DEPOSITIONAL LANDFORMS DEPOSITIONAL LANDFORMS DEPOSITIONAL LANDFORMS DEPOSITIONAL LANDFORMS
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C. Meanders
River meanders refers to the bends of longitudinal cources of the rivers. The shape of the meander is usually semi circular. It is governed by lithological characteristics, topographic characteristics, annual precipitation, cycle of erosion etc.
• Depositional Landforms A. Alluvial Fans
Alluvial fans are fan-shaped deposits of water-transported material (alluvium). They typically form at the base of topographic features where there is a marked break in slope.
Consequently, alluvial fans tend to be coarse-grained, especially at their mouths. At their edges, however, they can be relatively fine-grained.
B. Delta
Deltas are built from primarily river-borne sediment. It forms when the amount of sediment delivered at the mouth of a river exceeds the amount removed by waves and tidal currents
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C. Natural leeves
The narrow belt of ridges of low height built by the deposition of sediments by the spill water of the stream on the either bank is called natural leeve.
Karst Topography
Karst is a distinctive topography in which the landscape is largely shaped by the dissolving action of water on carbonate bedrock (usually limestone, dolomite, or marble).
This geological process, occurring over many thousands of years, results in unusual surface and subsurface features ranging from sinkholes, vertical shafts, disappearing streams, and springs, to complex underground drainage systems and caves.
The process of karst formation involves what is referred to as “the carbon dioxide cascade.”
As rain falls through the atmosphere, it picks up CO2 which dissolves in the droplets.
Once the rain hits the ground, it percolates through the soil and picks up more CO2 to form a weak solution of carbonic acid:
H2O+CO2=H2CO3.
The infiltrating water naturally exploits any cracks or crevices in the rock. Over long periods, with a continuous supply of CO2 - enriched water, carbonate bedrock begins to dissolve.
Openings in the bedrock increase in size and an underground drainage system begins to develop, allowing more water to pass, further accelerating the formation of karst. Eventually this leads to the development of subsurface caves.
• Erosional Landforms
A. Pools, Sinkholes, Lapies and Limestone Pavements
Small to medium sized round to sub-rounded shallow depressions called swallow holes form on the surface of limestones through solution. Sinkholes are very common in limestone/karst areas. A sinkhole is an opening more or less circular at the top and funnel-shapped towards the bottom with sizes varying in area from a few sq. m to a hectare and with depth from a less than half a metre to thirty metres or more. Some of these form solely through solution action (solution sinks) and others might startas solution forms first and if
the bottom of a sinkhole forms the roof of a void or cave underground, it might collapse leaving a large hole opening into a cave or a void below (collapse sinks). Quite often, sinkholes are covered up with soil mantle and appear as shallow water pools. The term doline is sometimes used to refer the collapse sinks. When sink holes and dolines join together because of slumping of materials along their margins or due to roof collapse of caves, long, narrow to wide trenches called valley sinks or Uvalas form.
Gradually, most of the surface of the limestone is eaten away by these pits and trenches, leaving it extremely irregular with a maze of points, grooves and ridges or lapies.
B. Caves
In areas where there are alternating beds of rocks (shales, sandstones, quartzites) with limestones or dolomites in between or in areas where limestones are dense, massive and occurring as thick beds, cave formation is prominent. Water percolates down either through the materials or through cracks and joints and moves horizontally along bedding planes. It is along these bedding planes that the limestone dissolves and long and narrow to wide gaps called caves result. There can be a maze of caves at different elevations depending upon the limestone beds and intervening rocks. Caves normally have an opening through which cave streams are discharged. Caves having openings at both the ends are called tunnels.
• Depositional Landforms
Many depositional forms develop within the limestone caves. The chief chemical in limestone is calcium carbonate which is easily soluble in carbonated water (carbon dioxide absorbed rainwater). This calcium carbonate is deposited
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when the water carrying it in solution evaporates or loses its carbon dioxide as it trickles over rough rock surfaces.
A. Stalactites, Stalagmites and Pillars Stalactites hang as icicles of different diameters. Normally they are broad at their bases and taper towards the free ends showing 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
Stalagmites may take the shape of a column, a disc, with either a smooth, rounded bulging end or a miniature crater like depression. The stalagmite and stalactites eventually fuse to give rise to columns and pillars of different diameters.
Coastal Geomorphology
Coastal Geomorphology encompasses the study of coastal processes and the evolution of landforms.
The formation and development of cliffs, beaches, salt marshes, reefs and other coastal landforms reflect the pressures and forces acting upon a coastline, both natural and manmade.
• Erosional Coastal Landforms
Some of the most spectacular scenery is found along coastlines and produced by the effects of wave erosion. Wave erosion undercuts steep shorelines creating coastal cliffs.
A. Sea cliff is a vertical precipice created by waves crashing directly on a steeply inclined slope. Hydraulic action, abrasion, and chemical solution all work to cut a notch at the high water level near the base of the cliff. Constant undercutting and erosion causes the cliffs to retreat landward.
B. Sea caves form along lines of weakness in cohesive but well-jointed bedrock. Sea caves are prominent headlands where wave refraction attacks the shore.
A sea arch forms when sea caves merge from opposite sides of a headland. If the arch collapses, a pillar of rock remains behind as a sea stack.
Seaward of the retreating cliffs, wave
erosion forms a broad erosional platform called a wave-cut bench or wave-cut platform. After the constant grinding and battering, eroded material is transported to adjacent bays to become beaches or seaward coming to rest as a wave-built terrace. If tectonic forces raise the bench above the water level a marine terrace forms. Some shorelines have several marine terraces creating during various episodes of uplift.
• Depositional Landforms A. Beaches and Dunes
Beaches are characteristic of shorelines that are dominated by deposition, but may occur as patches along even the rugged shores. Most of the sediment making up the beaches comes from land carried by the streams and rivers or from wave erosion. 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.
Most of the beaches are made up of sand sized materials. Beaches called shingle beaches contain excessively small pebbles and even cobbles.
B. Bars, Barriers and Spits
A ridge of sand and shingle formed in the sea in the off-shore zone (from the position of low tide waterline to seaward) lying approximately parallel to the coast is called an off-shore bar.
An off-shore bar which is exposed due to further addition of sand is termed a barrier bar.
The off-shore bars and barriers commonly form across the mouth of a river or at the entrance of a bay. Sometimes such barrier bars get keyed up to one end of the bay when they are called spits (Figure 7.15). Spits may also develop attached
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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.
Glaciated Topography
Glaciers have played an important role in the shaping of landscapes in the middle and high latitudes and in alpine environments. Their ability to erode soil and rock, transport sediment, and deposit sediment is extraordinary. During the last glacial period more than 50 million square kilometers of land surface were geomorphically influenced by the presence of glaciers.
• Erosional Landforms
Glacial erosion consists of two processes: (i) plucking or the tearing away of blocks of rock which have become frozen into the base and sides of a glacier, and (ii) abrasion or the wearing away of rocks beneath a glacier by the scouring action of the rocks embedded in the glacier.
A. Cirque
Cirques are the most common of landforms in glaciated mountains. The cirques quite often are found at the heads of glacial valleys. 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.
B. 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.
C. Glacial stairways
The advancing ice of glaciers carves out giant stairways through the process of abrasion and plucking of step faults coming across the path of moving glaciers
• Deposional Landforms A. Moraines
A valley glacier carries a large amount of rock waste called moraine. The moraine forming along the sides of a glacier is called lateral moraine; that along the front of a glacier is called terminal moraine; that at the bottom of a glacier is the ground moraine. When two glaciers join together, their inner lateral moraines coalesce to give a medial moraine. Terminal moraine material is carried down-valley by the melt waters issuing from the glacier’s snout (front) and is deposited as a layer called an outwash plain. One of the most conspicuous features of lowlands which have been glaciated by ice sheets is the widespread morainic deposits. Because of the numerous boulders in the clay these are called boulder clay deposits.
B. Drumlins
The swarms of rounded hummocks resulting from the deposition of glacial till are called drumlins. They look like inverted boat or spoon.
C. 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
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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.
D. Outwash fan
An outwash fan is a fan-shaped body of sediments deposited by braided streams from a melting glacier. Sediment locked within the ice of the glacier, gets transported by the streams
of meltwater, and deposits on the outwash plain, at the terminus of the glacier. The outwash, the sediment transported and deposited by the melt water and that makes up the fan, is usually poorly sorted due to the short distance traveled before being deposited.