Landforms and their Evolution

Landscapes evolve through three distinct chronological stages (William Morris Davis's Geographical Cycle):

• Youth Stage: Steep vertical gradients, fast turbulent flow, dominant vertical downcutting, V-shaped valleys, rapids, and waterfalls; zero floodplains.
• Mature Stage: Gradients moderate, lateral erosion widens valley floors, meanders develop, and extensive floodplains emerge.
• Old Age Stage: Flat, sluggish flow, dominant deposition, extensive oxbow lakes, natural levees, braiding channels, and the landscape is reduced near base level to a gently undulating Peneplain with residual monadnocks.

Fluvial Landforms (Running Water):

• Erosional Landforms:
  • Gorges & Canyons: Deep, narrow valleys with near-vertical rock walls. A Gorge has equal width at top and bottom formed in hard rocks; a Canyon is wider at top with stepped cliff slopes formed in horizontal sedimentary rocks (e.g., Grand Canyon of Colorado).
  • Potholes & Plunge Pools: Circular depressions ground into river beds by swirling pebbles (potholes); large deep basins at the base of waterfalls (plunge pools).
  • River Terraces: Step-like benches marking former floodplain levels resulting from vertical river rejuvenation.
• Depositional Landforms:
  • Alluvial Fans: Cone-shaped deposits formed where steep mountain streams abruptly emerge onto flat plains, suddenly dropping coarse sediment.
  • Deltas: Triangular alluvial deposits formed at river mouths entering oceans or lakes, where river velocity drops to zero and distributaries branch out.
  • Meanders & Oxbow Lakes: Loop-like curving bends in mature rivers. Erosion occurs on the outer concave cut-bank, while deposition occurs on the inner convex point-bar. Over time, the meander neck is severed during floods, leaving an isolated crescent-shaped Oxbow Lake.
  • Natural Levees: Low linear ridges of coarse alluvium deposited along river banks during repeated seasonal floods.

In regions underlain by thick, permeable, jointed Limestone or Dolomite ($CaCO_3$), percolating acidic groundwater dissolves the rock, creating Karst Topography (named after the Karst region in Slovenia):

A. Erosional Karst Features:

• Sinkholes (Swallow Holes): Funnel-shaped circular depressions formed on the surface where limestone has dissolved or cavern roofs have collapsed.
• Doline & Uvala: When multiple adjacent sinkholes collapse and merge, they form larger intermediate depressions called Dolines, and colossal compound trenches called Uvalas or Poljes.
• Caves: Subterranean tunnels and caverns dissolved along bedding planes where horizontal groundwater flows.

B. Depositional Karst Features (Speleothems):

Water dripping from cavern ceilings contains dissolved calcium bicarbonate. As water evaporates, it precipitates tiny crystals of calcium carbonate:

• Stalactites: Sharp, icicle-like stone formations hanging downward from the cavern ceiling.
• Stalagmites: Broad, rounded stone mounds growing upward from the cavern floor directly beneath dripping stalactites.
• Pillar / Column: When an ascending stalagmite fuses completely with a descending stalactite.

Glaciers are colossal masses of moving ice traveling downhill under gravity, scouring bedrock through plucking and abrasion:

A. Erosional Glacial Landforms:

• Cirque (Corrie): Deep, steep-sided, armchair-like amphitheater depressions carved into high mountain heads. When the glacier melts, a beautiful circular lake called a Tarn fills the basin.
• Arête & Horn: When three or more cirques erode headward toward each other, they carve knife-edged, serrated mountain ridges called Arêtes, terminating in a sharp, pyramidal horn peak (e.g., the Matterhorn in the Swiss Alps).
• U-Shaped Glacial Trough: Unlike V-shaped river valleys, glaciers scour deep, wide valleys with flat valley floors and vertical rock walls, creating a distinct U-shaped cross-section.
• Hanging Valleys: Tributary glacial valleys with smaller ice volume that cannot erode as deeply as the main trunk glacier. When ice melts, tributary streams plunge down sheer cliffs as spectacular waterfalls (e.g., Yosemite Falls).

B. Depositional Glacial Landforms (Glacial Drift):

• Moraines: Linear ridges of unsorted glacial till (boulders, gravel, sand):
  • Terminal / End Moraine: Ridge dumped at the farthest forward snout of the glacier.
  • Lateral Moraine: Ridges along the outer valley walls.
  • Medial Moraine: Ridge formed in the center when two lateral moraines merge.
• Eskers: Sinuous, winding ridges of gravel deposited by sub-glacial meltwater rivers flowing in tunnels beneath melting ice sheets.
• Drumlins: Smooth, inverted spoon-shaped oval hills of glacial till aligned parallel to ice flow, forming an "Egg-Basket Topography".

A. Coastal Landforms (Waves & Currents):

• Erosional:
  • Sea Cliffs & Wave-Cut Platforms: Steep rock faces notched by wave hydraulic pounding; as cliffs retreat landward, a flat rock bench (wave-cut platform) is left at water level.
  • Caves, Arches & Stacks: Waves attack headland weaknesses, carving Sea Caves. When caves erode through from both sides, an open Sea Arch forms. When the arch roof collapses, an isolated vertical stone pillar called a Sea Stack remains standing in the surf.
• Depositional:
  • Beaches & Dunes: Temporary accumulations of sand and shingle along shorelines.
  • Bars, Spits & Lagoons: Ridges of sand deposited parallel to the coast (bars). A bar attached to land at one end extending into the sea is a Spit. When a spit encloses a body of seawater, it creates a shallow coastal saltwater lake called a Lagoon (e.g., Chilika Lake in Odisha).

B. Arid Aeolian Landforms (Wind Action in Deserts):

• Erosional:
  • Pediments & Inselbergs: Broad, gently sloping rock floors at mountain bases; isolated, steep-sided residual rock hills rising from desert plains (Inselbergs).
  • Mushroom Rocks (Rock Pedestals): Wind-blown sand grains bounce within 1 to 2 meters of the ground, intensely eroding the lower trunk of a boulder while leaving the top intact, sculpting a mushroom shape.
  • Yardangs & Zeugens: Wind-carved ridges and troughs in alternating bands of hard and soft rock.
• Depositional (Sand Dunes):
  • Barchan: Crescent-shaped sand dunes with the horns (tips) pointing downwind (in the direction of wind flow), formed in areas of constant wind direction and moderate sand supply.
  • Seif (Longitudinal Dunes): Long, straight ridges of sand aligned parallel to prevailing wind direction.
  • Loess: Fine, unstratified, yellowish, wind-blown silt transported hundreds of kilometers from deserts and deposited in thick blankets (e.g., Loess Plateau of northern China).