Evolution of Glacial & Coastal Landforms

GEOMORPHOLOGY 201 READER

PART IV : EVOLUTION OF GLACIAL & COASTAL LANDFORMS

A landform is a small to medium tract or parcel of the earth’s surface.

After weathering processes have had their actions on the earth materials making up the surface of the earth, the geomorphic agents like running water, ground water, wind, glaciers, waves perform erosion. The previous sections covered the erosion and deposition and hence landforms caused by running water (fluvial geomorphology) and by wind (aeolian geomorphology).

Erosion causes changes on the surface of the earth and is followed by deposition, which likewise causes changes to occur on the surface of the earth.

Several related landforms together make up landscapes, thereby forming large tracts of the surface of the earth. Each landform has its own physical shape, size, materials and is a result of the action of certain geomorphic processes and agents. Actions of most of the geomorphic processes and agents are slow, and hence the results take a long time to take shape. Every landform has a beginning. Landforms once formed may change their shape, size and nature slowly or fast due to continued action of geomorphic processes and agents.

Due to changes in climatic conditions and vertical or horizontal movements of landmasses, either the intensity of processes or the processes themselves might change leading to new modifications in the landforms. Evolution here implies stages of transformation of either a part of the earth’s surface from one landform into another or transformation of individual landforms after they are once formed. That means, each and every landform has a history of development and changes through time. A landmass passes through stages of development somewhat comparable to the stages of life — youth, mature and old age.

The evolutionary history of the continually changing surface of the earth is essential to be understood in order to use it effectively without disturbing its balance and diminishing its potential for the future. Geomorphology deals with the reconstruction of the history of the surface of the earth through a study of its forms, the materials of which it is made up of and the processes that shape it.

Changes on the surface of the earth are due mostly to erosion by various geomorphic agents. The process of deposition also causes changes in the surface of the land by covering the land surfaces and filling basins, valleys or depressions. Deposition follows erosion and the depositional surfaces too are ultimately subjected to erosion. Running water, groundwater, glaciers, wind and waves are powerful erosional and depositional agents shaping and changing the surface of the earth, which in turn are aided by weathering and mass wasting processes. These geomorphic agents acting over long periods of time produce systematic changes leading to sequential development of landforms. Each geomorphic agent produces its own assemblage of landforms. Not only this, each geomorphic process and agent leave their distinct imprints on the landforms they produce. You know that most of the geomorphic processes are imperceptible functions and can only be seen and measured through their results. What are the results? These results are nothing but landforms and their characteristics. Hence, a study of landforms, will reveal to us the process and agent which has made or has been making those landforms.

As the geomorphic agents are capable of erosion and deposition, two sets — erosional or destructional and depositional or constructional — of landforms are produced by them. Many 2 varieties of landforms develop by the action of each of the geomorphic agents depending upon especially the type and structure i.e. folds, faults, joints, fractures, hardness and softness, permeability and impermeability, etc. There are some other independent controls like

(i) stability of sea level; (ii) tectonic stability of landmasses; (iii) climate, which influences the evolution of landforms.

A disturbance in any of these three controlling factors can upset the systematic and sequential stages in the development and evolution of landforms. In the following pages, under each of the geomorphic regimes i.e. groundwater, glaciers, waves, and winds, first a brief discussion is presented as to how landmasses are reduced in their relief through erosion and then, the development of some of the erosional and depositional landforms is dealt with.

GLACIAL GEOMORPHOLOGY

Masses of ice moving as sheets over the land (continental glacier or piedmont glacier if a vast sheet of ice is spread over the plains at the foot of mountains) or as linear flows down the slopes of mountains in broad trough-like valleys (mountain and valley glaciers) are called glaciers. The movement of glaciers is slow unlike water flow. The movement could be a few centimetres to a few metres a day or even less or more. Glaciers move basically because of the force of gravity.

What is a glacier?

A glacier is a large body of ice that formed on land from the compaction and recrystallization of snow, surviving year to year, and moves downhill due to gravity. They can be masses of ice moving as sheets over the land or as linear flows down the slopes of mountains in broad trough- like valley. The movement of glaciers is slow, unlike the flow of water, and could be a few centimetres to a few metres a day or even more.

A glacier starts when snow—delicate, feathery crystals of ice—falls in areas above the snow line, which is the elevation above which snow can form and remain all year. It takes snow on top of snow on top of more snow to create a glacier, followed by a long time of compaction. Snow will compact to 1/10 of its thickness as ice. In polar regions, annual snowfall is very low: it may take snow about 1 000 years to turn into ice, because the air is too cold to hold much moisture. Over time, the unmelted snow compresses beneath additional new snow layers added to it, forcing snow crystals to recrystallize, forming grains similar in size and shape to cane sugar. Grains grow larger with further snowfalls and compression, forcing out the air in the spaces between them. The compressed snow becomes granular firn or névé within a period of only a few years. Density of snow is 1/10 that of liquid water, with firn’s density being about half that of water; firn is therefore approximately five times heavier than snow. When the thickness of the overlying snow is about 50m or more, the firn turns into a solid mass of glacial ice. Glaciers are not landforms, but their action creates landforms; this action is termed glaciation. Glaciers are therefore landscape-forming agents, with glacial ice being an active agent of erosion as they scour the landscape and "carve out” landforms. Glaciers also deposit rocky material, creating even more features. At present the work done by glaciers is slow and restricted to certain areas of the planet, but there is plenty of evidence of past Ice Age glaciers which once covered almost one third of the planet's land surface. These extensive continental glaciers shaped the distinctive features of the northern landscapes of North America and Europe. 3

Erosion by glaciers is tremendous because of friction caused by the sheer weight of the ice. The material plucked from the land by glaciers - usually large-sized angular blocks and fragments - get dragged along the floors and sides of the valleys and cause great damage through abrasion and plucking. Glaciers can cause significant damage to even un-weathered rocks and can reduce high mountains into low hills and plains.

As glaciers continue to move, debris gets removed, divides get lowered and eventually the slope is reduced to such an extent that glaciers will stop moving leaving, only a mass of low hills and extensive outwash plains along with other depositional features. Figures 4.13 up to 4.16 show various glacial erosional and depositional forms described in the text.

Types of glaciers We identify three main types of glacier, which are –  Continental glaciers  Confined glaciers, such as mountain or alpine glaciers  Transitional glaciers, such as the piedmont glaciers; a form between the other two types.

Continental ice sheets cover large landmasses over 1000s of square kilometers (km2) and are unconfined at their margins, i.e. the glacier covers the topography. The Antarctic ice sheet is the largest single mass of ice on Earth, covering an area of almost 14 million square kilometres and contains over 29 million cubic kilometres (km3) of ice. Ice caps are similar, but smaller than ice sheets and usually cover less than 50 000 km2. Most of the glaciers in Iceland, such as the Vatnajokull that covers most of south-eastern portion of the country, are ice caps.

Figure 4.4 : Ice sheets and ice caps : Antarctica & Greenland are covered by Continental ice sheets while Iceland has a number of smaller smaller ice caps (landmasses not to scale).

Confined glaciers Alpine glaciers develop in mountainous regions. Unlike continental glaciers, their location and movement is restricted by topography - ridges and peaks . Glaciers are moving ice, ranging from small patches to ice sheets of millions of km2. The largest alpine glacier is the Siachen Glacier in the Karakoram Mountains of Pakistan. Measuring 75 km in length, it contains more than 56.7 km3 of ice. From smallest to largest, the various alpine glaciers are:  niche and bench glaciers  cirque glaciers  valley glaciers  ice fields, which are precursors to ice caps 4

Figure 4.5 : Clockwise from left: Valley glacier, Cirque glacier and niche and bench glaciers

Figure 4.6A : Heiltskuk Icefield, British Columbia

Transitional glaciers Transitional glaciers is the term used for glaciers that are in part unconfined but yet controlled by local topographic conditions. Piedmont glaciers are glacial fans formed where confined valley glaciers emerge and diverge from the mountain fronts, as illustrated in Figure 4. 6B below.

Figure 4.6B : Piedmont glaciers in southern Axel Heiberg Island, Canadian Arctic.

Glacial flow

Glaciers flow as a result of the gravity deforming ice and pulling it downhill

Glacial flow Glacial flow is largely accommodated by:  •internal plastic deformation (creep)  •basal sliding  •bed deformation  •fracturing where stresses exceed the ability of ice to deform plastically

The rate of plastic deformation is greatest near the bed.

Basal sliding: 1.enhanced basal creep = stress on the upstream side of an obstacle results in locally high strain rates causes ice to accelerate around the obstacle.

 Bed deformation: as sediment beneath a glacier deforms, it moves it with the ice sheet.

 Fracturing stresses exceed the ability of ice to plastically deform 7

EROSIONAL LANDFORMS

Cirque

Cirques are the most common landforms in glaciated mountains. They are often found at the heads of glacial valleys. See Figure 4.5 – top right, as well as Figure 4.7 below. The accumulated ice cuts these cirques while moving down the mountain tops. They are deep, long and wide troughs or basins with very steep concave to vertically dropping high walls at its head as well as sides. A lake of water can be seen quite often within the cirques after the glacier disappears. Such lakes are called cirque or tarn lakes. There can be two or more cirques one leading into another down below in a stepped sequence.

Figure 4.7 : Cirques and tarn lakes

Horns and Serrated Ridges

Horns form through head ward erosion of the cirque walls. If three or more radiating glaciers cut headward until their cirques meet, high, sharp pointed and steep sided peaks called horns form. The divides between cirque side walls or head walls get narrow because of progressive erosion and turn into serrated or saw-toothed ridges sometimes referred to as arêtes with very sharp crest and a zig-zag outline.

Figure 4.8 : Aretes or knife-edged ridges

The highest peak in the Alps, Matterhorn and the highest peak in the Himalayas, Everest are in fact horns formed through headward erosion of radiating cirques. As cirques cut back (headward), the remaining arêtes meet at the highest remaining peak – which is called a horn.

Figure 4.9 : Horn

Glacial Valleys/Troughs

Glaciated valleys are trough-like and U-shaped with broad floors and relatively smooth, and steep sides. The valleys may contain littered debris or debris shaped as moraines with swampy appearance. There may be lakes gouged out of rocky floor or formed by debris within the valleys. There can be hanging valleys at an elevation on one or both sides of the main glacial valley. The faces of divides or spurs of such hanging valleys opening into main glacial valleys are quite often truncated to give them an appearance like triangular facets. Very deep glacial troughs filled with sea water and making up shorelines (in high latitudes) are called fjords/fiords. 9

Figure 4.10 : U-shaped glacial valleys. When submerged, they are termed fjords.

Finger lakes in up-state New York formed by continental ice sheet

Figure 4.11 : Finger lakes

Striations (glacial grooves)

Figure 4.12 : Striations in Central Park

Figure 4. 13 : Erosional glacial landforms – during and after glaciation

DEPOSITIONAL LANDFORMS

The unassorted coarse and fine debris dropped by the melting glaciers is called glacial till. Most of the rock fragments in till are angular to subangular in form. Streams form by melting ice at the bottom, sides or lower ends of glaciers. Some amount of rock debris small enough to be carried by such melt-water streams is washed down and deposited. Such glaciofluvial deposits are called outwash deposits. Unlike till deposits, the outwash deposits are roughly stratified and assorted. The rock fragments in outwash deposits are somewhat rounded at their edges. Figures 4.14, 4.15 & 4.16 show a few depositional landforms commonly found in glaciated areas.

Moraines

They are long ridges of deposits of glacial till. Terminal moraines are long ridges of debris deposited at the end (toe) of the glaciers. Lateral moraines form along the sides parallel to the glacial valleys. The lateral moraines may join a terminal moraine forming a horse-shoe shaped ridge (Figure 4.14). There can be many lateral moraines on either side in a glacial valley. These moraines partly or fully owe their origin to glaciofluvial waters pushing up materials to the sides of glaciers. Many valley glaciers retreating rapidly leave an irregular sheet of till over their valley floors. Such deposits varying greatly in thickness and in surface topography are called ground moraines. The moraine in the centre of the glacial valley flanked by lateral moraines is called medial moraine. They are imperfectly formed as compared to lateral moraines. Sometimes medial moraines are indistinguishable from ground moraines.

Figure 4. 14 : Glacial moraine surrounding Jenny Lake & glacial moraines near Mamoth, CA 11

Glacial erratic

Figure 4. 15 : Some glacial depositional landforms after glacier retreat

End moraine = terminal moraine

Figure 4.16 : Glacial landscape with various depositional landforms

Eskers

When glaciers melt in summer, the water flows on the surface of the ice or seeps down along the margins or even moves through holes in the ice. These waters accumulate beneath the glacier and flow like streams in a channel beneath the ice. Such streams flow over the ground (not in a valley cut in the ground) with ice forming its banks. Very coarse materials like boulders and blocks along with some minor fractions of rock debris carried into this stream settle in the valley of ice beneath the glacier and after the ice melts can be found as a sinuous ridge called esker.

Figure 4.17 : Various examples of eskers and their formation

Outwash Plains

The plains at the foot of the glacial mountains or beyond the limits of continental ice sheets are covered with glacio-fluvial deposits in the form of broad flat alluvial fans which may join to form outwash plains of gravel, silt, sand and clay.

Figure 4. 18 : Moraines (left) and outwash plain with braided stream (right) visible at retreating glaciers 13

Drumlins

Drumlins are smooth oval shaped ridge-like features composed mainly of glacial till with some masses of gravel and sand. The long axes of drumlins are parallel to the direction of ice movement. They may measure up to 1 km in length and 30 m or so in height. One end of the drumlins facing the glacier called the stoss end is blunter and steeper than the other end called tail. The drumlins form due to dumping of rock debris beneath heavily loaded ice through fissures in the glacier. The stoss end gets blunted due to pushing by moving ice. Drumlins give an indication of direction of glacier movement.

Figure 4.19 : Drumlins, drumlin fields and their morphology

Kame

Kame: Dude Hill Yellowstone Park, US

Figure 4.20 : Kames 14

COASTAL GEOMORPHOLOGY

Coasts are among the most beautiful and inspiring landscapes on the planet, whether they are scenes of torrential storms or serene calm. Throughout recorded history, humans have sought to live on or near coasts. Although coasts account for only 10 percent of Earth's land surface, they serve as home to two-thirds of the world's human population.

Coasts mark the area where dry land meets oceans or other large bodies of water and they are affected by the gravitational interactions of Earth, the Moon, and the Sun. Coastal processes are the most dynamic and hence of the most destructive environments found on Earth. They reflect the conflicting processes of erosion (the gradual wearing away of Earth surfaces through the action of wind and water) and deposition (the accumulation and building up of natural materials). Some of the changes along the coasts take place very fast, with erosion taking place in one season and deposition in another. Most of the changes along the coasts are accomplished by waves. When waves break, the water is thrown with great force onto the shore, and simultaneously, there is a great churning of sediments on the sea bottom. Constant impact of breaking waves drastically affects the coasts. Storm and tsunami waves can cause more far-reaching changes in a short period of time than normal breaking waves can over a long period. As the wave environment changes, the intensity of the force of breaking waves changes. Wave size is determined by: (1) the strength of the winds, (2) the length of the time the wind blows, and (3)the distance of open water across which the winds blow.

Other than the action of waves, coastal landforms depend upon

 the configuration of land and sea floor;  whether the coast is advancing (emerging) seaward or retreating (submerging) landward.

Assuming sea level to be constant, two types of coasts are considered to explain the concept of evolution of coastal landforms:

 high, rocky coasts (emerged coasts);  low, smooth and gently sloping sedimentary coasts (submerged coasts)

Let us first turn to the landscape forming agents of coastal areas.

Waves: agents forming coastal landforms

Most waves get energy and motion from the wind. Wind blowing over the surface of an ocean or other large water body creates friction along that surface, producing tiny ripples. Further pushed by the wind, these ripples combine and increase in size. The size of waves depends on the strength of the winds, the length of the time the winds blow, and the distance of open water across which the winds blow. Large waves may be created by strong winds blowing for long periods of time across large areas of water.

The highest part of a wave is called the wave crest. The lowest part of a wave between two crests is the wave trough. The vertical distance between the wave crest and the wave trough is the wave height. The horizontal distance between two wave crests or troughs is the wavelength. As a wave travels across an ocean or other body of water, the water particles in the wave move in circular patterns, in loops. These loops extend down underneath the surface of the water only one-half the distance of the wavelength. Water beneath that is not disturbed by wave motion. As the wave form advances across the surface, its energy moves forward, not the water itself. The water particles in the loops essentially return to their original position after the wave has passed.

As a wave enters shallow water near a shoreline, the lower loops in the wave begin to drag on the bottom. This causes the wave to slow down. As it does so, its wavelength decreases and its height increases. When the wave can no longer support its height, the wave breaks. Most of the energy it has carried across the ocean is then transferred into the churning, turbulent water known as surf. The powerful surf either crashes into a cliff or runs up a beach. The water moving up the beach is known as 15 swash; the return flow toward the ocean is called the backwash. The speed of the swash is greater than that of the backwash.

How waves move sediment by littoral drift: (a) Swash and backwash move particles along the beach in beach drift.

(b) Waves set up a longshore current that move particles by longshore drift.

Most of the sand and other sediments making up a beach come from weathered and eroded rock from the mainland that is deposited by rivers at the coast. Material eroded from cliffs at the head of a beach itself may be abraded form the beach. Beach sand may also contain fragments of smoothed and rounded shells of marine creatures, such as clams. Tropical beaches often consist entirely of shell and coral fragments.

Figure 4.21 : Sediment mobility along the seashore due to wave action & littoral drift

Beaches in areas of volcanic activity, such as Hawaii, can be black since the sand is created by the erosion of volcanic rock. Because sand is in constant motion, beaches are often referred to as "rivers of sand." Depending on the action of waves, sand on a beach may travel along the shore hundreds of feet a day.

When waves approach a shoreline, they rarely do so parallel or straight on. Most often, they do so at a slight angle. After a wave breaks, the swash runs up the shore at that slight angle. Because of gravity, however, the back-wash runs straight back to the water directly, without any angle. As a result, the water moves the sand along the beach downwind in a zigzag pattern. This movement is known as beach drift. In addition to beach drift, sand and other sediment is transported downwind along the beach in the longshore current, a current formed by the angled rush of waves that runs close to and almost parallel to the shoreline. The movement of material in this current is known as longshore drift. Over long periods of time, beach drift and long-shore drift may combine to transport sand and other material great distances, eventually forming coastal features such as spits.

Beaches are not fixed features; they are dynamic environments. They expand and contract depending on wave conditions. In winter and during storms, when wind and wave action are more powerful and frequent, erosion often reduces the size of beaches. In summer and during calm weather, gentle waves deposit sand, creating wider beaches. All sandy landforms along a coast— spits, bars, barrier islands, tombolos—are subject to the erosional or depositional action of waves.

Although they do not change as quickly, rocky coasts will eventually change. An irregular coastline of headlands and bays will be straightened by erosion. Wave action will cut away at headlands, quickly break up rock debris and other material, then deposit it along the shoreline of the bay.

Defining the coast, shore and shoreline

The line that marks the boundary between water and land is the shoreline. It constantly fluctuates because of the regular action of waves and tides. The shore is that strip of land bordering a body of 16 water that is alternately covered or exposed by waves or tides. The boundaries of the shore are marked by the shoreline at its furthest seaward (low tide) and furthest landward (high tide).

Figure 4. 22 : Defining coast and shore: the tide, shore, coast and littoral zone.

Tide is the periodic rising and falling of water in oceans and other large bodies of water in response to the gravitational attraction of the Moon and the Sun upon the Earth. Although the Sun is larger than the Moon, the Moon is closer to Earth and, therefore, its gravitational force is approximately 2.2 times greater. The gravitational pull of the Moon creates both high and low tides. A tidal bulge occurs in the oceans on the side of Earth nearest the Moon; at the same time, a second tidal bulge occurs on the opposite side of Earth. These bulges are high tides. At the same time the areas between the tidal bulges experience low tide. As the Earth rotates, the tides move over its surface. It takes approximately 24 hours and 50 minutes for a given point on Earth to make a complete cycle relative to the Moon, and two lunar tidal cycles occur during this time. Thus, the average time between high tides is 12 hours and 25 minutes. This is a generalized explanation of tides. They do not move evenly and predictably over Earth's surface. Variations in the depth of the oceans and the distribution of landmasses combine with other factors to produce highly complex tidal behaviour.

The coast and coastline begin where the shore ends at its high tide mark (farthest landward). The line between the coast and the shore at high tide is the coastline. The coast extends landward from the coastline to the first major change in terrain features, which may be miles inland. This could be a highland or a forest or some other type of terrain. Sometimes, the change between the coast and the adjacent terrain is not so distinct. . Coastal landscapes may be broadly divided into rocky cliffs and sandy beaches and dunes. All coasts experience a combination of erosion and deposition to varying degrees. Coasts are generally classified into two types: emergent and submergent

HIGH ROCKY or EMERGENT COASTS

Emergent coasts are those in which land formerly under water has gradually risen above sea level through geologic uplift of the land or has been exposed because of a drop in sea level. Currently, sea level around the world is rising by an average of 2.5 millimeter per year because glaciers and ice sheets are melting due to global warming (an increase in the world's temperatures thought to be caused, in part, by the burning of fossil fuels and the depletion of the ozone layer). So an emergent coast in the present-day is one that is rising on average more than 2.5 millimeter per year. Coasts along Scandinavia, New England, California, and Hawaii are examples of emergent coasts.

Figure 4.23 : The Californian coast is an emergent coast, a formerly submerged area which has gradually risen above sea level through either geologic uplift or a drop in sea level. Corbis Corporation.

Emergent coasts are typically dominated by cliffs or high, steep faces of rock. Because the land is rising in these areas, its landforms are subject to erosion. As waves break against a cliff, certain features are formed, depending on the hardness of the rock. Initially, wave action may cut an indentation, called a wave-cut notch, at the base of the cliff. When the notch becomes larger, rock in the cliff face above the notch loses support and falls into the water where it is broken up by the action of the waves. This process continues and the cliff slowly retreats inland. As it does so, a horizontal bench of rock remains beneath the waves at high tide where the cliff once stood. This feature is called a wave-cut platform, seaward of the sea cliff. Over time, as the land continues to rise, this platform may be elevated and a new cliff face formed. Waves gradually minimise the irregularities along the shore.

Figure 4.24 : Headland erosion and accompanying coastal landforms

In areas where cliff rock is alternately hard and soft, headlands and bays may form. A headland is an elevated area of hard rock that projects out into an ocean or other large body of water. When soft rock is eroded away between headlands, a curved inlet that holds a body of water known as a bay forms. Because of its location, a headland receives the brunt of wave attack (wave energy is spread out and weaker in bays). Erosion by water and wind may create distinct features such as sea caves, sea arches, and sea stacks in a headland. Sea caves arise when waves hollow out weak areas of rock in headlands. Waves may then erode the cave through the headland, or caves on either side of 18 the headland may meet. In either case, a sea arch is formed. Erosion of the arch continues until its top portion collapses, leaving a column of hard rock known as a sea stack standing detached from the sea cliff. Continual wave erosion eventually reduces the stack into a stump.

Example of Emergent Coast: England’s White Cliffs of Dover

The celebrated White Cliffs of Dover have been an important part of England's natural and social history. Located on the southeast coast of England facing the English Channel, the more than 91- meter high cliffs have provided a natural barrier to foreign invaders throughout history.

The cliffs are composed of chalk, which is a particular type of sedimentary limestone that forms a brilliant white rock formed mainly from coccoliths, the fossilized remains of tiny, single-celled marine organisms. The coccoliths that form the cliffs were first laid down over a large area of shallow sea, eventually piling into layers.

Figure 4.25 : The 91 m White Cliffs of Dover, on the coast of England, are composed of chalk. The soft rock has formed from the fossilized remains of tiny marine organisms called coccoliths. Corbis Corporation.

Before the last ice age some 10,000 years ago, England and France were linked by a chalk landmass. Over time, as sea levels rose, tidal movements eroded the mass, creating the English Channel. Layers of chalk still lie under the seabed of the channel, which has an average depth of 110 metres. White cliffs along the French coast match the White Cliffs of Dover. Chalk is a soft rock, and the cliffs naturally erode as waves cut into lower levels while rain seeps into and weakens upper levels. On average, the cliffs are receding at a rate of 1.3 centimetres a year.

LOW SEDIMENTARY or SUBMERGENT COASTS

Submergent coasts are those in which formerly dry land has been gradually flooded, either by land sinking or by sea level rising. Along low sedimentary coasts the rivers appear to extend their length by building coastal plains and deltas. The coastline appears smooth with occasional incursions of water in the form of lagoons and tidal creeks. The land slopes gently into the water. Marshes and swamps may abound along the coasts. Depositional features dominate.

With continuing erosion occurring along the coast, a good supply of material becomes available to longshore currents and waves, which deposit it as beaches along the shore and as bars (long ridges of sand and/or shingle parallel to the coast) in the nearshore zone. Bars are submerged features and when bars show up above water, they are called barrier bars. A barrier bar which gets keyed up to the headland of a bay is called a spit. When barrier bars and spits form at the mouth of a bay and block it, a lagoon forms. The lagoons would gradually get filled up by sediments from the land, giving rise to a coastal plain. The maintenance of these depositional features depends upon the steady 19 supply of materials. Storm and tsunami waves cause drastic changes irrespective of supply of sediments. Large rivers which bring lots of sediments build deltas along low sedimentary coasts.

Example of submergent coast: Cape Cod, Massachusetts, USA

On the East Coast of the United States, just south of Boston, Massachusetts, a large peninsula resembling a flexed arm extends 96.5 kilometers into the Atlantic Ocean (a peninsula is a piece of land that projects from a mainland into a body of water).

Considered a premiere vacation resort, Cape Cod features the longest uninterrupted sandy shore in New England.

Cape Cod was formed more than 15,000 years ago by glaciers that covered the area, deposited sediment, then retreated. Ever since, waves and currents near the shore have shaped and reshaped the peninsula.

They continue to do so in the present, transporting sand along the shore to create a variety of coastal landforms, including chains of barrier islands, curved spits, and dunes. Cape Cod's wide sandy beaches are maintained by material that is eroded from cliffs that border the beaches.

Figure 4.26 : Although glaciers originally shaped the unusual landscape of Cape Cod some 15,000 years ago, waves and nearshore currents continue to sculpt the 70 km coastline. Corbis Corporation.

During calm summer months, the amount of beach material that is eroded is equal to the amount that is deposited. In winter months, dominated by severe storms from the northeast, erosion takes over. Prevailing waves drag away sand and other material. Overall, land is lost. On average, the cliffs retreat by 1 meter every year. Structures such as walls and jetties built to protect housing and other development from erosion actually increased its rate. To protect this delicate environment from further development, over 17,440 hectares along the outer portion of Cape Cod were designated a national seashore in 1961.

EROSIONAL LANDFORMS

Coasts and shores are constantly changing. The shoreline moves with the waves and the tides. Rock is eroded away, and gravel and sand are deposited onshore, only to be swept back offshore. Storms batter coasts, and tides flood areas on a daily basis. The premiere forces that shape the coastal landscape, however, are waves.

Breaking waves exert great force. A 3-meter wave can produce a force of 2.1 kilograms per square centimetre. In addition to the pressure exerted by their impact, waves erode by scouring rock cliffs and other coastal features with rock fragments they carry.

Cliffs, Terraces, Caves and Stacks

Wave-cut cliffs and terraces are two forms usually found where erosion is the dominant shore process. Almost all sea cliffs are steep and may range from a few m to 30 m or even more. At the foot of such 20 cliffs there may be a flat or gently sloping platform covered by rock debris derived from the sea cliff behind. Such platforms occurring at elevations above the average height of waves is called a wave- cut terrace. The lashing of waves against the base of the cliff and the rock debris that gets smashed against the cliff along with lashing waves create hollows and these hollows get widened and deepened to form sea caves. The roofs of caves collapse and the sea cliffs recede further inland. Retreat of the cliff may leave some remnants of rock standing isolated as small islands just off the shore. Such resistant masses of rock, originally parts of a cliff or hill are called sea stacks. Like all other features, sea stacks are also temporary and eventually coastal hills and cliffs will disappear because of wave erosion giving rise to narrow coastal plains, and with onrush of deposits from over the land behind may get covered up by alluvium or may get covered up by shingle or sand to form a wide beach.

Figure 4.27 : Some common coastal erosional landforms

DEPOSITIONAL LANDFORMS

Beaches and Dunes

Beaches are characteristic of shorelines that are dominated by deposition, but may occur as patches along even the rugged shores. Beaches occur when sand, gravel, and other loose material are deposited by waves along a shore. Most of the sediment making up the beaches comes from land carried by the streams and rivers or from wave erosion. A beach extends landward from the shoreline at low tide to the shoreline at high tide during storms, when waves are at their highest. In general, a beach is a sandy shore, but shingle beaches contain small pebbles and even cobbles. Beaches are commonly divided into two zones: the foreshore zone and the back-shore zone. The foreshore zone is the area between the ordinary low tide mark and the ordinary high tide mark. The backshore zone is the area normally affected by waves only during a storm at high tide. Behind the backshore zone may be cliffs, vegetation, or dunes created by winds moving sand from the beach. Commonly separating the two zones is a distinct mound of sand or gravel called a berm that runs parallel to the shoreline and which is created by the action of waves and tides.

Beaches are temporary features. The sandy beach which appears so permanent may be reduced to a very narrow strip of coarse pebbles in some other season.

Just behind the beach, the sands lifted and winnowed from over the beach surfaces will be deposited as sand dunes. Sand dunes forming long ridges parallel to the coastline are very common along low sedimentary coasts.

Figure 4.28 : General coastal landforms

Bars, Barriers, Spits and Tombolos

Wave activity keeps sand and other loose material in constant motion. As a consequence, it can create other features along the shore, such as spits, bars, barrier islands, and tombolos. A spit is a long, narrow deposit of sand or gravel that projects from land into open water. Spits normally form at the mouth of a bay and curve inward. If a spit extends across the entire mouth of a bay, it is called a baymouth bar or bay barrier. A bar, commonly known as a sand bar or off-shore bar, is a ridge or mound of sand or gravel that lies partially or completely underwater a short distance from and parallel to a beach in the off-shore zone (from the position of low tide waterline to seaward. If more sand is deposited on the bar so that it rises above the normal high tide level, the bar becomes a barrier bar or barrier island.

Figure 4. 29 : Depositional coastal landforms

Sometimes such barrier bars get keyed up to one end of the bay when they are called spits A spit is a long, narrow deposit of sand or gravel that projects from land into open water. Spits normally form at the mouth of a bay and curve inward (Figure 7.15). If a spit extends across the entire mouth of a bay, it is called a baymouth bar or bay barrier. A mound of sand or other beach material that rises above the water to connect an offshore island to the shore or to another island is called a tombolo. A tombolo can also be regarded as a spit that has grown until it reaches a nearby island or shore. 22

Spits may also develop attached to headlands/hills. The barriers, bars and spits at the mouth of the bay gradually extend leaving only a small opening of the bay into the sea and the bay will eventually develop into a lagoon. The lagoons get filled up gradually by sediment coming from the land or from the beach itself (aided by wind) and a broad and wide coastal plain may develop replacing a lagoon. The off-shore bars and barriers commonly form across the mouth of a river or at the entrance of a bay. Barrier islands range in length from 3 to 100 kilometers and in width from 1 to 3 kilometers. Separated from the shore by a shallow body of water known as a lagoon, a barrier island often helps protect the shore from the full force of waves.

Definitions

Backshore zone: The area of a beach normally affected by waves only during a storm at high tide.

Backwash: The return flow of water to the ocean following the swash of a wave.

Bar: A ridge or mound of sand or gravel that lies partially or completely underwater a short distance from and parallel to a beach; also commonly known as a sand bar.

Barrier island: A bar that has been built up so that it rises above the normal high tide level.

Bay: A body of water in a curved inlet between headlands.

Beach: A deposit of loose material on shores that is moved by waves, tides, and, sometimes, winds.

Beach drift: The downwind movement of sand along a beach as a result of the zigzag pattern created by swash and backwash.

Berm: A distinct mound of sand or gravel running parallel to the shoreline that divides the foreshore zone from the backshore zone of a beach.

Cliff: A high, steep face of rock.

Coast: A strip of land that extends landward from the coastline to the first major change in terrain features.

Coastline: The boundary between the coast and the shore.

Emergent coast: A coast in which land formerly under water has gradually risen above sea level through geologic uplift of the land or has been exposed because of a drop in sea level.

Erosion: 23

The gradual wearing away of Earth surfaces through the action of wind and water.

Foreshore zone: The area of a beach between the ordinary low tide mark and the high tide mark.

Headland: An elevated area of hard rock that projects out into an ocean or other large body of water.

Longshore current: An ocean current that flows close and almost parallel to the shoreline and is caused by the angled rush of waves toward the shore.

Longshore drift: The movement of sand and other material along a shoreline in the longshore current.

Sea arch: An arch created by the erosion of weak rock in a sea cliff through wave action.

Sea stack: An isolated column of rock, the eroded remnant of a sea arch, located in the ocean a short distance from the shoreline.

Shore: The strip of ground bordering a body of water that is alternately covered or exposed by waves or tides.

Shoreline: The fluctuating line between water and the shore.

Spit: A long, narrow deposit of sand or gravel that projects from land into open water.

Submergent coast: A coast in which formerly dry land has been gradually flooded, either by land sinking or by sea level rising.

Swash: The rush of water up the shore after the breaking of a wave.

Tide: The periodic rising and falling of water in oceans and other large bodies of water that results from the gravitational attraction of the Moon and the Sun upon Earth.

Tombolo: A mound of sand or other beach material that rises above the water to connect an offshore island to the shore or to another island.

Wave crest: The highest part of a wave.

Wave-cut notch: An indentation produced by wave erosion at the base of a sea cliff.

Wave-cut platform: A horizontal bench of rock formed beneath the waves at the base of a sea cliff as it retreats because of wave erosion.

Wave height: The vertical distance between the wave crest and the wave trough.

Wavelength: The horizontal distance between two wave crests or troughs.

Wave trough: The lowest part of a wave form between two crests.