5.2 Fluvial, glacial, coastal & eolian landforms

Fluvial, Glacial, Coastal, and Eolian Landforms

Surface processes sculpt landscapes through moving water, ice, waves, and wind. Each agent erodes, transports, and deposits sediment in characteristic ways that produce diagnostic landforms tested on the ASBOG exams.

Fluvial systems

Running water is the dominant erosional agent on land. A stream's gradient is the vertical drop per unit horizontal distance; it is steep in the headwaters and gentle near the mouth, producing a concave-up longitudinal profile. Base level is the lowest elevation to which a stream can erode. Ultimate base level is sea level, and local base levels include lakes and resistant rock ledges. Lowering base level rejuvenates a stream and triggers downcutting, while raising it promotes deposition.

Streams carry sediment as dissolved, suspended, and bed loads. Competence (the largest particle a stream can move) depends on velocity, while capacity (the total load) depends on discharge. Where velocity drops, sediment is deposited.

Key fluvial landforms include:

• Meanders: Sinuous bends that erode the outer bank (cut bank) and deposit sediment on the inner bank (point bar). Cutoff meanders form oxbow lakes.
• Floodplains: Flat valley floors built by overbank deposition, with natural levees flanking the channel.
• Deltas: Sediment deposited where a stream enters standing water, showing topset, foreset, and bottomset beds.
• Alluvial fans: Cone-shaped deposits where a steep stream emerges onto a plain, common in arid basins.
• Braided streams: Networks of shifting channels carrying coarse, abundant bedload.

Drainage patterns reflect the underlying geology: dendritic (branching, on uniform rock), trellis (parallel channels with right-angle tributaries, on folded or tilted beds), radial (outward from a volcano or dome), and rectangular (right-angle bends controlled by joints and faults).

A graded stream has adjusted its slope so that, over years, the sediment it receives is just transported through the system, balancing erosion and deposition. When base level falls or the land is uplifted, a stream downcuts and abandons its former floodplain as paired stream terraces; a meandering stream that downcuts into bedrock produces incised meanders. These features record changes in base level, tectonics, or climate and are important to Quaternary and engineering studies.

Glacial systems

Glaciers are moving masses of ice that erode by plucking (quarrying jointed bedrock) and abrasion (grinding that leaves striations and polish). Alpine (valley) glaciers carve distinctive erosional forms, whereas continental ice sheets blanket entire regions.

Erosional landforms include U-shaped valleys (glacial troughs, contrasting with the V-shaped valleys of streams), cirques (bowl-shaped headwalls), aretes (knife-edge ridges), horns (pyramidal peaks), and hanging valleys left by tributary glaciers. Where a glacial trough is later drowned by the sea, it becomes a fjord. Small crescent gouges, chatter marks, and roches moutonnees (asymmetric bedrock knobs) record ice-flow direction.

Glacial deposits are collectively called drift:

• Till: Unsorted, unstratified debris deposited directly by ice, forming moraines (terminal, lateral, medial, ground, and recessional).
• Outwash: Sorted, stratified sediment deposited by glacial meltwater on outwash plains.
• Erratics: Boulders transported far from their bedrock source.
• Eskers, kames, and kettles: Meltwater ridges, mounds, and depressions associated with stagnant ice.
• Drumlins: Streamlined till hills that indicate the direction of ice flow.

Coastal systems

Waves reshape shorelines. Waves approaching the shore at an angle drive longshore drift, the zigzag transport of sand along the beach within the surf zone; the related longshore current moves sediment parallel to the coast.

• Beaches: Zones of wave-worked sediment where swash and backwash sort the grains.
• Spits and baymouth bars: Elongated ridges built by longshore drift across bays; a spit that connects an island to the mainland is a tombolo.
• Barrier islands: Long offshore sand islands parallel to the coast and backed by lagoons.
• Sea stacks, arches, and wave-cut platforms: Erosional forms on rocky, exposed coasts.

Engineered structures such as groins, jetties, and seawalls interrupt longshore drift, trapping sand updrift while starving beaches downdrift, a key consideration in coastal-hazard practice.

Emergent coasts (from uplift or falling sea level) display marine terraces, while submergent coasts (from subsidence or rising sea level) display drowned river valleys called estuaries and rias.

Eolian systems

Wind moves fine sediment and is dominant in deserts and along coasts where vegetation is sparse. Sand travels by saltation (short bouncing hops), building dunes that migrate downwind; the gentle windward slope and steep leeward slip face record the wind direction. Dune types include barchan (crescent-shaped, limited sand), transverse (ridges, abundant sand), longitudinal (linear, parallel to wind), and star (formed under multidirectional winds). Loess is wind-blown silt that forms thick, fertile, vertically stable blankets, often derived from glacial outwash or deserts. Desert pavement (a surface armor of coarse clasts left after fines are removed) and ventifacts (wind-faceted stones) reflect deflation and abrasion, while streamlined bedrock ridges carved by wind are yardangs. Because eolian deposits are sensitive to wind and moisture, they are readily reactivated when vegetation is disturbed, a concern in land management and hazard assessment.

Summary table