the design of sea walls, breakwaters, and other coastal structures. about 20 degrees of obliquity. all right. the goal is to design structures that do the job with a minimal negative impact on the environment. jenkins and his colleagues use a wave tank and scale models to test their designs-- in this case, a breakwater. sensors placed around the tank measure the heights of the waves both inside and outside the breakwater,

giving jenkins an indication of its effectiveness at reducing wave energy. data from the experiment is fed into a computer, allowing the scientists to refine and retest the design before an actual prototype is built. when designing the sea wall, jenkins and his colleagues turned to nature for inspiration. the irregularly shaped surfaces of sea cliffs and coral reefs reflect a minimal amount of wave energy, so the scripps scientists decided to incorporate nature's energy-absorbing design into their sea wall. so far, this wall has been a success. the property has been protected from further erosion without destroying the beach at the wall's base. but while jenkins is committed to building the most effective sea walls he can, he recognizes that they are only a short-term solution, and he is sensitive to the arguments of those

who oppose attempts to redirect or in any way modify natural processes along the coast. there's a wide variety of environmental groups, and there's a wide range of government officials and university professors who oppose construction and structural intervention on the shoreline, and the reason is philosophical-- that we want to preserve the shoreline in its natural state. those who are going to lose property if erosion continues also have a concern, and those are the people who, of course, are going to favor these structures. and my personal belief is we should adopt the policy of maintaining the coastline in its natural state, and a large part of that policy would involve bypassing of sediments around dams and preventing further encroachment of coastal structures in the near shore area.

and then i would say, having made those fixes, let the system adjust to its own equilibrium. there's far too much energy out there for man to compete against. jenkins contends that doing a better job of transporting sediment around dams would be an important long-term solution to the problem of beach erosion. basically, it's an earth-moving problem, and we already have a well-developed technology in earth moving. now, in southern california and in many other areas as well, there are seasonal fluctuations in the lake level. typically, lake levels are low in the summer, but whatever season they're low, earth-moving equipment can come in and excavate these sands from the dry foreshore area. the foreshore deltas in these reservoirs contain most of the beach-size sand, and these will be high and dry when lake levels are low.

so they can be collected with standard earth-moving equipment and trucked directly to the beach or reintroduced to the stream beds downstream. if there is technology and engineering available for transporting sand around dams, why isn't this being done? one reason may be that many scientists originally rejected the idea that dams actually contribute to erosion. but that is no longer the case. the problem currently seems to be that the value of sand as a coastal resource may still not be fully recognized. a lot of this sand is already excavated by sand and gravel companies for construction material. it should be treated as a public resource and a fair market value paid for it. for instance, people on the beach would be willing to pay many dollars per cubic yard for nourishment sands, sands that sand and gravel companies haul away

at just a fraction of a dollar a cubic yard. so this needs to be regulated just like water-- treating sand as a public resource. regardless of how the battle over sea walls and sediment supply eventually turns out, coastal dwellers will always have to deal with incursions from the oceans. in addition to problems caused by crashing waves, there are a number of other factors that affect the level of the water. the most familiar is the action of the tides. tides are primarily the work of the moon, and to a lesser degree, the sun. as the moon orbits the earth, it exerts a powerful gravitational pull. this causes the ocean on the side of the earth facing the moon to bulge out slightly. another tidal bulge

occurs on the other side of the planet as water lags behind due to weaker gravitational attraction from the moon. these bulges create a high tide. high tides can create tremendous havoc, especially if they're combined with violent storms. this is what happened in 1970 in bangladesh when a cyclone combined with a spring tide flooded the delta of the ganges river, drowning a quarter of a million people. but such tidal disasters are rare. most of the time, the twice-daily ebb and flow of the tides only brings about small, brief changes in the water level, but there's also a long-term change going on all the while.

since the peak of the last ice age tens of thousands of years ago, melting glaciers have spilled immense quantities of water into the oceans, causing a rise in sea level of over 100 meters. such a global change in the volume of water in the ocean is known as a eustatic change. although today's sea level is much more stable than it was at the time the ice age ended, a small eustatic change is still going on. the glaciers of greenland and antarctica are continuing to melt faster than they grow. this causes a small but steady rise in sea level worldwide. however, between now and the year 2100, there may be a significant increase in sea level due to the so-called greenhouse effect.

carbon dioxide and water vapor in the atmosphere act like the glass of a greenhouse. they let in the sunlight, but trap some of the reradiated infrared heat energy. without this greenhouse effect, the earth would become too cold to support human life. but since the industrial revolution began to mechanize our world in the late 18th century, we've been adding tremendous quantities of carbon dioxide to the atmosphere by burning fossil fuels. the first of these was coal-- the fossil remains of vegetation. burning coal produced the steam which powered steamships, factories, and locomotives. it also released vast amounts of carbon dioxide, which until then had been stored underground for millions of years. since the early days of the industrial revolution, the world's reliance on fossil fuels has increased dramatically.

today, these fuels include not only coal, but gasoline and oil. if we continue to burn these at our present rate, the amount of carbon dioxide in the atmosphere will increase significantly. this could magnify the greenhouse effect to such an extent that air temperatures could rise by several degrees and accelerate polar ice melting, which would result in a risise in sea level of a few meters. this may not seem like much, but it would be enough to flood many of the world's coastal communities. although the coastline appears to be a stable and permanent fixture of the landscape, it's, in fact, a place of inevitable change. when people choose to live here, they become subject to that change and run the risk of losing everything,

either suddenly or steadily over time. permanent protection for coastal development simply doesn't exist, and many protection schemes actually degrade the quality of the beach that attracted people here in the first place. as a result, it's becoming increasingly important to develop a wise coastal management policy that incorporates the most current scientific knowledge with the needs of the environment and of our communities. it's clear that there's a significant role for geologists, and indeed, for all of us to play in learning to protect the coastline for ourselves and for future generations. captioning performed by the national captioning institute, inc. captions copyright 1991 the corporation for community college television

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