Landforms And Their Evolution
Running Water
Erosional Landforms
Valleys
Valleys are formed by the erosional activity of running water, primarily through vertical erosion and also by mass wasting.
The type of valley depends on the stage of the river’s course:
Young stage: Characterized by steep, narrow valleys with V-shaped cross-sections, often with waterfalls and rapids.
Mature stage: Valleys are wider, with gentler slopes and the river starts to meander.
Old stage: Valleys are very wide, broad, and flat-floored, with meandering rivers and extensive floodplains.
Potholes And Plunge Pools
Potholes: These are cylindrical depressions found at the bottom of a river valley, formed by the abrasive action of pebbles and boulders rotated by the swirling water in eddies.
Plunge Pools: At the base of waterfalls, the turbulent water, along with the eroded material, carves out a deep basin called a plunge pool.
Incised Or Entrenched Meanders
When a river flowing through a soft, flat plain starts to experience rejuvenation (uplift of the land), it cuts vertically downwards, deepening its existing meanders. These are called incised or entrenched meanders.
River Terraces
River terraces are flat or gently sloping surfaces found on either side of a river valley, representing former floodplains that have been uplifted or lowered by river incision due to rejuvenation. They are remnants of earlier, higher levels of the valley floor.
Depositional Landforms
Alluvial Fans
When a river emerges from a steep mountain region onto a plain, its speed decreases significantly. This causes it to deposit the eroded materials, forming a fan-shaped deposit of sediment known as an alluvial fan.
Deltas
Deltas are large, triangular-shaped landforms created by the deposition of sediments at the mouth of a river where it enters a larger body of water, such as an ocean, sea, or lake. The river’s velocity reduces, leading to the settling of sediment load.
Floodplains, Natural Levees And Point Bars
Floodplains: These are broad, flat areas adjacent to a river that are subject to flooding. During floods, the river overflows its banks, depositing a layer of fertile silt, making floodplains very productive for agriculture.
Natural Levees: As floodwaters spread out onto the floodplain, the velocity of the water decreases, causing it to deposit coarser sediments along the immediate banks of the river. These raised, parallel ridges are called natural levees.
Point Bars: These are depositional features found on the inside bends of meanders. As the river flows, it erodes the outer bank and deposits sediment on the inner bank, leading to the growth of point bars.
Meanders
Meanders are sinuous or looping bends in a river channel. They form in the middle and lower courses of a river where the land is relatively flat and the river has a lower gradient. Meandering is a result of erosion on the outer bank and deposition on the inner bank.
Groundwater
Erosional Landforms
Pools, Sinkholes, Lapies And Limestone Pavements
Pools: Small depressions in the bedrock formed by the chemical action of water, especially on soluble rocks like limestone.
Sinkholes: These are funnel-shaped or cylindrical depressions in the ground formed by the dissolution of soluble rocks, such as limestone, by groundwater. They can also form by the collapse of underground caves.
Lapies: These are irregular grooved or fluted surfaces found on exposed limestone bedrock, formed by the solution action of rainwater and groundwater.
Limestone Pavements: Extensive areas of exposed limestone bedrock exhibiting well-developed lapies.
Caves
Caves are large underground openings formed by the dissolution of soluble rocks, particularly limestone, by groundwater. Water, often slightly acidic due to dissolved carbon dioxide, dissolves the rock over long periods, creating caverns and passages.
Depositional Landforms
Stalactites, Stalagmites And Pillars
These are depositional features found within limestone caves, formed by the dripping of mineral-rich water.
Stalactites: Icicle-shaped formations that hang from the roof of a cave, formed as mineral deposits (primarily calcium carbonate) accumulate from dripping water.
Stalagmites: Upward-growing cone-shaped formations that rise from the floor of a cave, formed by the accumulation of mineral deposits from dripping water originating from stalactites or the cave roof.
Pillars: When a stalactite and a stalagmite meet and fuse, they form a pillar.
Glaciers
Erosional Landforms
Cirque
A cirque (also known as a corrie or cwm) is a bowl-shaped hollow or amphitheatre-like valley head formed by glacial erosion at the head of a glacial valley. It is characterized by steep, often cliff-like, walls and a flat or gently sloping floor.
Horns And Serrated Ridges
Horns: When several cirques erode backwards into a mountain mass from different sides, they can form sharp, pyramid-shaped peaks called horns (e.g., the Matterhorn).
Serrated Ridges: The sharp, saw-toothed ridges that separate adjacent cirques are called arêtes or serrated ridges, formed by the backwearing of glacial ice.
Glacial Valleys/Troughs
These are U-shaped valleys carved out by the immense erosive power of glaciers. As glaciers move down valleys, they widen and deepen them, removing pre-existing V-shaped river valleys and smoothing out the sides and floor. They often contain hanging valleys (smaller valleys that enter the main glacial valley at a higher level) and truncated spurs (the ends of ridges that have been eroded by the glacier).
Depositional Landforms
Moraines
Moraines are ridges or mounds of unconsolidated debris (till) deposited by a glacier. They are classified based on their position relative to the glacier:
Lateral Moraines: Formed along the sides of the glacier.
Medial Moraines: Formed when two glaciers merge, with the lateral moraines of each glacier joining to form a ridge in the middle of the combined glacier.
Terminal Moraines: Deposited at the furthest extent of a glacier's advance.
Ground Moraines: A widespread, irregular layer of till deposited as the glacier retreats.
Eskers
Eskers are long, winding ridges of stratified sand and gravel deposited by meltwater streams flowing within, under, or upon a receding glacier. They represent the channel of a subglacial or englacial stream.
Outwash Plains
These are broad, flat plains composed of stratified sand and gravel deposited by meltwater streams flowing away from the front of a glacier. They are characterized by braided stream patterns and are often found in front of terminal moraines.
Drumlins
Drumlins are streamlined, elongated hills composed of glacial till, typically with a smoother, gentler slope on the down-ice side and a steeper, more abrupt end on the up-ice side. They are formed by the moulding action of a glacier moving over unconsolidated till.
Waves And Currents
High Rocky Coasts
These coasts are characterized by steep slopes, cliffs, and a lack of extensive beaches. They are often formed in areas where the land has been uplifted or where erosion is dominant over deposition.
Low Sedimentary Coasts
These coasts are typically low-lying and dominated by deposition, with features like beaches, sand dunes, and barrier islands. They are often found in areas where sediment supply is high and wave energy is moderate.
Erosional Landforms
Cliffs, Terraces, Caves And Stacks
Cliffs: Steep, vertical or near-vertical slopes formed by the undercutting action of waves at the coastline. As waves repeatedly strike the base of a cliff, they erode the rock, causing it to collapse and retreat.
Terraces (Wave-cut Terraces): Flat or gently sloping platforms formed at the base of a retreating cliff, marking the former sea level. As the cliff erodes back, the terrace is left behind, often at a higher elevation.
Caves (Sea Caves): Formed by the action of waves exploiting weaknesses in the rock along the coastline. Hydraulic action, abrasion, and corrosion all contribute to the enlargement of these openings.
Stacks: Isolated pillars of rock standing offshore, formed when a sea arch (a cave that has eroded through a headland) collapses. The remaining isolated masses of rock are stacks.
Depositional Landforms
Beaches And Dunes
Beaches: Accumulations of sand, pebbles, or shells deposited by waves along the coastline. They are dynamic features, constantly modified by wave action and currents.
Dunes: Mounds of sand formed by the wind, typically found behind beaches. Vegetation often colonizes dunes, helping to stabilize them.
Bars, Barriers And Spits
Spits: Elongated, narrow deposits of sand or shingle that project from the coast into the sea. They are formed by longshore drift, where material is carried parallel to the coast by waves and deposited where the coastline changes direction.
Bars: Similar to spits but often extend across the mouth of a bay or estuary, or connect an island to the mainland. Offshore bars are submerged ridges of sand parallel to the coast.
Barriers: Long, narrow, sandy islands that run parallel to the coast, separated from the mainland by a lagoon or bay. They are formed by the accumulation of sand deposited by waves and currents.
Winds
Erosional Landforms
Pediments And Pediplains
Pediments: Gently sloping, eroded surfaces found at the base of mountains or hills in arid and semi-arid regions. They are formed by the combined action of weathering, mass wasting, and occasional ephemeral stream erosion.
Pediplains: Extensive, undulating, and relatively flat plains formed by the coalescence of pediments. They represent a significant stage in the dissection of a mountain mass in arid environments.
Playas
Playas are flat, low-lying, ephemeral lake basins or dry lake beds found in arid and semi-arid regions, particularly in depressions within basins. They are characterized by fine-grained sediments and evaporite deposits, formed by the evaporation of seasonal or intermittent lakes.
Deflation Hollows And Caves
Deflation Hollows: Depressions formed by the removal of loose, fine-grained material (sand and dust) by the wind, a process called deflation. These hollows can vary in size and depth.
Caves (Wind-Eroded Caves): Formed by the abrasive action of wind-blown sand and dust. The sand particles strike and wear away the rock surface, creating cavities and alcoves. These are often found in areas with a strong, consistent wind and abundant sand.
Mushroom, Table And Pedestal Rocks
These are distinctive rock formations created by differential weathering and erosion by wind-blown sand. The wind, carrying abrasive particles, erodes the base of an isolated rock mass more intensely than the upper parts, resulting in a shape that resembles a mushroom, table, or pedestal.
Depositional Landforms
Sand Dunes
Sand dunes are accumulations of sand formed by the wind. They are common in deserts and coastal areas. The shape and size of sand dunes depend on factors such as wind velocity, sand supply, and the presence of vegetation. Common types include barchans (crescent-shaped), transverse dunes (perpendicular to the wind), longitudinal dunes (parallel to the wind), and star dunes.