Erosional & Depostional Landforms

Introduction to Landforms and Landscapes

• Introduction to Landforms and Landscapes:
  • Landform: A small to medium-sized tract of the earth's surface.
  • Landscape: A large area composed of several related landforms.
  • Each landform has its own physical shape, size, materials, and is formed by the action of geomorphic processes and agents.
  • Landforms are constantly evolving, changing in shape, size, and nature over time due to continued geomorphic activity.
• Evolution of Landforms:

  

  • Landforms evolve through stages, akin to the life stages of youth, maturity, and old age.
  • Climatic changes and tectonic movements can influence the intensity and nature of geomorphic processes, resulting in new modifications to landforms.
• Key Aspects of Landform Evolution:
  • Degradation: Erosion and degradation of the earth's surface through processes like weathering, mass-wasting, and erosion.
  • Deposition: Deposition follows erosion, resulting in the accumulation of materials and forming new landforms.



• Possibility of Complete Relief Reduction:
  • Complete reduction of relief of a highland mass is unlikely, as some resistant remnants (monadnocks) remain after erosion.

Running Water as Geomorphic Agent

• Running Water as a Geomorphic Agent:
  • In humid regions with heavy rainfall, running water is a primary geomorphic agent responsible for surface degradation.
  • Running water has two components: overland flow (sheet flow) and linear flow (streams and rivers) in valleys.
  • Running water causes erosion in steep gradients, but as the gradient becomes gentler, deposition becomes more prominent.
  • Gentle slopes promote deposition, whereas steeper gradients result in erosion.
• Overland Flow and Erosion:
  • Sheet Erosion: Overland flow causes sheet erosion, where water removes material from the land surface, forming narrow rills.
  • Rills to Gullies: Rills gradually develop into gullies, which deepen and widen, eventually forming a network of valleys.
  • As erosion progresses, waterfalls and cascades are removed, and valleys become more pronounced.
• Stages of Landscape Development in Running Water Regimes:

  

  • Youth Stage:
    • Streams are few, and valleys are shallow and V-shaped with narrow floodplains.
    • Stream divides are broad and flat with marshes, swamps, and lakes.
    • Waterfalls and rapids may be present where hard rock bodies are exposed.
  • Mature Stage:
    • Streams are more numerous with good integration, and valleys are V-shaped and deeper.
    • Floodplains are wider, and meanders develop within valleys.
    • Waterfalls and rapids disappear.

  

  • Old Stage:
    • Smaller tributaries with gentle gradients and meandering streams.
    • Vast floodplains are formed, showing features like natural levees and oxbow lakes.
    • Stream divides are broad and flat, and landscapes are generally at or slightly above sea level.

    

Erosional Landforms by Running water

• Valleys:
  • Valleys form from small rills that develop into gullies, which deepen, widen, and lengthen to form valleys.
  • Common valley types include V-shaped valleys, gorges, and canyons.
  • Gorge: A deep valley with very steep to straight sides.
  • Canyon: Characterized by steep, step-like side slopes and is usually wider at the top than at the bottom.
  • The type of valley depends on the rock structure and type, e.g., gorges form in hard rocks, while canyons form in horizontal bedded sedimentary rocks.

  

• Potholes and Plunge Pools:
  • Potholes form over rocky beds of hill-streams due to stream erosion and abrasion by rock fragments.
  • Small depressions in the stream bed collect pebbles and boulders, which rotate, enlarging the depression over time.
  • Large and deep potholes may form at the base of waterfalls and are called plunge pools due to the impact and rotation of boulders.
• Incised or Entrenched Meanders:
  • In steep gradient streams, erosion is focused at the stream channel's bottom, while lateral erosion is less significant.
  • Meanders form in streams with gentle gradients, but deep and wide meanders can occur in hard rocks, called incised or entrenched meanders.
• River Terraces:
  • River terraces are surfaces that mark old valley floors or floodplain levels, resulting from vertical erosion by streams into their own floodplains.
  • Terraces can be bedrock surfaces or alluvial deposits.
  • Multiple terraces at different heights indicate former river bed levels, and paired terraces occur at the same elevation on both sides of the river.

Depostional Landforms by Running Water

• Deposition Landforms
  • Alluvial Fans

    

    • Formed when streams flow from higher levels and break into foot slope plains of low gradient.
    • Very coarse load is carried by streams flowing over mountain slopes.
    • This load becomes too heavy for the streams to be carried over gentler gradients, forming a broad cone-shaped deposit called an alluvial fan.
    • Streams often shift their position across the fan, forming many channels known as distributaries.
    • In humid areas, alluvial fans have low cones with gentle slopes, while in arid and semi-arid climates, they form high cones with steep slopes.
  • Deltas
    • Similar to alluvial fans, but develop at a different location where the load is dumped and spread into the sea.
    • The deposits in deltas are very well sorted, with clear stratification.
    • The coarsest materials settle first, followed by finer materials like silt and clay.
    • As the delta grows, river distributaries increase in length and the delta continues to build up into the sea.
  • Floodplains, Natural Levees, and Point Bars
    • Floodplains develop as a result of deposition, with fine materials like sand, silt, and clay carried by slow-moving waters in gentler channels.
    • The active floodplain is the riverbed, while the inactive floodplain is the area above the bank.
    • Inactive floodplains contain two types of deposits: flood deposits and channel deposits.
    • In plains, channels shift laterally, leaving cut-off courses that gradually fill with coarse deposits.
    • The flood deposits of spilled waters carry finer materials like silt and clay.
    • Floodplains in deltas are called delta plains.
    • Natural Levees are low, linear ridges of coarse deposits found along the banks of large rivers.
    • Point Bars, also known as meander bars, are sediments deposited on the concave side of river meanders.
  • Meanders

    

    • Meanders are loop-like channel patterns that develop in large floodplains.
    • They form due to the lateral movement of water over very gentle gradients.
    • When the gradient becomes low, slight irregularities in the banks lead to curvature due to deposition on the inside and erosion on the outside.
    • If no deposition or erosion occurs, meandering tendencies are reduced.
    • In meanders, deposition occurs along the concave bank and undercutting along the convex bank.
    • The concave bank is known as the cut-off bank, which appears as a steep scarp, while the convex bank presents a gentle profile.
    • As meanders grow into deep loops, they may be cut-off due to erosion at the inflection points, forming ox-bow lakes.

    

Ground Water as Geomorphic Agent

• Groundwater
  • The focus is on groundwater’s role in erosion and landform evolution, rather than as a resource.
  • Water percolates through permeable, thinly bedded, and highly jointed or cracked rocks.
  • Water moves horizontally through bedding planes and joints, causing erosion.
  • The physical removal of materials by groundwater is insignificant in developing landforms.
  • Groundwater causes erosion and deposition primarily in rocks like limestone and dolomite rich in calcium carbonate.
  • The chemical processes of solution and precipitation lead to the development of a variety of landforms.
  • Regions with typical landforms produced by groundwater are called Karst topography, after the Karst region in the Balkans.
  • Karst topography is characterized by both erosional and depositional landforms.

  

• Erosional Landforms
  • Pools, Sinkholes, Lapies, and Limestone Pavements
    • Swallow Holes
      • Small to medium-sized round to sub-rounded shallow depressions form on the surface of limestones through solution.
    • Sinkholes
      • Very common in limestone/karst areas.
      • Sinkhole is an opening more or less circular at the top and funnel-shaped towards the bottom.
      • Sizes vary from a few square meters to a hectare, and depths range from less than half a meter to over 30 meters.
      • Types of Sinkholes
        • Solution Sinks form solely through solution action.
        • Collapse Sinks form if the bottom of a sinkhole is the roof of a void or cave, leading to collapse.
    • Appearance of Sinkholes
      • Sinkholes can be covered with soil, appearing as shallow water pools.
      • Stepping over such pools may lead to a collapse, like quicksand in deserts.
      • Doline is a term sometimes used to refer to collapse sinks.
      • Solution sinks are more common than collapse sinks.
    • Water Flow and Underground Streams
      • Surface runoff often flows into swallow and sinkholes, becoming underground streams.
      • The water may re-emerge at a distance downstream through a cave opening.
    • Valley Sinks (Uvalas)
      • Form when sinkholes and dolines join due to slumping or collapse of cave roofs.
      • These long, narrow to wide trenches are called valley sinks or Uvalas.
    • Lapies
      • The surface of limestone becomes irregular with a maze of pits, grooves, and ridges due to erosion.
      • These ridges or lapies form due to differential solution activity along parallel or sub-parallel joints.
    • Limestone Pavements
      • Lapies may eventually form smooth limestone pavements.
  • Caves
    • Cave Formation
      • Caves form in areas with alternating rock beds (shales, sandstones, quartzites) and limestones or dolomites.
      • Caves also form in dense, massive limestone beds occurring as thick layers.
    • Water Percolation
      • Water percolates down through the rocks or cracks and joints, moving horizontally along bedding planes.
      • It is along these bedding planes that the limestone dissolves, forming caves.
    • Types of Caves
      • Narrow and Wide Caves are formed along bedding planes.
      • Caves can have a maze-like structure at different elevations depending on the rock layers.
    • Cave Openings
      • Caves usually have openings through which cave streams are discharged.
      • If a cave has openings at both ends, it is referred to as a tunnel.
• Depositional Landforms
  • Formation of Depositional Landforms in Caves
    • Depositional forms develop within limestone caves.
    • The main chemical in limestone is calcium carbonate, which is soluble in carbonated water (rainwater with absorbed carbon dioxide).
    • Calcium carbonate is deposited when the water carrying it evaporates or loses its carbon dioxide as it trickles over rough rock surfaces.
  • Stalactites, Stalagmites, and Pillars

    

    • Stalactites
      • Stalactites hang like icicles, with varying diameters.
      • They are broad at their bases and taper towards the free ends, forming various shapes.
    • Stalagmites
      • Stalagmites rise from the floor of the caves, forming due to water dripping from stalactites or from the cave ceiling.
      • They can take shapes like columns, discs, or even miniature crater-like depressions.
    • Formation of Pillars
      • Stalagmites and stalactites eventually fuse to form columns and pillars of different diameters.

Glaciers as Geomorphic Agent

• Glaciers
  • Definition of Glaciers
    • Masses of ice moving over the land are called glaciers.
    • They can move as sheets (continental glaciers or piedmont glaciers) or as linear flows down mountain slopes (mountain and valley glaciers).
  • Movement of Glaciers
    • Glaciers move slowly, unlike water flow, and the movement can range from a few centimeters to a few meters per day.
    • Glaciers move due to the force of gravity.

  

  • Erosion by Glaciers
    • Glacial erosion is tremendous due to the friction caused by the sheer weight of the ice.
    • The material plucked by glaciers, including large angular blocks and fragments, gets dragged along the floors or sides of valleys.
    • This erosion can cause significant damage, reducing high mountains to low hills and plains.
  • Deposition and Glacier Movement
    • As glaciers move, debris is removed, divides are lowered, and slopes are reduced.
    • Eventually, glaciers stop moving, leaving behind low hills, vast outwash plains, and other depositional features.



• EROSIONAL LANDFORMS
  • Cirque
    • Cirques are the most common landforms in glaciated mountains.
    • They are often found at the heads of glacial valleys.
    • The accumulated ice cuts these cirques while moving down the mountain tops.
    • Cirques are deep, long, and wide troughs or basins with steep, concave to vertically dropping high walls at the head and sides.
    • A lake, called a cirque or tarn lake, is often seen within the cirque after the glacier disappears.
    • There may be multiple cirques arranged in a stepped sequence, with one leading into another down below.
  • Horns and Serrated Ridges
    • Horns
      • Horns form through headward erosion of cirque walls.
      • If three or more radiating glaciers cut headward until their cirques meet, sharp-pointed and steep-sided peaks called horns form.
    • Serrated Ridges
      • The divides between cirque side walls get narrower due to progressive erosion, forming serrated or saw-toothed ridges called arêtes.
      • Arêtes have a very sharp crest and a zig-zag outline.
  • Glacial Valleys/Troughs
    • Glaciated valleys are trough-like and U-shaped with broad floors and relatively smooth, steep sides.
    • These valleys may contain debris or moraines that have a swampy appearance.
    • Lakes can be formed by debris within the valleys or gouged out of the rocky floor.
    • Hanging valleys may be present at an elevation on one or both sides of the main glacial valley.
    • The divides or spurs of hanging valleys opening into main glacial valleys are often truncated, giving them a triangular facet appearance.
    • Deep glacial troughs filled with sea water, forming shorelines in high latitudes, are called fjords/fiords.



• DEPOSITIONAL LANDFORMS
  • Glacial Till
    • The unassorted coarse and fine debris dropped by melting glaciers is called glacial till.
    • Most of the rock fragments in till are angular to sub-angular in form.
  • Outwash Deposits
    • Streams form by melting ice at the bottom, sides, or lower ends of glaciers.
    • Rock debris small enough to be carried by meltwater streams is washed down and deposited.
    • These deposits are called outwash deposits, which are roughly stratified and assorted.
    • The rock fragments in outwash deposits are somewhat rounded at their edges.
  • Moraines
    • Moraines are long ridges of deposits of glacial till.
    • Terminal Moraines
      • Terminal moraines are long ridges of debris deposited at the end (toe) of glaciers.
    • Lateral Moraines
      • Lateral moraines form along the sides of glaciers, parallel to glacial valleys.
      • These may join terminal moraines to form a horseshoe-shaped ridge.
    • Ground Moraines
      • Ground moraines are deposits left by rapidly retreating valley glaciers, covering valley floors irregularly.
    • Medial Moraines
      • The moraine in the center of a glacial valley, flanked by lateral moraines, is called a medial moraine.
      • Medial moraines are imperfectly formed compared to lateral moraines and sometimes indistinguishable from ground moraines.
  • Eskers
    • Eskers are sinuous ridges formed by streams flowing beneath glaciers.
    • They are composed of coarse materials like boulders and rock debris settled in valleys of ice beneath glaciers.
  • Outwash Plains
    • Outwash plains are flat areas covered with glacio-fluvial deposits such as gravel, silt, sand, and clay.
    • They typically form at the foot of glacial mountains or beyond the limits of continental ice sheets.
  • Drumlins
    • Drumlins are smooth, oval-shaped ridges composed mainly of glacial till, with some gravel and sand.
    • The long axes of drumlins are parallel to the direction of ice movement.
    • Drumlins are formed by the dumping of rock debris beneath heavily loaded ice.
    • One end of the drumlin, called the stoss end, is blunter and steeper than the tail end.

Waves & Currents as Geomorphic Agent

• WAVES AND CURRENTS
  • Coastal processes are dynamic and can be destructive, with changes occurring rapidly at some places.
  • The primary force behind coastal changes is waves, which impact the shore with great force, while also churning sediments on the sea bottom.
  • Generating Forces Behind Waves and Currents
    • Waves are generated by wind, with their intensity and force varying based on environmental conditions.
    • Storm waves and tsunami waves can cause more drastic changes compared to regular waves.
  • Coastal landforms are also influenced by the configuration of land and sea floors, as well as whether the coast is advancing (emerging) or retreating (submerging).
  • Types of Coasts
    • High Rocky Coasts (Submerged Coasts)
      • High rocky coasts feature irregular coastlines with drowned rivers and glacial valleys (fjords).
      • The land drops steeply into the water, and erosion features dominate.
      • Waves break forcefully against the land, shaping the cliffs and causing erosion, leaving behind wave-cut platforms.
      • Materials from eroded cliffs form deposits in the offshore, leading to the development of wave-built terraces.
      • Longshore currents and waves deposit materials as beaches, bars, and spits, with barrier bars potentially forming lagoons.
    • Low Sedimentary Coasts (Emerging Coasts)
      • Low sedimentary coasts are characterized by smooth coastlines, often with marshes, swamps, and deltas.
      • When waves break on these coasts, the sediments churn, forming bars, barrier bars, spits, and lagoons.
      • Lagoons can eventually turn into swamps and coastal plains.
      • Large rivers build deltas along low sedimentary coasts, further contributing to the coastal landscape.
• EROSIONAL LANDFORMS
  • Cliffs, Terraces, Caves and Stacks
    • Wave-Cut Cliffs
      • Wave-cut cliffs are steep coastal landforms, ranging from a few meters to 30 meters or more in height.
      • They are created by the erosion of the coastline by the force of waves.
    • Wave-Cut Terraces
      • Wave-cut terraces are flat or gently sloping platforms found at the base of cliffs.
      • These terraces are covered by rock debris derived from the sea cliff behind.
      • They are often located at elevations above the average height of the waves.
    • Sea Caves
      • Sea caves form due to the action of waves smashing against the cliff base.
      • Hollows are created and widened by the constant pounding of waves and rock debris.
      • Over time, these caves deepen and may collapse, further retreating the sea cliff inland.
    • Sea Stacks
      • Sea stacks are isolated, resistant rock formations left standing when the surrounding cliff retreats.
      • They are usually small islands just off the shore, formed by the erosion of coastal hills and cliffs.
      • Sea stacks are temporary features and will eventually erode away completely, as coastal hills and cliffs disappear.
    • Coastal Plains
      • As wave erosion continues, coastal hills and cliffs retreat, leading to the formation of narrow coastal plains.
      • These plains can become covered by alluvial deposits or by shingle or sand to form a wide beach.
• DEPOSITIONAL LANDFORMS
  • Beaches and Dunes
    • Beaches
      • Beaches are typical of shorelines dominated by deposition but can also appear on rugged shores.
      • Most beach sediment comes from land, carried by streams and rivers, or from wave erosion.
      • Beaches are temporary features, and the composition may change with seasons, from sandy beaches to narrow strips of coarse pebbles.
      • Most beaches consist of sand-sized materials, while shingle beaches are made of small pebbles and cobbles.
    • Sand Dunes
      • Sand dunes form just behind beaches when sand lifted and winnowed from the beach is deposited.
      • These dunes often form long ridges parallel to the coastline, especially along low sedimentary coasts.

      

  • Bars, Barriers, and Spits
    • Off-shore Bars
      • Off-shore bars are ridges of sand and shingle that form in the sea in the offshore zone, roughly parallel to the coast.
    • Barrier Bars
      • If an off-shore bar is exposed due to the addition of sand, it becomes a barrier bar.
      • Barrier bars commonly form across the mouths of rivers or at the entrance of bays.
    • Spits
      • Spits develop when barrier bars extend across the mouth of a bay or get keyed up to one end of the bay or headland.
      • Spits may eventually block the bay, transforming it into a lagoon.
    • Lagoons and Coastal Plains
      • As barrier bars and spits extend, they gradually leave only a small opening, leading to the formation of a lagoon.
      • Eventually, lagoons fill up with sediment, leading to the development of coastal plains.

Winds as Geomorphic Agent

• WINDS
  • Role of Wind in Deserts
    • Wind is one of the two dominant agents in hot deserts.
    • The desert floor heats up quickly due to dryness and barrenness, causing the air above to warm and rise, creating turbulence.
    • This turbulence leads to eddies, whirlwinds, updrafts, and downdrafts, with winds moving swiftly along the desert floor.
    • Storm winds can be very destructive in deserts.
  • Processes Driven by Winds
    • Deflation
      • Deflation involves the lifting and removal of dust and smaller particles from the surface of rocks.
    • Abrasion
      • In abrasion, wind-driven sand and silt act as tools to wear away the land surface.
    • Impact
      • The impact is the sheer force of momentum when sand is blown into or against a rock surface, similar to a sandblasting operation.
  • Erosional and Depositional Features
    • Wind action in deserts creates a range of interesting erosional and depositional features.
  • Mass Wasting and Sheet Floods
    • Many desert features are formed by mass wasting and sheet floods.
    • Despite scarce rainfall, when it does occur, it comes down torrentially in short periods.
    • Desert rocks, exposed to weathering due to drastic temperature changes, decay faster, and torrential rains help remove weathered materials.
  • Movement of Weathered Debris
    • The weathered debris in deserts is moved not only by wind but also by rain and sheet wash.
  • Stream Channels in Deserts
    • Stream channels in desert areas are broad, smooth, indefinite, and typically flow briefly after rainfall.

• EROSIONAL LANDFORMS
  • Pediments and Pediplains
    • Landscape evolution in deserts is primarily concerned with the formation and extension of pediments.
    • Pediments are gently inclined rocky floors near mountains, sometimes with a thin cover of debris.
    • Pediments form through lateral erosion by streams and sheet flooding.
    • Erosion begins along steep margins of landmasses or tectonically controlled steep incisions.
    • Once pediments form, the steep wash slope and free face retreat backwards. This is called parallel retreat of slopes through backwasting.
    • Through parallel retreat, pediments extend backward at the expense of the mountain front, gradually reducing the mountain to an inselberg.
    • Eventually, high relief in desert areas is reduced to low, featureless plains called pediplains.
  • Playas
    • Plains are the most prominent landforms in deserts, often found in basins surrounded by mountains and hills.
    • Drainage in these basins flows toward the center, where sediment gradually deposits, forming a nearly level plain.
    • When there is sufficient water, the plain can be covered by a shallow water body, creating playas—shallow lakes that retain water only for short durations due to evaporation.
    • Playas often contain salt deposits and the playa plain covered with salts is called alkali flats.
  • Deflation Hollows and Caves
    • Deflation occurs when weathered material is blown away by persistent wind currents, creating shallow depressions known as deflation hollows.
    • Wind abrasion on rock surfaces creates small pits or cavities, which are the initial form of blowouts.
    • Some blowouts deepen and widen, evolving into caves.
  • Mushroom, Table and Pedestal Rocks
    • Many rock outcrops in deserts are easily worn down by wind deflation and abrasion, leaving behind remnants of resistant rocks.
    • These remnants are often shaped like mushrooms, with a slender stalk and a broad, rounded cap above.
    • Sometimes, the tops of rocks are flat like a table, or the remnants stand like a pedestal.

• Depositional Landforms
  • Wind as a Sorting Agent
    • Wind is a good sorting agent, capable of moving different sizes of grains along the floors by rolling, saltation, and suspension.
    • As the wind slows or dies down, the grains settle based on their size and critical velocity, leading to sorting in the deposition process.
    • Deposition by wind results in well-sorted grains, and these features can develop anywhere in arid regions where there is a good source of sand and constant wind direction.
  • Sand Dunes
    • Hot, dry deserts are ideal for sand dune formation.
    • Obstacles play a crucial role in initiating dune formation, leading to various dune shapes:
    • Barchans: Crescent-shaped dunes with points (wings) directed downwind. These form where the wind direction is constant and moderate, and the surface is uniform.
    • Parabolic Dunes: Formed when sandy surfaces are partially covered with vegetation, these dunes are reversed barchans, with the wind direction being the same.
    • Seif Dunes: Similar to barchans, but with only one wing or point due to a shift in wind conditions. These can grow long and high.
    • Longitudinal Dunes: Form when the sand supply is poor, but wind direction remains constant. These dunes are long ridges with considerable length but low height.
    • Transverse Dunes: Aligned perpendicular to the wind direction. They form when the wind is constant and the sand source is elongated at right angles to the wind direction. These dunes are often long and low in height.
    • When there is an abundance of sand, dunes may coalesce and lose their individual characteristics.
    • In some areas, dunes shift over time, while others stabilize, particularly near human habitation.