Erosion and Deposition

Action of Running Water and Groundwater

Running water, which doesn’t need any further explanation, has two components: one is overland flow on the general land surface as a sheet and the other is linear flow as streams and rivers in valleys.

The overland flow causes sheet erosion and depending upon the irregularities of the land surface, the overland flow may concentrate into narrow to wide paths. During the sheet erosion, minor or major quantities of materials from the surface of the land are removed in the direction of flow and gradual small and narrow rills will form.

These rills will gradually develop into long and wide gullies, the gullies will further deepen, widen and lengthen and unite to give rise to a network of valleys. (Note: A valley can be formed in various ways like faulting, but here we are dealing only with the formation by means of exogenic geomorphic agent). Once a valley is formed, it later develops into a stream or river.

Courses of a river

A river, which is the best example of the linear flow of running water through a valley, can be divided into three, on the basis of its course – upper course, middle course and lower course.

Upper Course / Stage of Youth (Erosion dominates):
It starts from the source of the river in hilly or mountainous areas. The river flows down the steep slope and, as a result, its velocity and eroding power are at their maximum. Streams are few, with poor integration.
As the river flows down with high velocity, vertical erosion or downward cutting will be high which results in the formation of V-Shaped Valleys. Waterfalls, rapids, and gorges exist where the local hard rock bodies are exposed.

Middle Course/ Stage of Maturity (Transportation dominates):
In this stage, vertical erosion slowly starts to replace with lateral erosion or erosion from both sides of the channel. Thus, the river channel causes the gradual disappearance of its V-shaped valley (not completely). Streams are plenty at this stage with good integration. Wider flood plains start to visible in this course and the volume of water increases with the confluence of many tributaries. The work of river predominantly becomes transportation of the eroded materials from the upper course (little deposition too).
Landforms like alluvial fans, piedmont alluvial plains, meanders etc. can be seen at this stage.

Lower Course/ Stage of Old (Deposition dominates):
The river starts to flow through a broad, level plain with heavy debris brought down from upper and middle courses. Vertical erosion has almost stopped and lateral erosion still goes on. The work of the river is mainly deposition, building up its bed and forming an extensive flood plain.
Landforms like braided channels, floodplains, levees, meanders, oxbow lakes, deltas etc. can be seen at this stage.

Erosional Landforms due to Running Water

1. Valleys, Gorges, Canyon
valleys are formed as a result of running water. The rills which are formed by the overland flow of water later develop into gullies. These gullies gradually deepen and widen to form valleys.
A gorge is a deep valley with very steep to straight sides.
A canyon is characterized by steep step-like side slopes and may be as deep as a gorge.
A gorge is almost equal in width at its top as well as bottom and is formed in hard rocks while a canyon is wider at its top than at its bottom and is formed in horizontal bedded sedimentary rocks.

2. Potholes, Plunge pools
Potholes are more or less circular depressions over the rocky beds of hills streams. Once a small and shallow depression forms, pebbles and boulders get collected in those depressions and get rotated by flowing water. Consequently, the depressions grow in dimensions to form potholes. Plunge pools are nothing but large, deep potholes commonly found at the foot of a waterfall.
They are formed because of the sheer impact of water and rotation of boulders.

3. Incised or Entrenched Meanders
They are very deep wide meanders (loop-like channels) found cut in hard rocks. In the course of time, they deepen and widen to form gorges or canyons in hard rock. The difference between a normal meander and an incised/entrenched meander is that the latter found on hard rocks.

4. River Terraces
They are surfaces marking old valley floor or flood plains. They are basically the result of vertical erosion by the stream. When the terraces are of the same elevation on either side of the river, they are called as paired terraces. When the terraces are seen only on one side with none on the other or one at quite a different elevation on the other side, they are called as unpaired terraces.

Depositional Landforms due to Running Water

1. Alluvial Fans
They are found in the middle course of a river at the foot of slope/ mountains. When the stream moves from the higher level break into foot slope plain of low gradient, it loses its energy needed to transport much of its load. Thus, they get dumped and spread as a broad low to the high cone-shaped deposits called an alluvial fan. The deposits are not roughly very well sorted.

2. Deltas
Deltas are like an alluvial fan but develop at a different location. They are found in the mouth of the river, which is the final location of depositional activity of a river. Unlike alluvial fans, the deposits making up deltas are very well sorted with clear stratification. The coarser material settle out first and the finer materials like silt and clay are carried out into the sea.

3. Flood Plains, Natural Levees
Deposition develops a flood plain just as erosion makes valleys. A riverbed made of river deposits is the active flood plain and the flood plain above the bank of the river is the inactive flood plain. Natural levees are found along the banks of large rivers. They are low, linear and parallel ridges of coarse deposits along the banks of a river. The levee deposits are coarser than the deposits spread by flood water away from the river.

4. Meanders and oxbow lakes
Meanders are loop-like channel patterns develop over the flood and delta plains.
They are actually not a landform but only a type of channel pattern formed as a result of deposition.
They are formed basically because of three reasons:
(i) propensity of water flowing over very gentle gradient to work laterally on the banks;
(ii) unconsolidated nature of alluvial deposits making up the bank with many irregularities;
(iii) Coriolis force acting on fluid water deflecting it like deflecting the wind.
The concave bank of a meander is known as cut-off bank and the convex bank is known as a slip-off.

As meanders grow into deep loops, the same may get cut-off due to erosion at the inflection point and are left as oxbow lakes. For large rivers, the sediments deposited in a linear fashion at the depositional side of a meander are called as Point Bars or Meander Bars.

5. Braided Channels
When selective deposition of coarser materials causes the formation of a central bar, it diverts the flow of river towards the banks, which increases lateral erosion. Similarly, when more and more such central bars are formed, braided channels are formed. Riverine Islands are the result of braided channels.

What does Groundwater do?
The part of rain or snow-melt water which accumulates in the rocks after seeping through the surface is called underground water or simply groundwater. The rocks through which water can pass easily are called as permeable rocks while the rocks which do not allow water to pass are called as impermeable rocks. After vertically going down to some depth, the water under the ground flows horizontally through the bedding planes, joints or through the materials themselves. Although the amount of groundwater varies from place to place, its role in shaping the surface features of the earth is quite important.

The works of groundwater are mainly seen in rocks like limestone, gypsum or dolomite which are rich in calcium carbonate.Any limestone, dolomite or gypsum region showing typical landforms produced by the action of groundwater through the process of solution and deposition is called as Karst Topography (Karst region in the Balkans).
The zones or horizons of permeable and porous rocks which are fully filled with water are called as the Zones of Saturation.
The marks which show the upper surface of these saturated zones of the groundwater are called as the Water Tables.
The water table is generally higher in the areas of high precipitation and also in areas bordering rivers and lakes.
On the basis of variability, water tables are of two types:
(i) Permanent water table, in which the water will never fall below a certain level and wells dug up to this depth provide water in all seasons;
(ii) Temporary water tables, which are seasonal water tables.

Springs: They are the surface outflow of groundwater through an opening in a rock under hydraulic pressure.
When such springs emit hot water, they are called as Hot Springs. They generally occur in areas of active or recent volcanism.
When a spring emits hot water and steam in the form of fountains or jets at regular intervals, they are called as geysers. In a geyser, the period between two emissions is sometimes regular (Yellowstone National Park of USA is the best example).

Erosional Landforms due to Groundwater
Sinkholes and caves are erosional landforms formed due to the action of ground water.

1. Sinkholes
Small to medium sized rounded to sub-rounded shallow depressions called swallow holes forms on the surface of rocks like limestone by the action of the solution. A sinkhole is an opening more or less circular at the top and funnel-shaped towards the bottom. When as sinkhole is formed solely through the process of solution, it is called as a solution sink.
Some sinkhole starts its formation through the solution process but later collapse due to the presence of some caves or hollow beneath it and becomes a bigger sinkhole. These types are called as collapse sinks.
The term Doline is sometimes used to refer collapse sinks.
Solution sinks are more common than collapse sinks.

When several sink holes join together to form valley of sinks, they are called as Lapies are the irregular grooves and ridges formed when most of the surfaces of limestone are eaten by solution process.

2. Caves
In the areas where there are alternative beds of rocks (non-soluble) with limestone or dolomite in between or in areas where limestone are dense, massive and occurring as thick beds, cave formation is prominent.
Caves normally have an opening through which cave streams are discharged.
Caves having an opening at both the ends are called tunnels.

Depositional Landforms of Groundwater
1. Stalactites and stalagmites
They are formed when the calcium carbonates dissolved in groundwater get deposited once the water evaporates. These structures are commonly found in limestone caves.
Stalactites are calcium carbonate deposits hanging as icicles while Stalagmites are calcium carbonate deposits which rise up from the floor.

Erosion and Deposition: Action of Glaciers

What is a Glacier?

Glaciers are a mass of ice moving under its own weight. They are commonly found in the snow-fields.
The landmass on the earth is not entirely the same as we see around. Some areas are covered by thick green forests, some with dry hot deserts, some with permanent ice covers etc. Among these varied landmasses, the permanently ice-covered regions on the earth surface are called as snow-fields. The lowest limit of permanent snow or snow-field is called as the snowline.
A Glacier forms in areas where the accumulation of snow exceeds its ablation (melting and sublimation) over many years, often centuries. They form features like crevasses, seracs etc.
A crevasse is a deep crack, or fracture, found in an ice sheet or glacier, as opposed to a crevice that forms in rock. A serac is a block or column of glacial ice, often formed by intersecting crevasses on a glacier.

Ogives are alternating wave crests and valleys (troughs) that appear as dark and light bands of ice on glacier surfaces. They are linked to seasonal motion of glaciers; the width of one dark and one light band generally equals the annual movement of the glacier.
Glaciers cover about 10 percent of Earth’s land surface and they are the largest freshwater reservoirs on earth.

On the basis of the location of the glacier, they can be classified as:
Continental Glacier/Piedmont Glacier: they move outward in all directions
Valley/Mountain Glaciers: Move from higher elevation to lower.

Erosional landforms due to Glaciers

1. Cirque or Corris
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. They are simply a bowl-shaped depression formed due to the erosional activity of glaciers. When these depressions are filled with water, they are called as Cirque lake or Corrie Lake or Tarn Lakes.

2. Hanging Valleys or U-shaped Valleys, Fjords/fiords
The Glacier doesn’t create a new valley like a river does but deepens and widens a pre-existing valley by smoothening away the irregularities. These valleys, which are formed by the glacial erosions assume the shape of letter ‘U’ and hence are called as U-shaped Valleys or Hanging Valleys. A fjord is a very deep glacial trough filled with sea water and making up shorelines.
A fjord is formed when a glacier cuts a U-shaped valley by ice segregation and abrasion of the surrounding bedrock and this valley gradually gets filled with the seawater (formed in mountains nearby sea).

3. Horns and Aretes
Horns are sharp pointed and steep-sided peaks.They are formed by headward erosion of cirque wall.
When the divide between two cirque walls gets narrow because of progressive erosions, it results in the formation of a saw-toothed ridge called Arete.

Erosion and Deposition: Action of Wind and Waves

Action of Winds:

The wind is the main geomorphic agent in the hot deserts. Winds in hot deserts have greater speed which causes erosional and depositional activities in the desert. The landforms which are created by erosional and depositional activities of wind are called as Aeolian Landforms.
An erg (also known as sand sea / dune sea / sand sheet if it lacks dunes) is a broad, flat area of desert covered with wind-swept sand with little or no vegetative cover. It is defined as a desert area that contains more than 125 square kilometres of aeolian or wind-blown sand and where sand covers more than 20% of the surface. Smaller areas are known as “dune fields”. The largest hot desert in the world, the Sahara, contains several ergs.

Erosional Landforms due to Wind

1. Pediplains
When the high relief structures in deserts are reduced to low featureless plains by the activities of wind, they are called as Pediplains.

2. Deflation Hollows
Deflation is the removal of loose particles from the ground by the action of wind.
When deflation causes a shallow depression by persistent movements of wind, they are called as deflation hollows.

3. Mushroom Tables
Ventifacts are rocks that have been abraded, pitted, etched, grooved, or polished by wind-driven sand or ice crystals. These geomorphic features are most typically found in arid environments where there is little vegetation to interfere with aeolian particle transport, where there are frequently strong winds, and where there is a steady but not overwhelming supply of sand.
Mushroom Tables / Mushroom rocks are Ventifacts in the shape of a mushroom. In deserts, a greater amount of sand and rock particles are transported close to the ground by the winds which cause more bottom erosion in overlying rocks than the top. This result in the formation of rock pillars shaped like a mushroom with narrow pillars with broad top surfaces.

Depositional Landforms of Wind

1. Sand dunes
Dry hot deserts are good places for sand dune formation. According to the shape of a sand dune, there are varieties of sand dune forms like Barchans, Seifs etc.
The crescent-shaped dunes are called as Barchans and they are the most common one.
Seif is similar to Barchans but has only one wing or point.

2. Loess
In several large areas of the world, the surface is covered by deposits of wind-transported silt that has settled out from dust storms over many thousands of years. These depositions are called as Loess.