EolianSystems

Locations of Major Deserts

Deserts are found primarily in latitudinal belts between 100 and 200 north and south of the equator.

Changing Environments

Climatic conditions can vary between humid and arid at a single location because of actual fluctuations in the climate or by migration of plates through different climatic belts.

Photo by W. W. Little

Rain Shadows

Mountain ranges can create barriers that force humid air higher into the atmosphere where it condenses and falls as precipitation. Dry air then descends on the opposite side absorbing moisture from the air and surface.

Coastal Dune System

Dune fields are common features in sandy coastal regions, where fine-grained material is blown landward from beaches.

Photo by Ian & Tonya WestPilot Point (Shell Bay)

Simplified Model

Eolian deposits tend to be relatively uniform in character, consisting of a single major facies association.

Eolian Processes

Wind is the primary hydrologic agent operating in desert regions. The low viscosity of wind makes it a highly effective sorting agent. Particles that cannot be transported, such as gravel and course sand are left behind as desert pavement. Medium and fine sand are piled into dunes. Silt and clay are blown high into the atmosphere and settle elsewhere as loess.

Dune Processes

Dunes are the primary depositional feature related to eolian processes and are formed as sand grains migrate up the stoss side of the macroform and avalanche down the lee face.

Saint Anthony Sand Dunes at Egin Lakes

Dune Origin

Sand will often begin to accumulate behind a barrier, such as a fence post, a rock, or a bush, that creates a wind shadow.

Dune Growth

Once a small pile of sand has accumulated, it begins to move as single mass, called a dune. Small dunes coalesce into larger dunes.

Dune Fields

Because small dunes migrate more rapidly than large dunes, they eventually combine to form a larger body called a dune field, which migrates very slowly relative to the smaller features.

Sand Seas (draas)

Very large sand bodies, such as that which covers the Sahara Desert, are referred to as sand seas.

Dune Field Margins

The margins of dunes and dune fields are often very sharp and distinct.

Dune Migration

Dunes are asymmetrical features. Sand is eroded from and blown upward along the gentle side of the dune, where It piles up at the crest before avalanching down the steep “slip face.” This forms a diagonal pattern referred to as cross-bedding.

Flow Separation

Already well-sorted sediment becomes even better sorted at the dune crest, where finer grains are suspended by momentum and settle onto the back of a leading dune; whereas, coarser particles avalanche down the dune front.

Eolian cross-bed sets can be exceptionally thick, potentially exceeding 15 m.

Common Eolian Facies

Eolian deposits are dominated by very fine-grained, trough or planar (depending upon dune morphology) cross-bedded sand that is both compositionally- and texturally-mature. Thin beds of parallel-laminated sand can separate major cross-bed sets. Occasional interdune lacustrine deposits can also be found. In some areas, alluvial fan, braided stream, and playa deposits can be found in association with eolian deposits.

Planar Cross-stratified Sand

Planar cross-stratified sand forms by migration of straight-crested (transverse) dunes.

Straight-crested (transverse) Dunes

Transverse dunes are elongate ridges with relatively-straight fronts oriented perpendicular to wind direction. These dunes form under conditions of an abundant sand supply and a consistent wind direction.

Trough Cross-stratified Sand

Trough cross-stratified sand forms by migration of sinuous-crested (barchan, barchanoid, & parabolic) dunes.

Sinuous-crested (barchanoid) Dunes

Barchanoid dunes are elongate ridges with curved fronts oriented perpendicular to wind direction. These dunes form under conditions of a moderate sand supply and a consistent wind direction. They are a transitional form between barchan and transverse dunes.

Barchan Dunes

Barchan dunes are crescent-shaped with the horns pointing down wind. These dunes form under conditions of a limited sand supply and a consistent wind direction.

Parabolic Dunes

Parabolic dunes are crescent-shaped with the horns pointing up wind. These dunes form under conditions of a limited sand supply and a consistent wind direction. The horns are reversed from barchan dunes because of anchoring by vegetation.

Dune Morphology

There are several common shapes for sand dunes. These shapes are controlled by sand abundance, wind velocity and duration, consistency of wind direction, and presence of vegetation.

Longitudinal Dunes

Longitudinal dunes are narrow, elongate ridges oriented parallel to wind direction. These dunes form under conditions of a limited sand supply and at least two prominent, sub-parallel wind directions.

Star Dunes

Star dunes form central peaks with three or more arms that radiate outward. These dunes form under conditions of an abundant sand supply and a variable wind direction.

Ripple-bedded Sand

The most common feature on the surface of sand dunes in ripple marks. These are essentially small-scale dunes that form from gentler winds between major storms.

Avalanche Structures

Sand on the dune slip face accumulates at the angle of repose, leading to common avalanching during and between storm episodes.

Contorted-bedding in Sand

The most common feature on the surface of sand dunes in ripple marks. These are essentially small-scale dunes that form from gentler winds between major storms.

Tracks & Trails

Despite the arid conditions, many plants and animals make their home on sand dunes. Because of the very fine nature of the sand, great detail can be preserved.

Rain Imprints

Interdune Ponds

During the wet part of climatic cycles, ponds often form in the troughs between dunes. Vegetation begins to grow and helps to, at least temporarily, stabilize the dunes and prevent their migration.

Blowouts

Blowing wind may form erosional depressions called blowouts that can later be filled, forming lens-shaped deposits.

Large-scale Architecture

In areas of abundant sand and high winds dune trains can form, where one dune climbs up the back of another. The result is a pattern of cross-bed sets separated by erosional surfaces.

Loess

Silt and clay are picked up and blown high into the atmosphere. It settles far from the source, producing thick accumulations called loess.

Photo by W. W. Little

Photo by W. W. Little

The Dust Bowl