Landforms and Their Evolution PDF

CHAPTER LANDFORMS AND THEIR EVOLUTION d...

CHAPTER LANDFORMS AND THEIR EVOLUTION d he A fter weathering processes have had a part of the earth’s surface from one landform their actions on the earth materials into another or transformation of individual making up the surface of the earth, the landforms after they are once formed. That is geomorphic agents like running water, ground means, each and every landform has a history water, wind, glaciers, waves perform erosion. of development and changes through time. A It is already known to you that erosion causes landmass passes through stages of bl changes on the surface of the earth. Deposition development somewhat comparable to the follows erosion and because of deposition too, stages of life — youth, mature and old age. changes occur on the surface of the earth. pu As this chapter deals with landforms and their evolution ‘first’ start with the question, What are the two important aspects of the evolution of landforms? what is a landform? In simple words, small to be T medium tracts or parcels of the earth’s surface The evolutionary history of the continually re are called landforms. changing surface of the earth is essential to be o R understood in order to use it effectively without If landform is a small to medium sized disturbing its balance and diminishing its part of the surface of the earth, what is a potential for the future. Geomorphology deals tt E landscape? with the reconstruction of the history of the surface of the earth through a study of its Several related landforms together make forms, the materials of which it is made up of C up landscapes, (large tracts of earth’s surface). and the processes that shape it. Each landform has its own physical shape, size, Changes on the surface of the earth owe materials and is a result of the action of certain mostly to erosion by various geomorphic no N geomorphic processes and agent(s). Actions agents. Of course, the process of deposition too, of most of the geomorphic processes and by covering the land surfaces and filling the agents are slow, and hence the results take a basins, valleys or depressions, brings changes long time to take shape. Every landform has a in the surface of the land. Deposition follows © beginning. Landforms once formed may erosion and the depositional surfaces too are change in their shape, size and nature slowly ultimately subjected to erosion. Running water, or fast due to continued action of geomorphic ground-water, glaciers, wind and waves are processes and agents. powerful erosional and depositional agents Due to changes in climatic conditions and shaping and changing the surface of the earth vertical or horizontal movements of land- aided by weathering and mass wasting masses, either the intensity of processes or the processes. These geomorphic agents acting processes themselves might change leading to over long periods of time produce systematic new modifications in the landforms. Evolution changes leading to sequential development of here implies stages of transformation of either landforms. Each geomorphic agent produces  LANDFORMS AND THEIR EVOLUTION 59 its own assemblage of landforms. Not only this, is linear flow as streams and rivers in valleys. each geomorphic process and agent leave their Most of the erosional landforms made by distinct imprints on the landforms they running water are associated with vigorous and produce. You know that most of the youthful rivers flowing over steep gradients. geomorphic processes are imperceptible With time, stream channels over steep gradients functions and can only be seen and measured turn gentler due to continued erosion, and as a through their results. What are the results? consequence, lose their velocity, facilitating These results are nothing but landforms and active deposition. There may be depositional their characteristics. Hence, a study of forms associated with streams flowing over steep d landforms, will reveal to us the process and slopes. But these phenomena will be on a small agent which has made or has been making scale compared to those associated with rivers flowing over medium to gentle slopes. The he those landforms. gentler the river channels in gradient or slope, Most of the geomorphic processes are the greater is the deposition. When the stream imperceptible. Cite a few processes which beds turn gentler due to continued erosion, can be seen and a few which can’t be downward cutting becomes less dominant and is seen. lateral erosion of banks increases and as a consequence the hills and valleys are reduced As the geomorphic agents are capable of to plains. bl erosion and deposition, two sets — erosional or destructional and depositional or Is complete reduction of relief of a high constructional — of landforms are produced pu land mass possible? by them. Many varieties of landforms develop by the action of each of the geomorphic agents Overland flow causes sheet erosion. depending upon especially the type and Depending upon irregularities of the land be T structure i.e. folds, faults, joints, fractures, surface, the overland flow may concentrate into hardness and softness, permeability and narrow to wide paths. Because of the sheer re friction of the column of flowing water, minor o R impermeability, etc. There are some other independent controls like (i) stability of sea or major quantities of materials from the level; (ii) tectonic stability of landmasses; (iii) surface of the land are removed in the direction of flow and gradually small and narrow rills tt E climate, which influence the evolution of landforms. Any disturbance in any of these will form. These rills will gradually develop into three controlling factors can upset the long and wide gullies; the gullies will further C systematic and sequential stages in the deepen, widen, lengthen and unite to give rise development and evolution of landforms. to a network of valleys. In the early stages, In the following pages, under each of the down-cutting dominates during which no N geomorphic regimes i.e. running water, irregularities such as waterfalls and cascades groundwater, glaciers, waves, and winds, first a will be removed. In the middle stages, streams brief discussion is presented as to how cut their beds slower, and lateral erosion of landmasses are reduced in their relief through valley sides becomes severe. Gradually, the valley sides are reduced to lower and lower © erosion and then, development of some of the erosional and depositional landforms is dealt with. slopes. The divides between drainage basins are likewise lowered until they are almost RUNNING WATER completely flattened leaving finally, a lowland of faint relief with some low resistant remnants In humid regions, which receive heavy rainfall called monadnocks standing out here and running water is considered the most important there. This type of plain forming as a result of of the geomorphic agents in bringing about the stream erosion is called a peneplain (an almost degradation of the land surface. There are two plain). The characteristics of each of the stages components of running water. One is overland of landscapes developing in running water flow on general land surface as a sheet. Another regimes may be summarised as follows:  60 FUNDAMENTALS OF PHYSICAL GEOGRAPHY Youth Streams are few during this stage with poor integration and flow over original slopes showing shallow V-shaped valleys with no floodplains or with very narrow floodplains along trunk streams. Streams divides are broad and flat with marshes, swamp and lakes. Meanders if present develop over these broad d upland surfaces. These meanders may eventually entrench themselves into the uplands. Waterfalls and rapids may exist where he local hard rock bodies are exposed. Mature During this stage streams are plenty with good is integration. The valleys are still V-shaped but deep; trunk streams are broad enough to have bl wider floodplains within which streams may flow in meanders confined within the valley. The flat and broad inter stream areas and swamps and marshes of youth disappear and pu the stream divides turn sharp. Waterfalls and rapids disappear. be T Old re o R Smaller tributaries during old age are few with Figure 7.1 : The Valley of Kaveri river near Hogenekal, gentle gradients. Streams meander freely over Dharmapuri district, Tamilnadu in the form of gorge vast floodplains showing natural levees, oxbow tt E lakes, etc. Divides are broad and flat with lakes, swamps and marshes. Most of the landscape is at or slightly above sea level. C EROSIONAL LANDFORMS no N Valleys Valleys start as small and narrow rills; the rills will gradually develop into long and wide © gullies; the gullies will further deepen, widen and lengthen to give rise to valleys. Depending Figure 7.2 : An entrenched meander loop of river Colorado in USA showing step-like side slopes of its valley upon dimensions and shape, many types of typical of a canyon valleys like V-shaped valley, gorge, canyon, etc. can be recognised. A gorge is a deep valley is wider at its top than at its bottom. In fact, a with very steep to straight sides (Figure 7.1) and canyon is a variant of gorge. Valley types depend a canyon is characterised by steep step-like upon the type and structure of rocks in which side slopes (Figure 7.2) and may be as deep as they form. For example, canyons commonly a gorge. A gorge is almost equal in width at its form in horizontal bedded sedimentary rocks top as well as its bottom. In contrast, a canyon and gorges form in hard rocks.  LANDFORMS AND THEIR EVOLUTION 61 Potholes and Plunge Pools River Terraces Over the rocky beds of hill-streams more or less River terraces are surfaces marking old valley circular depressions called potholes form floor or floodplain levels. They may be bedrock because of stream erosion aided by the abrasion surfaces without any alluvial cover or alluvial of rock fragments. Once a small and shallow terraces consisting of stream deposits. River depression forms, pebbles and boulders get terraces are basically products of erosion as collected in those depressions and get rotated they result due to vertical erosion by the stream by flowing water and consequently the into its own depositional floodplain. There can depressions grow in dimensions. A series of such be a number of such terraces at different d depressions eventually join and the stream heights indicating former river bed levels. The valley gets deepened. At the foot of waterfalls river terraces may occur at the same elevation he also, large potholes, quite deep and wide, form on either side of the rivers in which case they because of the sheer impact of water and are called paired terraces (Figure 7.3). rotation of boulders. Such large and deep holes at the base of waterfalls are called plunge pools. These pools also help in the deepening of valleys. is Waterfalls are also transitory like any other landform and will recede gradually and bring the floor of the valley above waterfalls to the bl level below. Incised or Entrenched Meanders pu In streams that flow rapidly over steep gradients, normally erosion is concentrated on be T the bottom of the stream channel. Also, in the Figure 7.3 : Paired and unpaired river terraces case of steep gradient streams, lateral erosion re on the sides of the valleys is not much when o R compared to the streams flowing on low and When a terrace is present only on one side gentle slopes. Because of active lateral erosion, of the stream and with none on the other side streams flowing over gentle slopes, develop or one at quite a different elevation on the other tt E sinuous or meandering courses. It is common side, the terraces are called unpaired terraces. to find meandering courses over floodplains Unpaired terraces are typical in areas of slow and delta plains where stream gradients are uplift of land or where the water column C very gentle. But very deep and wide meanders changes are not uniform along both the banks. can also be found cut in hard rocks. Such The terraces may result due to (i) receding water meanders are called incised or entrenched after a peak flow; (ii) change in hydrological no N meanders (Figure 7.2). Meander loops develop regime due to climatic changes; (iii) tectonic over original gentle surfaces in the initial stages uplift of land; (iv) sea level changes in case of of development of streams and the same loops rivers closer to the sea. get entrenched into the rocks normally due to © erosion or slow, continued uplift of the land DEPOSITIONAL LANDFORMS over which they start. They widen and deepen over time and can be found as deep gorges and canyons in hard rock areas. They give an Alluvial Fans indication on the status of original land Alluvial fans (Figure 7.4) are formed when surfaces over which streams have developed. streams flowing from higher levels break into foot slope plains of low gradient. Normally very What are the differences between incised coarse load is carried by streams flowing over meanders and meanders over flood and mountain slopes. This load becomes too heavy delta plains? for the streams to be carried over gentler  62 FUNDAMENTALS OF PHYSICAL GEOGRAPHY gradients and gets dumped and spread as a as a low cone. Unlike in alluvial fans, the broad low to high cone shaped deposit called deposits making up deltas are very well sorted alluvial fan. Usually, the streams which flow with clear stratification. The coarsest materials over fans are not confined to their original settle out first and the finer fractions like silts channels for long and shift their position across and clays are carried out into the sea. As the the fan forming many channels called delta grows, the river distributaries continue distributaries. Alluvial fans in humid areas to increase in length (Figure 7.5) and delta show normally low cones with gentle slope from continues to build up into the sea. d Floodplains, Natural Levees and Point Bars Deposition develops a floodplain just as he erosion makes valleys. Floodplain is a major landform of river deposition. Large sized materials are deposited first when stream channel breaks into a gentle slope. Thus, normally, fine sized materials like sand, silt and is clay are carried by relatively slow moving waters in gentler channels usually found in the plains and deposited over the bed and when bl the waters spill over the banks during flooding Figure 7.4 : An alluvial fan deposited by a hill stream above the bed. A river bed made of river on the way to Amarnath, Jammu and Kashmir pu deposits is the active floodplain. The floodplain above the bank is inactive floodplain. Inactive head to toe and they appear as high cones with floodplain above the banks basically contain steep slope in arid and semi-arid climates. two types of deposits — flood deposits and be T channel deposits. In plains, channels shift Deltas re laterally and change their courses occasionally o R Deltas are like alluvial fans but develop at a leaving cut-off courses which get filled up different location. The load carried by the rivers gradually. Such areas over flood plains built is dumped and spread into the sea. If this load up by abandoned or cut-off channels contain tt E is not carried away far into the sea or distributed coarse deposits. The flood deposits of spilled along the coast, it spreads and accumulates waters carry relatively finer materials like silt and clay. The flood plains in a delta are called C delta plains. Natural levees and point bars (Figure 7.6) are some of the important landforms found no N associated with floodplains. Natural levees are found along the banks of large rivers. They are low, linear and parallel ridges of coarse deposits along the banks of rivers, quite often cut into © individual mounds. During flooding as the water spills over the bank, the velocity of the water comes down and large sized and high specific gravity materials get dumped in the immediate vicinity of the bank as ridges. They are high nearer the banks and slope gently away from the river. The levee deposits are coarser than the deposits spread by flood Figure 7.5 : A satellite view of part of Krishna river waters away from the river. When rivers shift delta, Andhra Pradesh laterally, a series of natural levees can form.  LANDFORMS AND THEIR EVOLUTION 63 Meander is not a landform but is only a type of channel pattern. This is because of (i) propensity of water flowing over very gentle gradients to work laterally on the banks; (ii) unconsolidated nature of alluvial deposits making up the banks with many irregularities which can be used by water exerting pressure laterally; (iii) coriolis force acting on the fluid d water deflecting it like it deflects the wind. When the gradient of the channel becomes extremely he low, water flows leisurely and starts working Figure 7.6 : Natural levee and point bars laterally. Slight irregularities along the banks slowly get transformed into a small curvature Point bars are also known as meander bars. in the banks; the curvature deepens due to They are found on the convex side of meanders deposition on the inside of the curve and is of large rivers and are sediments deposited in erosion along the bank on the outside. If there a linear fashion by flowing waters along the is no deposition and no erosion or undercutting, bank. They are almost uniform in profile and in bl width and contain mixed sizes of sediments. If the tendency to meander is reduced. Normally, there more than one ridge, narrow and elongated in meanders of large rivers, there is active depressions are found in between the point bars. pu deposition along the convex bank and Rivers build a series of them depending upon undercutting along the concave bank. the water flow and supply of sediment. As the rivers build the point bars on the convex side, be T the bank on the concave side will erode actively. re In what way do natural levees differ from o R point bars? tt E Meanders In large flood and delta plains, rivers rarely flow C in straight courses. Loop-like channel patterns called meanders develop over flood and delta plains (Figure 7.7). no N © Figure 7.7 : A satellite scene showing meandering Burhi Gandak river near Muzaffarpur, Bihar, showing Figure 7.8 : Meander growth and cut-off loops and a number of oxbow lakes and cut-offs slip-off and undercut banks  64 FUNDAMENTALS OF PHYSICAL GEOGRAPHY The concave bank is known as cut-off bank is more in the valley, channel bars and islands which shows up as a steep scarp and the of sand, gravel and pebbles develop on the floor convex bank presents a long, gentle profile and of the channel and the water flow is divided is known as slip-off bank (Figure 7.8). As into multiple threads. These thread-like streams meanders grow into deep loops, the same may of water rejoin and subdivide repeatedly to give get cut-off due to erosion at the inflection points a typical braided pattern (Figure 7.9). and are left as ox-bow lakes. Braided Channels d When rivers carry coarse material, there can be he selective deposition of coarser materials causing formation of a central bar which diverts the flow towards the banks; and this flow increases lateral erosion on the banks. As the valley widens, the water column is reduced and more is and more materials get deposited as islands and lateral bars developing a number of separate channels of water flow. Deposition bl and lateral erosion of banks are essential for Figure 7.9 : Satellite scenes showing braided channel the formation of braided pattern. Or, segments of Gandak (right) and Son (left) rivers alternatively, when discharge is less and load pu Arrows show the direction of flow be T re o R tt E C no N © Figure 7.10 : Various karst features  LANDFORMS AND THEIR EVOLUTION 65 GROUNDWATER as solution forms first and if the bottom of a sinkhole forms the roof of a void or cave Here the interest is not on groundwater as a underground, it might collapse leaving a large resource. Our focus is on the work of hole opening into a cave or a void below groundwater in the erosion of landmasses and (collapse sinks). Quite often, sinkholes are evolution of landforms. The surface water covered up with soil mantle and appear as percolates well when the rocks are permeable, shallow water pools. Anybody stepping over thinly bedded and highly jointed and cracked. such pools would go down like it happens in After vertically going down to some depth, the quicksands in deserts. The term doline is water under the ground flows horizontally d sometimes used to refer the collapse sinks. through the bedding planes, joints or through Solution sinks are more common than collapse the materials themselves. It is this downward sinks. Quite often the surface run-off simply he and horizontal movement of water which goes down swallow and sink holes and flow as causes the rocks to erode. Physical or underground streams and re-emerge at a mechanical removal of materials by moving distance downstream through a cave opening. groundwater is insignificant in developing When sink holes and dolines join together landforms. That is why, the results of the work because of slumping of materials along their is of groundwater cannot be seen in all types of margins or due to roof collapse of caves, long, rocks. But in rocks like limestones or dolomites narrow to wide trenches called valley sinks or rich in calcium carbonate, the surface water Uvalas form. Gradually, most of the surface of bl as well as groundwater through the chemical the limestone is eaten away by these pits and process of solution and precipitation trenches, leaving it extremely irregular with a deposition develop varieties of landforms. These pu maze of points, grooves and ridges or lapies. two processes of solution and precipitation are Especially, these ridges or lapies form due to active in limestones or dolomites occurring differential solution activity along parallel to either exclusively or interbedded with other sub-parallel joints. The lapie field may be T rocks. Any limestone or dolomitic region eventually turn into somewhat smooth showing typical landforms produced by the limestone pavements. re action of groundwater through the processes o R of solution and deposition is called Karst Caves topography after the typical topography developed in limestone rocks of Karst region In areas where there are alternating beds of tt E in the Balkans adjacent to Adriatic sea. rocks (shales, sandstones, quartzites) with The karst topography is also characterised limestones or dolomites in between or in areas where limestones are dense, massive and C by erosional and depositional landforms. occurring as thick beds, cave formation is EROSIONAL LANDFORMS prominent. Water percolates down either through the materials or through cracks and no N joints and moves horizontally along bedding Pools, Sinkholes, Lapies and planes. It is along these bedding planes that Limestone Pavements the limestone dissolves and long and narrow to wide gaps called caves result. There can be Small to medium sized round to sub-rounded © a maze of caves at different elevations shallow depressions called swallow holes form depending upon the limestone beds and on the surface of limestones through solution. intervening rocks. Caves normally have an Sinkholes are very common in limestone/karst opening through which cave streams are areas. A sinkhole is an opening more or less discharged. Caves having openings at both the circular at the top and funnel-shapped towards ends are called tunnels. the bottom with sizes varying in area from a few sq. m to a hectare and with depth from a Depositional Landforms less than half a metre to thirty metres or more. Some of these form solely through solution Many depositional forms develop within the action (solution sinks) and others might start limestone caves. The chief chemical in limestone  66 FUNDAMENTALS OF PHYSICAL GEOGRAPHY is calcium carbonate which is easily soluble in GLACIERS carbonated water (carbon dioxide absorbed rainwater). This calcium carbonate is deposited Masses of ice moving as sheets over the land when the water carrying it in solution (continental glacier or piedmont glacier if a vast evaporates or loses its carbon dioxide as it sheet of ice is spread over the plains at the foot trickles over rough rock surfaces. of mountains) or as linear flows down the slopes of mountains in broad trough-like Stalactites, Stalagmites and Pillars valleys (mountain and valley glaciers) are called glaciers (Figure 7.12). The movement of glaciers Stalactites hang as icicles of different d diameters. Normally they are broad at their bases and taper towards the free ends showing he up in a variety of forms. Stalagmites rise up from the floor of the caves. In fact, stalagmites form due to dripping water from the surface or through the thin pipe, of the stalactite, immediately below it (Figure 7.11). is bl Figure 7.12 : A glacier in its valley pu is slow unlike water flow. The movement could be T be a few centimetres to a few metres a day or even less or more. Glaciers move basically re because of the force of gravity. o R We have many glaciers in our country moving down the slopes and valleys in tt E Himalayas. Higher reaches of Uttaranchal, Himachal Pradesh and Jammu and Kashmir, are places to see some of them. C Do you know where one can see river Bhagirathi is basically fed by meltwaters from under the snout (Gaumukh) of the no N Gangotri glacier. In fact, Alkapuri glacier feeds waters to Alakananda river. Rivers Alkananda and Bhagirathi join to make river Ganga near Deoprayag. © Erosion by glaciers is tremendous because of friction caused by sheer weight of the ice. Figure 7.11 : Stalactites and stalagmites in limestone caves The material plucked from the land by glaciers (usually large-sized angular blocks and Stalagmites may take the shape of a fragments) get dragged along the floors or sides column, a disc, with either a smooth, rounded of the valleys and cause great damage through bulging end or a miniature crater like abrasion and plucking. Glaciers can cause depression. The stalagmite and stalactites significant damage to even un-weathered rocks eventually fuse to give rise to columns and and can reduce high mountains into low hills pillars of different diameters. and plains.  LANDFORMS AND THEIR EVOLUTION 67 As glaciers continue to move, debris gets the glacier disappears. Such lakes are called removed, divides get lowered and eventually cirque or tarn lakes. There can be two or more the slope is reduced to such an extent that cirques one leading into another down below glaciers will stop moving leaving only a mass in a stepped sequence. of low hills and vast outwash plains along with other depositional features. Figures 7.13 and Horns and Serrated Ridges 7.14 show various glacial erosional and Horns form through head ward erosion of the depositional forms described in the text. cirque walls. If three or more radiating glaciers cut headward until their cirques meet, high, d EROSIONAL LANDFORMS sharp pointed and steep sided peaks called horns form. The divides between cirque side he Cirque walls or head walls get narrow because of Cirques are the most common of landforms in progressive erosion and turn into serrated or glaciated mountains. The cirques quite often saw-toothed ridges sometimes referred to as are found at the heads of glacial valleys. The arêtes with very sharp crest and a zig-zag is accumulated ice cuts these cirques while outline. moving down the mountain tops. They are deep, long and wide troughs or basins with The highest peak in the Alps, Matterhorn bl and the highest peak in the Himalayas, very steep concave to vertically dropping high Everest are in fact horns formed through walls at its head as well as sides. A lake of water headward erosion of radiating cirques. can be seen quite often within the cirques after pu be T re o R tt E C no N © Figure 7.13 : Some glacial erosional and depositional forms (adapted and modified from Spencer, 1962)  68 FUNDAMENTALS OF PHYSICAL GEOGRAPHY Glacial Valleys/Troughs Some amount of rock debris small enough to be carried by such melt-water streams is Glaciated valleys are trough-like and U-shaped washed down and deposited. Such glacio- with broad floors and relatively smooth, and fluvial deposits are called outwash deposits. steep sides. The valleys may contain littered Unlike till deposits, the outwash deposits are debris or debris shaped as moraines with roughly stratified and assorted. The rock swampy appearance. There may be lakes fragments in outwash deposits are somewhat gouged out of rocky floor or formed by debris rounded at their edges. Figure 7.14 shows a within the valleys. There can be hanging valleys few depositional landforms commonly found at an elevation on one or both sides of the main d in glaciated areas. glacial valley. The faces of divides or spurs of such hanging valleys opening into main glacial he Moraines valleys are quite often truncated to give them an appearance like triangular facets. Very deep They are long ridges of deposits of glacial till. glacial troughs filled with sea water and Terminal moraines are long ridges of debris making up shorelines (in high latitudes) are deposited at the end (toe) of the glaciers. Lateral is called fjords/fiords. moraines form along the sides parallel to the glacial valleys. The lateral moraines may join a What are the basic differences between terminal moraine forming a horse-shoe shaped bl glacial valleys and river valleys? ridge (Fig. 7.13). There can be many lateral moraines on either side in a glacial valley. These moraines partly or fully owe their origin to glacio- Depositional Landforms pu The unassorted coarse and fine debris dropped fluvial waters pushing up materials to the sides of glaciers. Many valley glaciers retreating by the melting glaciers is called glacial till. Most rapidly leave an irregular sheet of till over their be T of the rock fragments in till are angular to sub- valley floors. Such deposits varying greatly in angular in form. Streams form by melting ice thickness and in surface topography are called re ground moraines. The moraine in the centre of the o R at the bottom, sides or lower ends of glaciers. tt E C no N © Figure 7.14 : A panoramic diagram of glacial landscape with various depositional landforms (adapted and modified from Spencer, 1962)  LANDFORMS AND THEIR EVOLUTION 69 glacial valley flanked by lateral moraines is What is the difference between till and called medial moraine. They are imperfectly alluvium? formed as compared to lateral moraines. Sometimes medial moraines are indistinguishable from ground moraines. WAVES AND CURRENTS Eskers Coastal processes are the most dynamic and hence most destructive. So, don’t you think it When glaciers melt in summer, the water flows is important to know about the coastal d on the surface of the ice or seeps down along processes and forms? the margins or even moves through holes in Some of the changes along the coasts take the ice. These waters accumulate beneath the he place very fast. At one place, there can be glacier and flow like streams in a channel erosion in one season and deposition in beneath the ice. Such streams flow over the another. Most of the changes along the coasts ground (not in a valley cut in the ground) with are accomplished by waves. When waves break, ice forming its banks. Very coarse materials like the water is thrown with great force onto the is boulders and blocks along with some minor shore, and simultaneously, there is a great fractions of rock debris carried into this stream churning of sediments on the sea bottom. settle in the valley of ice beneath the glacier Constant impact of breaking waves drastically bl and after the ice melts can be found as a affects the coasts. Storm waves and tsunami sinuous ridge called esker. waves can cause far-reaching changes in a pu short period of time than normal breaking Outwash Plains waves. As wave environment changes, the intensity of the force of breaking waves changes. The plains at the foot of the glacial mountains be T or beyond the limits of continental ice sheets Do you know about the generating forces are covered with glacio-fluvial deposits in the behind waves and currents? If not, refer re form of broad flat alluvial fans which may join o R to the chapter on movements in ocean to form outwash plains of gravel, silt, sand and waters. clay. tt E Other than the action of waves, the coastal Distinguish between river alluvial plains landforms depend upon (i) the configuration and glacial outwash plains. of land and sea floor; (ii) whether the coast is C advancing (emerging) seaward or retreating (submerging) landward. Assuming sea level to Drumlins be constant, two types of coasts are considered no N Drumlins are smooth oval shaped ridge-like to explain the concept of evolution of coastal features composed mainly of glacial till with landforms: (i) high, rocky coasts (submerged some masses of gravel and sand. The long axes coasts); (ii) low, smooth and gently sloping of drumlins are parallel to the direction of ice sedimentary coasts (emerged coasts). © movement. They may measure up to 1 km in length and 30 m or so in height. One end of HIGH ROCKY COASTS the drumlins facing the glacier called the stoss Along the high rocky coasts, the rivers appear end is blunter and steeper than the other end to have been drowned with highly irregular called tail. The drumlins form due to dumping coastline. The coastline appears highly of rock debris beneath heavily loaded ice indented with extension of water into the land through fissures in the glacier. The stoss end where glacial valleys (fjords) are present. The gets blunted due to pushing by moving ice. hill sides drop off sharply into the water. Shores Drumlins give an indication of direction of do not show any depositional landforms glacier movement. initially. Erosion features dominate.  70 FUNDAMENTALS OF PHYSICAL GEOGRAPHY Along high rocky coasts, waves break with Storm and tsunami waves cause drastic great force against the land shaping the hill changes irrespective of supply of sediments. sides into cliffs. With constant pounding by Large rivers which bring lots of sediments build waves, the cliffs recede leaving a wave-cut deltas along low sedimentary coasts. platform in front of the sea cliff. Waves gradually minimise the irregularities along the The west coast of our country is a high shore. rocky retreating coast. Erosional forms The materials which fall off, and removed dominate in the west coast. The east from the sea cliffs, gradually break into smaller coast of India is a low sedimentary coast. d fragments and roll to roundness, will get Depositional forms dominate in the east deposited in the offshore. After a considerable coast. he period of cliff development and retreat when coastline turns somewhat smooth, with the addition of some more material to this deposit What are the various differences between in the offshore, a wave-built terrace would a high rocky coast and a low sedimentary coast in terms of processes and is develop in front of wave-cut terrace. As the erosion along the coast takes place a good landforms? supply material becomes available to longshore bl currents and waves to deposit them as beaches EROSIONAL LANDFORMS along the shore and as bars (long ridges of sand and/or shingle parallel to the coast) in the pu Cliffs, Terraces, Caves and Stacks nearshore zone. Bars are submerged features and when bars show up above water, they are Wave-cut cliffs and terraces are two forms called barrier bars. Barrier bar which get keyed usually found where erosion is the dominant shore process. Almost all sea cliffs are steep be T up to the headland of a bay is called a spit. When barrier bars and spits form at the mouth and may range from a few m to 30 m or even re of a bay and block it, a lagoon forms. The more. At the foot of such cliffs there may be a o R lagoons would gradually get filled up by flat or gently sloping platform covered by rock sediments from the land giving rise to a coastal debris derived from the sea cliff behind. Such plain. platforms occurring at elevations above the tt E average height of waves is called a wave-cut LOW SEDIMENTARY COASTS terrace. The lashing of waves against the base of the cliff and the rock debris that gets C Along low sedimentary coasts the rivers appear smashed against the cliff along with lashing to extend their length by building coastal waves create hollows and these hollows get no N plains and deltas. The coastline appears widened and deepened to form sea caves. The smooth with occasional incursions of water in roofs of caves collapse and the sea cliffs recede the form of lagoons and tidal creeks. The land further inland. Retreat of the cliff may leave slopes gently into the water. Marshes and some remnants of rock standing isolated as © swamps may abound along the coasts. small islands just off the shore. Such resistant Depositional features dominate. masses of rock, originally parts of a cliff or hill When waves break over a gently sloping are called sea stacks. Like all other features, sedimentary coast, the bottom sediments get sea stacks are also temporary and eventually churned and move readily building bars, coastal hills and cliffs will disappear because barrier bars, spits and lagoons. Lagoons of wave erosion giving rise to narrow coastal would eventually turn into a swamp which plains, and with onrush of deposits from over would subsequently turn into a coastal plain. the land behind may get covered up by The maintenance of these depositional features alluvium or may get covered up by shingle or depends upon the steady supply of materials. sand to form a wide beach.  LANDFORMS AND THEIR EVOLUTION 71 DEPOSITIONAL LANDFORMS develop attached to headlands/hills. The barriers, bars and spits at the mouth of the Beaches and Dunes bay gradually extend leaving only a small opening of the bay into the sea and the bay Beaches are characteristic of shorelines that are will eventually develop into a lagoon. The dominated by deposition, but may occur as lagoons get filled up gradually by sediment patches along even the rugged shores. Most of coming from the land or from the beach itself the sediment making up the beaches comes (aided by wind) and a broad and wide coastal from land carried by the streams and rivers or plain may develop replacing a lagoon. d from wave erosion. Beaches are temporary features. The sandy beach which appears so Do you know, the coastal off-shore bars permanent may be reduced to a very narrow he offer the first buffer or defence against strip of coarse pebbles in some other season. storm or tsunami by absorbing most of Most of the beaches are made up of sand sized their destructive force. Then come the materials. Beaches called shingle beaches barriers, beaches, beach dunes and contain excessively small pebbles and even mangroves, if any, to absorb the is cobbles. destructive force of storm and tsunami Just behind the beach, the sands lifted and waves. So, if we do anything which winnowed from over the beach surfaces will be disturbs the ‘sediment budget’ and the bl deposited as sand dunes. Sand dunes forming mangroves along the coast, these coastal long ridges parallel to the coastline are very forms will get eroded away leaving human common along low sedimentary coasts. pu habitations to bear first strike of storm and tsunami waves. Bars, Barriers and Spits WINDS be T A ridge of sand and shingle formed in the sea in the off-shore zone (from the position of low Wind is one of the two dominant agents in hot re tide waterline to seaward) lying approximately o R deserts. The desert floors get heated up too parallel to the coast is called an off-shore bar. much and too quickly because of being dry An off-shore bar which is exposed due to and barren. The heated floors heat up the air further addition of sand is termed a barrier tt E directly above them and result in upward bar. The off-shore bars and barriers commonly movements in the hot lighter air with form across the mouth of a river or at the turbulence, and any obstructions in its path C entrance of a bay. Sometimes such barrier bars sets up eddies, whirlwinds, updrafts and get keyed up to one end of the bay when they downdrafts. Winds also move along the desert are called spits (Figure 7.15). Spits may also floors with great speed and the obstructions no N in their path create turbulence. Of course, there are storm winds which are very destructive. Winds cause deflation, abrasion and impact. Deflation includes lifting and removal of dust © and smaller particles from the surface of rocks. In the transportation process sand and silt act as effective tools to abrade the land surface. The impact is simply sheer force of momentum which occurs when sand is blown into or against a rock surface. It is similar to sand- blasting operation. The wind action creates a number of interesting erosional and Figure 7.15 : A satellite picture of a part of Godavari depositional features in the deserts. river delta showing a spit In fact, many features of deserts owe their  72 FUNDAMENTALS OF PHYSICAL GEOGRAPHY formation to mass wasting and running water deposition of sediment from basin margins, a as sheet floods. Though rain is scarce in deserts, nearly level plain forms at the centre of the it comes down torrentially in a short period of basin. In times of sufficient water, this plain is time. The desert rocks devoid of vegetation, covered up by a shallow water body. Such exposed to mechanical and chemical types of shallow lakes are called as playas weathering processes due to drastic diurnal where water is retained only for short duration temperature changes, decay faster and the due to evaporation and quite often the playas torrential rains help in removing the weathered contain good deposition of salts. The playa materials easily. That means, the weathered plain covered up by salts is called alkali flats. d debris in deserts is moved by not only wind but also by rain/sheet wash. The wind moves Deflation Hollows and Caves he fine materials and general mass erosion is Weathered mantle from over the rocks or bare accomplished mainly through sheet floods or soil, gets blown out by persistent movement sheet wash. Stream channels in desert areas of wind currents in one direction. This process are broad, smooth and indefinite and flow for may create shallow depressions called a brief time after rains. is deflation hollows. Deflation also creates numerous small pits or cavities over rock EROSIONAL LANDFORMS surfaces. The rock faces suffer impact and bl abrasion of wind-borne sand and first shallow Pediments and Pediplains depressions called blow outs are created, and Landscape evolution in deserts is primarily pu some of the blow outs become deeper and concerned with the formation and extension of wider fit to be called caves. pediments. Gently inclined rocky floors close to the mountains at their foot with or without Mushroom, Table and Pedestal Rocks be T a thin cover of debris, are called pediments. Many rock-outcrops in the deserts easily Such rocky floors form through the erosion of re susceptible to wind deflation and abrasion are mountain front through a combination of o R worn out quickly leaving some remnants of lateral erosion by streams and sheet flooding. resistant rocks polished beautifully in the Erosion starts along the steep margins of shape of mushroom with a slender stalk and a the landmass or the steep sides of the tt E broad and rounded pear shaped cap above. tectonically controlled steep incision features Sometimes, the top surface is broad like a table over the landmass. Once, pediments are formed top and quite often, the remnants stand out C with a steep wash slope followed by cliff or free like pedestals. face above it, the steep wash slope and free face retreat backwards. This method of erosion is List the erosional features carved out by no N termed as parallel retreat of slopes through wind action and action of sheet floods. backwasting. So, through parallel retreat of slopes, the pediments extend backwards at the Depositional Landforms expense of mountain front, and gradually, the © mountain gets reduced leaving an inselberg Wind is a good sorting agent. Depending upon which is a remnant of the mountain. That’s how the velocity of wind, different sizes of grains are the high relief in desert areas is reduced to low moved along the floors by rolling or saltation featureless plains called pediplains. and carried in suspension and in this process of transportation itself, the materials get sorted. Playas When the wind slows or begins to die down, Plains are by far the most prominent landforms depending upon sizes of grains and their in the deserts. In basins with mountains and critical velocities, the grains will begin to settle. hills around and along, the drainage is towards So, in depositional landforms made by wind, the centre of the basin and due to gradual good sorting of grains can be found. Since  LANDFORMS AND THEIR EVOLUTION 73 wind is there everywhere and wherever there are equally important. There can be a great is good source of sand and with constant wind variety of dune forms (Figure 7.16). directions, depositional features in arid regions Crescent shaped dunes called barchans can develop anywhere. with the points or wings directed away from wind direction i.e., downwind, form where the Sand Dunes wind direction is constant and moderate and Dry hot deserts are good places for sand dune where the original surface over which sand is formation. Obstacles to initiate dune formation moving is almost uniform. Parabolic dunes form when sandy surfaces are partially covered d with vegetation. That means parabolic dunes are reversed barchans with wind direction he being the same. Seif is similar to barchan with a small difference. Seif has only one wing or point. This happens when there is shift in wind conditions. The lone wings of seifs can grow is very long and high. Longitudinal dunes form when supply of sand is poor and wind direction is constant. They appear as long bl ridges of considerable length but low in height. Transverse dunes are aligned perpendicular pu to wind direction. These dunes form when the wind direction is constant and the source of sand is an elongated feature at right angles to the wind direction. They may be very long and be T low in height. When sand is plenty, quite often, re the regular shaped dunes coalesce and lose o R their individual characteristics. Most of the dunes in the deserts shift and a few of them will get stabilised especially near human tt E Figure 7.16 : Various types of sand dunes Arrows indicate wind direction habitations. C EXERCISES no N 1. Multiple choice questions. (i) In which of the following stages of landform development, downward cutting is dominated? (a) Youth stage (c) Early mature stage © (b) Late mature stage (d) Old stage (ii) A deep valley characterised by steep step-like side slopes is known as (a) U-shaped valley (c) Blind valley (b) Gorge (d) Canyon (iii) In which one of the following regions the chemical weathering process is more dominant than the mechanical process? (a) Humid region (c) Arid region (b) Limestone region (d) Glacier region  74 FUNDAMENTALS OF PHYSICAL GEOGRAPHY (iv) Which one of the following sentences best defines the term ‘Lapies’ ? (a) A small to medium sized shallow depression (b) A landform whose opening is more or less circular at the top and funnel shaped towards bottom (c) A landform formed due to dripping water from surface (d) An irregular surface with sharp pinnacles, grooves and ridges (v) A deep, long and wide trough or basin with very steep concave high walls d at its head as well as in sides is known as: (a) Cirque (c) Lateral Moraine he (b) Glacial valley (d) Esker 2. Answer the following questions in about 30 words. (i) What do incised meanders in rocks and meanders in plains of alluvium indicate? is (ii) Explain the evolution of valley sinks or uvalas. (iii) Underground flow of water is more common than surface run-off in bl limestone areas. Why? (iv) Glacial valleys show up many linear depositional forms. Give their pu locations and names. (v) How does wind perform its task in desert areas? Is it the only agent responsible for the erosional features in the deserts? be T 3. Answer the following questions in about 150 words. (i) Running water is by far the most dominating geomorphic agent in shaping re the earth’s surface in humid as well as in arid climates. Explain. o R (ii) Limestones behave differently in humid and arid climates. Why? What is the dominant and almost exclusive geomorphic process in limestone areas tt E and what are its results? (iii) How do glaciers accomplish the work of reducing high mountains into low hills and plains? C Project Work no N Identify the landforms, materials and processes around your area. ©