Aeolian Geomorphology, Glaciers, and Geomorphology of South Carolina
Aeolian Geomorphology: Wind as a Geomorphic Agent
Aeolian geomorphology studies wind as a geomorphic agent, focusing on erosion, transportation, and deposition processes.
Factors Affecting Aeolian Erosion
Wind speed: Higher wind speeds result in more erosion.
Surface cover and roughness: These factors alter wind speed by absorbing and deflecting wind energy away from erodible soil.
Soil texture and ‘clumpiness’: Affects cohesion and erodibility of soils (e.g., sands vs. clays).
Soil moisture: Moist soil is more cohesive and experiences less wind-based erosion than dry soil.
Fetch: Distance the wind blows without encountering a barrier.
Two Processes of Aeolian Erosion
Deflation: Erosion of particles as wind picks them up and moves them.
Abrasion: Erosion by windblown particles ("sand-blasting").
Transportation
Wind moves particles in three main ways:
Suspension (lightest):
Fine particles (silt, clay) are lifted and carried high and far in the air.
Can travel hundreds or even thousands of kilometers (e.g., dust storms).
Saltation:
Medium-sized particles (sand) are lifted briefly and hop along the ground.
Most common form of wind transport.
Surface creep (heaviest):
Larger particles (coarse sand, small pebbles) roll or slide along the ground.
Moved by impact from saltating particles.
Transport distance and height depend on:
Particle size (finer = longer/farther transport)
Wind speed
Turbulence in the air
Deposition
Wind deposits particles when:
Wind speed decreases, reducing energy.
Vegetation or obstacles trap moving particles.
Moisture increases, binding particles.
Topography changes (e.g., windward side of dunes slows wind).
This leads to formations like dunes, loess deposits, and sand sheets.
Erosional Landforms
Desert pavement: Removal of fine particles by wind and surface flow leaves a surface of larger stones.
Ventifact: A rock that has been abraded, pitted, etched, grooved, or polished by wind-driven factors.
Balanced rocks (mushroom rocks): A special type of ventifact where abrasion is maximized near the base due to the material-carrying capacity of the wind being at its maximum.
Yardangs: Formed where winds are strong, unidirectional, and carry an abrasive sediment load; the wind cuts down low-lying areas into parallel ridges which gradually erode into separate hills.
Depositional Features – Sand Dunes
Sand dunes: Wind-sculpted accumulation of sand.
Sand dunes form where:
There is a plentiful supply of sand.
There is little surface protection and cohesion.
A natural trap causes wind to drop the sand.
Dune Migration
Dunes can move while maintaining a constant shape.
Erosion and saltation up the windward slope (stoss slope).
Deposition on the backside (slip face) and dune advancement.
Live dunes vs. fixed dunes:
"Live" or mobile dunes are actively shifting and being shaped by wind, while "fixed" dunes are stabilized by vegetation, preventing significant movement.
Wind Direction
Wind Roses:
Graphs showing wind directions over time.
Winds are named in the direction from which they come.
North is oriented to the top; south to the bottom.
The length of the bar is proportional to the frequency of winds.
Dune Types
Barchans: Crescent-shaped dunes with horns pointing downwind from the dominant wind direction.
Transverse Dunes: Barchans can become aligned together along a plane perpendicular to the wind. If the line becomes somewhat straight, the resulting dune ridges form transverse dunes.
Longitudinal Dunes (Seifs): Long, straight, sharp-crested dunes with two slip faces that are aligned parallel to the wind direction. They are associated with bidirectional winds.
Parabolic Dunes and Coastal Blowouts: Dunes formed from "blowouts" where the erosion of vegetated sand leads to a U-shaped depression. The elongated arms, which are located on the upwind side, are often held in place by vegetation.
Star Dunes: Pyramidal dunes with slip faces on three or more arms that radiate from the high center of the mount. Occurs where winds shift and there is no prevailing wind direction.
Aeolian Features: Deposition
Loess: Wind-deposited blankets of silt and clay (smaller particles than sand).
Loess originated as silt ground up and deposited by glaciers during Ice Ages.
It was then picked up and deposited further east by wind, forming blankets of silt.
Long-distance aeolian transportation: extrabasin transfers of fine materials are important to explaining local soil chemistry and nutrient budgets.
Erosional Desert Surfaces: Regs and Hamadas
Reg: Stony depressions and flats
A reg is a desert surface covered with closely packed, angular or rounded gravel and pebbles, often left behind after finer particles are removed by wind.
Created by deflation, where wind blows away fine sand and dust, leaving behind coarser fragments.
Over time, the surface becomes compact and resistant to further erosion.
Hamada: High, barren rocky plateaus
A hamada is a barren, hard, rocky desert surface composed of exposed bedrock or large rock fragments.
Formed when wind and occasional water erosion remove sand and dust, exposing the underlying rock.
May also result from weathering and breakdown of rock in place.
Erg: Broad, sand seas covered with wind-swept sand with little or no plant cover
Strictly speaking, an erg is a desert area that contains more than 48 square miles of aeolian sand and where sand covers more than 20\% of the surface; smaller areas are known as dune fields.
Formed by the accumulation of sand transported by wind (via saltation and suspension).
Dunes within ergs can take various shapes (e.g., barchan, star, transverse).
Types of Desert Surfaces
Deserts are often characterized by different types of surfaces in different settings:
Ergs: depositional
Regs: depression with desert pavement
Algodones and Imperial Dunes in Southern California
These dunes are Ergs/Dune Fields.
These dunes are part of a large aeolian sand system stretching over 40 miles (64 km).
They are composed primarily of fine to medium sand transported and deposited by prevailing northwesterly winds.
The area features various types of sand dunes, including crescent-shaped barchan dunes and linear dunes.
Fluvial Processes in Arid Lands
Humid areas: rivers recharged by groundwater.
Deserts: deeper water table means that rivers flow only after rain.
Such dry rivers are called washes or arroyos.
Effects of Water in Deserts on Landforms
Even though deserts are defined by their dryness, water is a powerful agent of erosion and landform development—often more so than in wetter climates due to the intensity of rare storms.
Low precipitation.
Episodic rainfall (flash floods).
High evaporation rates.
Infiltration and porous soils.
Lack of perennial water sources.
Mesas, Buttes, and Pinnacles
Mesas: Tablelands/plateaus with a flat top and sides that are usually steep cliffs.
Buttes: Isolated “hills” with steep, often vertical sides and a small, relatively flat top.
Pinnacle (‘needle’): An individual column of rock, isolated from other rocks or groups of rocks, in the shape of a vertical shaft or spire.
Depositional Features in Deserts
When water flows out of mountains onto the desert floor, it slows down and deposits material forming a fan-shaped feature similar to a delta (alluvial fan).
Bajada: Merging of alluvial fans creates a “ramp” or “apron” of material at the base of slopes.
Playas and Salt Flats
Playa: A (usually) dry lakebed underlain by fine sediment or salts deposited by shallow lake waters on the floor of a closed basin.
Salt flat: A flat expanse of ground covered with salt and other minerals formed where water pools and evaporates.
Pluvial Lakes
Pluvial lakes: Lakes formed by higher rainfall and/or lower evaporation under past climates.
Pluvial lakes are evidence that many areas now dry or desert-like were once much wetter.
Their presence shows that climate has shifted dramatically over thousands of years.
Examples:
Lake Bonneville (ancestor of the modern Great Salt Lake in Utah).
Lake Lahontan (in present-day Nevada).
These lakes left behind features like:
Shoreline terraces
Clay flats and playas
Salt deposits
Wave-cut cliffs
Long Term Environmental Change
Geological, climatological, and biological proxies (natural archives): indicators or phenomena used to infer past conditions
Examples:
Fossils
Geological deposits
Pollen, tree rings, corals
Ice cores
Direct measurements of past climates don't exist before the modern instrumental record.
Proxies indirectly record conditions, helping us understand long-term environmental change.
Ice cores: Record past greenhouse gas concentrations, showing how atmospheric composition has influenced glacial/interglacial cycles and geomorphology.
Climate change and geomorphology
Time scales of change:
Really long scales (10s to 100s of millions of years)
Long time scales (millions to 100,000’s of years)
Historic time scales (10,000’s to dozens of years)
Drivers of change:
Factors that change geography
Plate tectonics alters latitudes of landmasses, reshapes topography, and changes currents.
Changes ocean currents, atmospheric circulation, and location of landmasses and mountains.
Factors that affect how solar energy is received, reflected, absorbed, and stored
Atmospheric composition
Milankovitch cycles and feedback
Climate Change and Atmospheric Composition
Greenhouse effect: Heat is trapped in the lower atmosphere by certain gases (greenhouse gases) that allow shortwave, incoming sunlight to pass but absorb longer wave outgoing energy from the planet’s surface.
Greenhouse gas concentrations have varied naturally throughout earth’s history, contributing to changes in earth’s climate.
Trap heat in the atmosphere via the greenhouse effect.
Allow incoming shortwave radiation but absorb outgoing longwave radiation.
Volcanism
Gases injected into the upper atmosphere reflect incoming solar radiation back into space, causing cooling.
Reflect sunlight causing cooling.
Climate Change and Milankovitch Cycles
Forcings:
Eccentricity: ellipticity of the Earth’s orbit
Obliquity: tilt of the earth’s axis: 22.1°-24.5°
Precession: orientation of the earth’s axis
Milankovitch cycles interact with feedback in:
Concentrations of greenhouse gases
Albedo (reflectivity)
Oceanic circulation
Positive feedbacks can accelerate climate change:
Ice melt → lower albedo → more heat absorbed → more melting.
Greenhouse gas release from warming permafrost, etc.
The Quaternary Period
Pleistocene epoch: 2.6 million-11,000 years ago
Holocene epoch: last 11,000 years
This period has experienced fluctuations between colder phases when ice and snow accumulate (“ice ages”) and warmer phases associated with melting (“interglacial”).
Marked by cycles of glaciation and interglacial warming.
Glaciers expanded and retreated multiple times.
Since then, warming, global retreat, and sea level rise.
Glaciers
Glaciers: large mass of perennial ice formed by the accumulation of recrystallization of snow under pressure.
Alpine glaciers: located in mountains, largely confined to areas within valley walls.
Continental ice sheets: large, unconfined glaciers that cover continental area.
How Glaciers Form
As snow accumulates, it is compressed, becoming denser and forming neve and eventually glacial ice. This process requires a snow thickness of > 150-200 ft.
Glacial Movement: Internal Plastic Flow
Internal plastic flow: weight of the overlying glacier causes glacial ice to move like a slow-moving ‘plastic river’. (ice deforms under pressure and flows slowly like plastic)
Near the surface of the glacier, the ice is more brittle and cracks forming crevasses.
The glacial terminus (the end of the glacier) does not necessarily advance in response to flow; it may even retreat if the flow rate < melt.
Crevasses: surface cracks in glaciers due to brittle deformation of the upper ice
Glacial Movement: Basal Sliding
Basal sliding: the weight of a glacier exerts enough pressure to melt the ice where it comes into contact with the ground. The meltwater acts as a lubricant, allowing the glacier to slide forward until its own weight and gravity.
Glacial expansion/contraction (mass balance)
Accumulation: new inputs of snow
Ablation: losses of ice
Mass balance = accumulation – ablation
In glacier dynamics, a positive net balance means a glacier is gaining more mass than it's losing, leading to growth and potential advance. Conversely, a negative net balance indicates a glacier is losing more mass than it's gaining, resulting in shrinkage and retreat.
Glacial expansion and contraction (mass balance)
Accumulation: new inputs of snow
Ablation (wastage): losses of ice
Mass balance = accumulation – ablation
Spatially, average annual conditions define zones of accumulation and ablation
Positive mass balance: glacier grows
Negative mass balance: glacier retreats
Glacial Mass Balance
Zone of accumulation: snow input > melt; net accumulation feeds ice flow downslope.
Zone of ablation: snow input < melt: melting is more rapid than accumulation, so relies on ice flows above.
Alpine glaciers are found in mountains (ex. Rockies, Appalachian mountains).
Glacial Erosion
Plucking: rocks are loosened, detached, and carried away by the glacier's movement.
Abrasion: plucked debris grinds across bedrock, scouring away material.
Glacial Transportation and Deposition
Material can be transported in two ways:
By meltwater streams running through and out of a glacier
By the ice (within or on top) of the glacier
Depositional features follow Transportation Type
Deposition by meltwater streams running through and out of a glacier: glaciofluvial deposits, valley train.
Alpine Glacial Geomorphology – A Checklist of Features
Erosional Features – alpine glaciers:
Cirque: A “scooped-out” bowl-shaped basin at the head of an alpine glacier.
Horn: A pyramidal peak that results when several glaciers create cirques on various sides of a mountain.
Arête: A sharp ridge dividing two cirques or glacial valleys.
Tarn: A small glacial lake.
U-shaped glacial valley (trough): Glaciated valley that takes on the shape of a “U”.
Hanging valley: Valleys of tributaries left “stranded” high above a glaciated valley floor.
Paternoster lakes: Small stair-stepped lakes running down a glaciated valley.
Fjord: A long, deep, narrow sea inlet formed by submergence of a glaciated valley formed by sea level rise.
Depositional features: alpine glaciers:
Valley train deposit: Glacially-eroded material that is deposited by meltwater streams beyond the furthest extent of glaciation.
Till: Blanket of eroded material of all sizes that is dropped as a glacier retreats.
Moraine: Ridge of unsorted glacial material piled up at the edge of a glacier by internal plastic flow (terminal moraine forms at the farthest extent of glaciation).
Continental Glaciers
Continental glaciers: (ice sheets) large, unconfined glaciers that cover continental areas.
Can be > 2 miles thick.
Isostatic depression of the crust due to the weight of overlying ice; much of the continental crust is below sea level.
General affect: smoothing the existing terrain through plucking and abrasion; and deepening and widening of existing rivers and lakes.
Depositional Features: Continental Glaciers
Ice transports sediment to the terminus like a conveyor belt
If the terminus is stable for a long period, the result is a moraine
Retreat refers to the melting back of the glacial terminus. Ice does not reverse flow direction; it continues to flow forward but melts too quickly to be replaced.
Terminal moraine: Forms at the farthest ice position
Recessional moraine: Ice retreats then restabilizes at a new position
Outwash plain: Glacifluvial deposits created by meltwater rivers beyond the extent of glaciation
Drumlin: A depositional hill streamlined in the direction of glacial movement
Drumlin formation continues to be debated, but the most prevalent theory is that they’re created by sub-glacial deposition and its impact on aqueous flow.
Blunt up-ice end, tapered down-ice end (opposite Roche moutonnee)
Moulin: Internal drainageway in a glacier that carries water and sediment to the glacial base
Kame: Hill of stratified sediment deposited by meltwater in a stagnant glacier below a mountain
Esker: A sinuous, narrow ridge of a stratified sand and gravel deposited in an ice tunnel within a glacier
Kettle lake: A glacial lake created by deposition of till around (and on) melting blocks of glacial ice
Glacial erratic: A boulder deposited by ice that differs from local bedrock
Anthropocene: The most recent geological epoch starting in the 18th century when human activities first began to have a significant impact on the earth’s atmosphere, geology, and ecosystems.
Since the last glacial period: Glaciers have retreated significantly since the last glacial maximum, retreat has accelerated in recent decades due to warming, and the Anthropocene
Geomorphology
Geomorphology: The study of landforms, their form, process, and pattern
Form: the shape, structure, or appearance of a landform or physical feature
Process: the forces or actions that create, modify, or destroy landforms over time
Pattern: the spatial arrangement of the distribution of landforms or features across a landscape or region
Geo = Earth; Morph = shape; Ology = study/ science of
Science of landforms: origin, evolution, form, and spatial distribution of landforms and related features
South Carolina Physiography
Blue Ridge: Mountains, rugged topography.
Piedmont: Rolling hills with iron-rich clay soils
Sandhills: Hilly, discontinuous band of sand and sandstone.
Coastal Plain: Lower elevation plain formed by deposited sediments and shaped by ocean advance and retreat.
Coastal Zone: The narrow interface along the ocean dominated by coastal processes and landforms.
Blue Ridge: Mountainous region dominated by folded and faulted metamorphic and intrusive igneous rocks
Piedmont: Fairly rugged area underlain by metamorphic rocks that were folded by plate convergence and later affected by rifting as Pangea broke apart, forming the Atlantic Ocean.
Looking from the blue ridge onto the piedmont, with lakes Hartwell and Keowee in the distance.
The piedmont contains a number of accreted terranes, including the Carolina terrane, an old volcanic island arc that was added to north America through subduction.
Regions and Features
Blue Ridge: Mountains; igneous & metamorphic rocks.
Piedmont: Rolling hills; iron-rich soils; site of historic erosion.
Sandhills: Ancient ocean/beach deposits.
Coastal Plain: Low relief; sedimentary rocks from marine deposits.
Coastal Zone: Dynamic shoreline influenced by tides, storms.
Due to erosion, weathering, deposition, these processes add or take away to landforms, completely reshaping and forming them.
The fall line: Transition from the piedmont (composed of continental bedrock) to the coastal plain (created by alluvial sediments).
The fall line was as far as most boats could sail safely up or down rivers. At the fall line, they would unload their goods from the boat and transport them by land.
The Columbia Canal: Built in 1824 to provide direct routes between upstate and downstate settlements
The canal is now used to generate hydroelectric power and provides recreation along the three rivers greenway
Brown water streams: Originate in upstate areas beyond the coastal plain. The name comes from the suspended sediment eroded from piedmont soils.
Black water streams: Originate on the coastal plain and have lower suspended loads. The water is ‘tea-colored’ from the breakdown of leaves in the swamp settings.
Sandhills: Region formed by ancient ocean deposits from ca. 85-100 million years ago when sea level was much higher, along with more recent wind-deposited sands brought in during ice ages
Soils are generally sandy, and sandstone formations at the edge of the sandhills are riddled with fossils of ancient marine creatures.
Coastal Plain: A low relief plain stretching from the Fall Line to the Atlantic Ocean.
Repeated inundation (when the ocean was higher) and exposure (when the ocean was lower) led to deposition, compaction, and ‘lithification’ of sands, clays, silts, and other materials, creating sedimentary rocks.
The Inner Coastal Plain (near the Fall line) was exposed first as the ocean receded, allowing weathering, erosion, and soil formation to operate the longest.
The outer coastal plain is more flat and swampy, transitioning into marshy areas, bays, and inlets. Soils are poorly drained and not suitable for farming.
Soft shoreline protection techniques: Use of natural materials, methods, and processes to prevent erosion and trap materials
Beach/dune stabilization
Living shoreline: A protected, stabilized coastal edge made of natural materials such as plants, sand, or rock. Unlike a concrete seawall or other hard structure, which impedes the growth of plants and animals, living shorelines grow over time.
Beach nourishment: A process by which sediment (usually sand) lost through longshore drift and erosion is replaced by material from another area
Benefit of beach nourishment: erosion control and property protection
Downside of beach nourishment: temporary and expensive
Why is beach nourishment important to SC: With valuable real estate, tourism, and ecosystems at stake, beach nourishment is seen as a critical management strategy to maintain beach width, reduce erosion, and support the economy.