The adhering to is informed by definitions from theGlossary of Geology created by the American Geological institute (recommended to authors by the Geological inspection of Canada), and by consumption in current literature. Many of these terms have actually synonyms, and also are likewise used in various other contexts.

You are watching: Which side of a dune typically has a slope angle equal to that of the angle of repose for sand?

Aeolian (also order eolian) way "wind-blown"; Aeolus was leader of the winds in Greek mythology. Aeolian transverse and barchan dune types have one idealized cross-section as presented in number 1 below, v a slipface on their lee (downwind) side.


Fig 1. Cross-section the a transverse dune

The acute edge of the slipface through respect to horizontal (as measure in a vertical plane normal come the slipface) is the dip; the dip of the slipface in number 1 is about 40 degrees. The dip direction is the compass direction toward which the slope faces (eg., the direction towards which water would flow, if the slope to be impermeable). A line that strike is a line created by the intersection of the plane of the dipping surface (eg., the slipface) and a horizontal plane; in the situation of a dune, the brink is a line of strike. (Lines that strike room perpendicular come the direction the dip.)

The slope angle at which sand starts to avalanche is the angle the yield. The edge at i beg your pardon avalanching sand pertains to rest is the angle the repose. This angles depend on attributes of the sand such as grain sizes, serial shapes, and moisture content. Dry aeolian dune sand frequently has an edge of repose of around 34 degrees. The edge of yield is usually a few degrees greater than the angle of repose.

Transverse dunes migrate, as illustrated in figure 2, when strong winds pick up sand grains from the stoss steep of the dune (upwind or windward side) and blow them across the crest and brink, whereby they loss onto the slipface. Safeguarded from the wind, they accumulate top top the top slipface come a low, wide mound or cornice1. Ultimately the cornice deposit grows also steep (its lower slope over the angle of yield) and also its sand starts to flow as an avalanche under the slipface, prolonging the dune downwind.

All other determinants being equal, a dune"s development depends top top its size, because it have the right to only development by including a great of sand to its slipface -- the longer (higher) the slipface, the larger the volume that sand compelled per unit the advance. Travel at a speed (celerity) of tens of meters per year would be a lot of travel for a dune many meters high.


Fig 2. Dune hike by avalanching (press Esc to prevent animation, PageRefresh to resume)

If erosion the the stoss surface is not complete, as in number 2, a migrating dune leaves behind a class consisting the what to be the reduced parts of the dune (left next of number 2). A subsequent dune (not have to the next one) may later on travel over this left-over layer also leaving a great consisting of its reduced parts; in this manner, sand have the right to accumulate in layers and become preserved. Figure 3 below shows three such layers (and the partly eroded remains of a fourth, at the top) in Navajo Sandstone (Utah, USA). This layers are countless meters special (there is a person sitting in ~ the edge of the shadow at the lower-right). The kept layers were no necessarily left through sequential dunes; dunes can pass there is no leaving a deposit, or a deep trough can even eliminate previous deposits.


Fig 3. Navajo Sandstone, Utah, USA

Seasonal Cycles that Deposition

The illustration listed below shows the idealized result on a dune of strong winter wind alternative with weaker reverse-direction summer wind. The weaker summer winds in this example cannot completely reverse the hike that occurred during the winter, however do transform the structure of the dune and also leave a wedge-shaped deposit in ~ the toe that the slipface, whereby they are often preserved.


Fig 4. Seasonal cycles of deposition (press Esc to stop animation, PageRefresh to resume)

Descriptive Terminology

Descriptive terminology gives a vocabulary for talking about sandstone outcrops without interpretation.

A cross-stratum (plural: cross-strata) is a layeror bed that is inclined loved one to a bigger context (eg., the floor of a dune).A set the cross-strata is the entire collection of nearby cross-strata between two surfaces.Two or much more adjacent to adjust are referred to as cosets ("co-sets"). Therefore the bulk of the outcrop in number 3 is composed of three sets the cross-strata.


Fig 5. Cross-strata, sets, and also cosets

A stratum less than 1cm special is a lamina; however, the ax "lamina" is frequently used together a synonym of stratum. The many of lamina is laminae.

A pinstripe (or "pin stripe") is a slim lamina, in the variety of 1mm thick.

The cross-strata in number 5 are stated to downlap, due to the fact that they room inclined layers terminating versus a less inclined surface.

Two shapes are frequently distinguished as soon as viewedin cross-section (perpendicular to your plane): tabular (rectangular) orwedge (triangular) -- or, referring to their form in three dimensions,tabular-planar or wedge-planar.

The surface dividing sets (visible together a curve in cross-sectional view) is a bounding surface. Thus figure 5 over has 3 sets of cross-strata; the top three-quarters of every set"s cross-strata is tabular, and the reduced one-quarter of each set has a slowly decreasing dip, downlapping asymptotically come a horizontal bounding surface; the cross-strata are concave-up (have a curve v its inside, the concave part, facing up).

Interpretative Terminology

Interpretative terminology offers a vocabulary the communicates the outcomes of interpretation and also recognition. Translate attempts to add value to the ceiling facts: "bounding surface" may come to be after translate "interdune migration surface".

Aeolian sandstones space those interpreted as being lithified remains of aeolian sand dunes. Dune terminology can be loosely grouped into "geomorphic" and also "genetic".


Fig 6. Geomorphic terms

Geomorphic dune hatchet is based upon the outside shape and form of a sand dune. The lee side (or leeward slope) is downwind that the crest, and the windward side or stoss slope is upwind (windward) the the crest. The brink divides the optimal of the dune native the lean slipface. Sand blowingover the brink falls into a wind shadow and accumulates in a cornice,1 a low mound on the upper slopeof the slipface. At some point the cornice grows too huge and that sand avalanches down the slipface, leaving arelatively level surface inclined in ~ the edge of repose. At other times, turning back winds or cross-windscan reason sand come accumulate in ~ the base of the dune, creating an apron. The allude between the curved (concave-up) apron surface and also the slipface is the toe. The area between dunes, generally flat and often erosion-resistant, is the interdune.


Fig 7. Genetic terms

Genetic terminology emphasizeshow frameworks formed. Grainflow(or "sandflow") layers are produced by sand seed flowing under a slope.Grainfall shop are developed by sand seed dropping an ext or much less ballistically native the waiting (such together occurs to form a cornice in ~ the top of the slipface,prior come avalanching). Wind-ripple laminae are left by migrating dry-sand ripples.

Bounding surfaces room surfaces separating distinct sets the strata. Bounding surfaces are recognized to be developed by (at least) the following mechanisms (referring to number 8 below):

An interdune hike surface (or first-order bounding surface)is the interdune surface ar (left behind by stoss-side erosion that a happen dune, and possiblyfurther eroded together an interdune surface). Moist sand is much more resistant to erosion thandry sand, so occasionally the water table controls the extent of interdune erosion.Interdune hike surfaces tend to be close to horizontal.A superimposition surface (or second-order bounding surface) iscreated through the migrate of one dune (usually a second smaller dune) end anotherdune. This can occur without erosion that the basic dune.A reactivation surface (or third-order bounding surface) iscreated as soon as a deposition procedure is temporarilyinterrupted by a adjust of wind direction resulting in erosion. Theerosion surface becomes a reactivation surface ar if later deposition resumes in theoriginal direction, laying down more deposits in the initial orientation.

Fig 8. Bounding surface ar types


Fig 9. Fluvial terms

Fluvial bedform terminology, developed to describe the interior structure the subaqueous ("in water") flow delta deposits, is periodically borrowedto describe comparable structures that subaerial ("in air") dunes (it was just in 1977 that geologists noticed methods toreliably distinguish fluvial and aeolian sandstones; fluvial ax is a hold-over and should probably be avoided when discussing aeolian features). Flow deltas prosper by sediments flowing to the edge of a delta and then avalanching down into deeper water -- a procedure similar come wind blow sand over a brink. Layers left bymaterial flowing along the height (of the delta, or the the dune) room topset strata (or "beds").Layers left by product avalanching under the slipface, at the edge of repose, room foresetstrata (or beds). Material recorded at in the corner in between the slipface and also the interdune area is bottomset strata. A foreset (or topset, etc) is a three-dimensional object (with a form that isgenerally tabular-planar), whereas a slipface is a surface ar (two-dimensional).

Wind-Ripple Deposits

A flat sand surface is not steady with wind blowing end it; the sand will certainly developripples, or wind-ripples (prefixed through "wind" come explicitly differentiate them native subaqueous ripples developed by water flowing end sand, which have superficially comparable appearance however differ in detail).

Wind-ripples seem at the very least superficially prefer small-scale dunes; they have actually a similar shape, and maymigrate in a comparable way, by erosion from your stoss side and deposition ontheir lee side. If erosion on the stoss next is no complete, the every ripple will take trip upon the stays of the coming before ripple"s base, and there will be net deposition.

Grains of various size tend to be deposited in different parts of a ripple. This sorting leader to textural laminations in the deposits created by wind-ripples. To watch why, take into consideration the 3 climbing ripples in figure 10 below, whereby colour variations have been introduced in the ripple surface ar make it simpler to monitor which part of the ripple remains after erosion that the stoss side of each ripple:

Fig 10. Wind-ripples leaving a deposit. Push Esc tostop animation, PageRefresh to resume

Notice that at this specific angle the climb, some parts of the ripples room not kept (the red and purple, equivalent to the stoss slope and also crest).

The computer animation moves in discrete jumps, leaving feather artifacts, but in nature, the ripples erode grain-by-grain, typically leaving no map of the previous surfaces -- just a smooth lamination as illustrated in figure 11 below:


Fig 11. Strata (apparent) in wind-ripple deposit(inset mirrors originating ripple and climb angle)


Fig 12. Wind-ripple deposit

The animation over exaggerates the upright climb; in nature, the edge of rise isusually rather shallow. Figure 12 mirrors the millimeter-scale layers of a sandstone wind-ripple deposit.

The fabricated laminae left by wind-ripples are referred to as climbing translatent strata ("translatent" is a recommendation to the geometric operation of "translation", showing that the deposits have the appearance of having been developed by the translation of the ripple surface, together in the computer animation of figure 10). The bounding surface of every lamina is the erosion surface ar upon i beg your pardon the ripple climbed (the blue line, in figure 11). Every bounding surface ar is a first-order bounding surface ("interdune hike surface").

The great in wind-ripple store were formed by rise ripples, therefore the edge of climb determines the angle of the layers. This method the dip (with respect to the surface ar hosting the ripples, which maybe be inclined) will certainly be modest, due to the fact that ripples deserve to only climb as rapid as net deposition permits. In contrast, grainflow layers are constantly near the angle of repose (before compaction).

Wind-ripple laminations, if planar, can not be thicker 보다 the height of the ripples that formed them, which method they are generally less than around 0.5cm thick. In contrast, grainflow layers are generally much more than 0.5cm thick (the angle of repose is greater for layers less than ~10 serial diameters, so flows are generally thicker than 10 diameters ~= 0.5cm).

Wind-ripple laminations dip, generally at shallow angle, the contrary the wind direction (ie., towards upwind), whereas grainflow strata dip, normally 25 come 35 degrees, aligned the wind direction (ie., towards downwind).

A class of grainflow sand (avalanched sand) i do not care a grainflow cross-stratum, but a "layer" (surface) the wind-ripple does not become a wind-ripple stratum. Every of what look favor layers in figures 11 and 12 space translatent strata (sometimes referred to as pseudo-layers), made up of contributions from numerous successive wind-ripple surfaces. For example, a environment-friendly stratum in figure 11 is consisted of of grains from the troughs of succeeding surfaces transparent the computer animation in number 10. The discontinuous black line in number 13 listed below shows contributions to the final deposit of a single, isochronous wind-ripple surface:


Fig 13. Black line fragments mark the donation of asingle ripple surface minute to the final deposit

The ripple trace is not continuous in number 13 due to the fact that in this situation the purple and red portions were eroded by succeeding ripple development, yet if the deposition rate were high enough, entire isochronous surfaces can be preserved.

Particle Transport

A an essential mode that sand transport by wind is saltation (from Latin saltere, come dance, jump, leap), whereby grains lifted right into the airstream are blown and also bounced downwind in low, arcing trajectories. A saltating serial falling earlier to the surface may strike and also dislodge various other grains in a "splash".

Grains rolling follow me the surface ar under the pressure of the wind or impacts from saltating grains, probably smaller, experience creep or reptation (from Latin reptare, come creep).

Smaller seed may enter suspension and also be scattered as dust if they are light sufficient to be carried by turbulent air.

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I"d appreciate suggestions: Comments

Jim Elder2006 Dec


1) "Cornice" in this paper definition is not in the navard of Geology however I think it"d it is in a great addition. A cornice is a deposit in the wind zero (recirculation eddy) that a ridge or crest. The shape of a cornice deposit relies on features of the wind and the wind-blown material -- non-cohesive products like dried granular eye or dry sand kind a short mound (the stronger the wind, the additional from the brink), and cohesive materials like wet snow can form a ledge or also overhang, shaped by the eddy-current flow. ~ above the slipface the a dried sand dune, a cornice will develop until at some point its reduced slope over the vital angle the yield and also then the sand will certainly avalanche in a tongue that grainflow, fed by a receding scarp that eats with the cornice toward the brink.