National Park Service White U.S. Department of the Interior White Sands National Monument Geology of While one-quarter to one-third of the world’s deserts are covered with sand, little research has taken place in ergs (sand-covered desert areas) relative to non-sandy areas. The great distances and hardships involved in reaching sandy areas, the general lack of wind data and other meteorological records, the almost total lack of human activ- ity in ergs, and the difficulty of getting a macro-scale view of sand seas from the surface have all contributed to the lack of knowledge of the movement and accumulation of sand in deserts (McKee, 1979).

The single major work on sand done in the application of aerial in suspension are therefore formation was written by photography (Smith, 1968) and classified as or dust, while R. A. Bagnold and published Landsat imagery (Fryberger heavier particles unaffected by in 1941. Bagnold used wind and Dean, 1979; Breed and wind are classified as or tunnel experiments to make Grow, 1979) in the study of (Bagnold, 1941; Cooke quantitative predictions erg morphology. This paper and Warren, 1973). about sand movement and will summarize the findings of accumulation and then research carried out in inland Neither mineral composition successfully corroborated most (non-coastal) desert eolian sand nor particle shape appears of those predictions in field deposits. to have any significant tests in the Libyan desert. He effect on sand movement or also speculated about some of THE NATURE OF SAND accumulation. Any solid, non- the larger-scale phenomena cohesive particle, natural or that he was not able to test in Sand is defined by its size, human-made, that falls within his wind tunnel. Despite the although exact quantitative the above-mentioned range fact that Bagnold’s work is over ranges vary with author. is technically sand, including forty years old, it is still the most Bagnold defines sand as any dry granular snow which widely quoted source on eolian particle between .02 mm and can build up into dune-like sand deposits. According to a 1.0 mm in diameter, while drifts. While sand can be modern-day student of sand Ahlbrandt (1979) uses the range composed of various minerals, dunes, Bagnold’s work “with of .1 mm to 1.6 mm. Despite quartz makes up the bulk of relatively few modifications, has discrepancies in quantitative the world’s sand grains. This stood the test of checking since definition, which is often due dominance is primarily due to publication” (McKee, 1979, p.5). to grain-size differences in the the widespread distribution individual dunefields studied of quartz-containing rocks Since Bagnold, additional by each researcher, the authors and to the chemical nature of important contributions to agree that sand is qualitatively quartz. Unlike other particles, the study of eolian sands have defined as any particle that is quartz sand grains resist both been made in the field by light enough to be moved by the mechanical and chemical Sharp (1963; 1966; 1978) and wind but too heavy to be held breakup into smaller sizes McKee (1979), among others. in suspension in the air. Very (Bagnold, 1941). Important work has also been fine particles that can be held

Geology of Sand Dunes Page 1 “Saltation of sand While wind plays the major coarse, immobile particles, such grains along the surface role in sand accumulation, the as pebbles, the sand grains will actual erosion of rock material bounce directly off the hard accounts for about 75% into sand-size grains is primarily surface and back into the air, of all sand movement by due to the action of water and where the wind will once again wind.” ice (Bagnold, 1941). Wind does provide a forward momentum. play a role in the abrading and These bouncing grains can rounding of sand grains, which move downwind at about half results in inland dune fields the speed of the wind. If the having more rounded grains surface is composed of finer than coastal dunes, due to the sand grains, however, a saltating increased distance of movement sand grain will not bounce off by wind (Ahlbrandt, 1979). the surface; rather, it will strike the sandy surface and bury itself. Sand composing inland The impact will eject a second dunefields generally varies more grain into the air to be blown widely than coastal-dune sand downwind. This “splashing” in both and degree form of saltation results in of sorting due to the greater a slower rate of downwind variety of sources for inland- movement than the bouncing dune sand and the varying motion on hard surfaces. Either distances from source to dune process falls under the definition field. Source areas for inland of saltation (Bagnold, 1941). sand seas include lake deposits, river deposits, alluvial fans, Saltation of sand grains along playas, glacial till, and the surface accounts for about bedrock (Ahlbrandt, 1979). 75% of all sand movement by wind. However, due to the fact WIND-INDUCED SAND that sand grains average about MOVEMENT two thousand times the weight of the atmosphere, not all winds Individual sand grains are will move sand. Wind speeds moved under the force of the must reach what Bagnold (1941) wind in two distinct ways: calls a “fluid threshold,” defined saltation and surface creep. as the wind speed necessary for The primary method of sand sand to start saltating under the movement is saltation. As wind direct pressure of the wind. The moves over a sand deposit, it is fluid threshold varies in direct able to pick up grains from the proportion to the predominant surface and give them a forward grain size of the sand surface, momentum, but the weight generally ranging from ten to of the sand grains soon bring twenty miles per hour (Bagnold, the grains back to the surface. 1941; Sharp, 1963). If the surface is composed of

Geology of Sand Dunes Page 2 “Grains larger than one After sand grains start moving with wind velocity, i.e. as wind millimeter in diameter under direct wind pressure, speeds increase, the downwind wind speeds lower than the fluid rate of sand movement increases are generally moved by threshold can maintain sand exponentially. Even during a second process called movement. Once saltation has intense “sand storms,” however, surface creep.” begun, direct wind pressure is at maximum wind speeds and no longer necessary to lift sand sand movement, saltating grains grains into the air. The impact of rarely exceed two meters in height saltating grains provides enough (Fryberger et al., 1979). energy to knock new grains into the air (assuming a sandy surface); Due to an increased fluid thus, the wind need provide threshold, heavier sand grains only enough energy to move the are rarely moved directly by wind airborne grains downwind. The pressure. Only intense storm winds wind speed necessary to maintain can lift the heavier grains off the saltation once it has begun is surface. Grains larger than one termed the “impact threshold” and millimeter in diameter are generally defined by Bagnold (1941, p.32) as moved by a second process called the velocity at which “the energy surface creep (Bagnold, 1941; received by the average saltating Sharp, 1966). When saltating sand grains becomes equal to that lost grains strike these heavy grains (by impact), so that motion is on the surface, they don’t have sustained.” Like the fluid threshold, enough energy to knock them the impact threshold increases with into the air, but they do impart to increasing grain size. the heavy grains a slight forward momentum along the surface. In Saltating sand grains usually stay this way, heavy sand grains up to close to the surface. In his wind two hundred times the mass of tunnel experiments, Bagnold the saltating grains can be slowly (1941) found the average height moved downwind. Up to 25% of all of windblown sand to be about wind-transported sand is moved by ten centimeters, although both surface creep (Bagnold, 1941). height and speed of saltating grains increased with wind speed. At SAND ACCUMULATION Kelso dunes, a fifteen-year study indicated that 90% of saltating Two primary factors are necessary grains moved within sixty-four for the accumulation of sand centimeters of the surface, with into sand sheets and dunes: 1) an maximum sand-blast effect at adequate supply of sand, and 2) twenty-three centimeters (Sharp winds strong enough and persistent and Saunders, 1978). enough to move the sand (McKee, 1979). If these two conditions are The overall volume of sand moved met, large quantities of sand can has an exponential relationship be transported hundreds and even

Geology of Sand Dunes Page 3 “Obstacles, however, thousands of miles (Fryberger and surface generally takes the form of are not needed for sand Ahlbrandt, 1979). repeated bouncing of individual grains. In such cases, most of the accumulation.” What makes sand accumulate wind-imparted momentum is into piles rather than spread out conserved and grains move rapidly evenly over an area? In general, downwind. In saltation over a sand will tend to accumulate any sandy surface, however, sand place “where a sufficient reduction grains impact into the surface, of wind energy exists along the transferring some energy to the direction of sand drift in an active surface (via surface creep) and extensive system” (Fryberger and some energy to dislodging other Ahlbrandt, 1979, p. 454). Any grains into the air. This process obstacle, such as a rock outcrop produces a slower downwind or a stand of vegetation, can force movement of sand (Bagnold, 1941). sand accumulation by lowering wind speeds and creating a “sand A second factor favoring self- shadow” to the lee of the obstacle. accumulation of sand is “saltation Any small depression or gentle dip drag.” Friction produced by the in an otherwise flat surface can saltating sand grains slows down fill with sand due to lower wind wind speed near the surface. Thus, velocity within the depression despite the smoother surface of (Cooke and Warren, 1973). a sandy area, “a given wind can Large areas of persistent wind drive sand over a hard immobile deceleration, such as a basin or the surface at a considerably greater base of a plateau, can spawn the rate than is allowed by the loose creation of large ergs. In fact, most sandy surface” (Bagnold, p. 72). desert eolian sand seas do occur in This negative feedback effect of basins (Fryberger and Ahlbrandt, saltating sand is so great, in fact, 1979; Cooke and Warren, 1973). that it places a maximum limit on near-surface wind velocity. In Obstacles, however, are not needed effect, increasing wind velocity also for sand accumulation. According increases saltation and saltation to Bagnold (1941, p.6), sand “alone drag to the point where energy of all artificial solids (has) the lost by increasing friction equals power of self-accumulation.” This the energy gained by increasing self-accumulation results from two velocity. At that point, further processes: 1) the differential speed increases in wind velocity outside of saltating sand over sandy versus the sandy area do not produce any non-sandy surfaces, and 2) the drag increase in velocity over the sand; effect of saltating sand grains on in fact, increased saltation drag may wind velocity. actually reduce near-surface wind speed over the sand and result As mentioned in the previous in vigorous deposition of grains. section, saltation over a coarse Hence, strong winds tend to favor

Geology of Sand Dunes Page 4 “Even an apparently sand accumulation in areas already size predominates with a normal completely flat sand sheet sand-covered (Bagnold, 1941). distribution around the peak size, saltating sand grains are striking the is inherently unstable. SMALL-SCALE SAND surface at a relatively uniform angle Due to variations in grain ACCUMULATION FEATURES (approximately ten degrees for the size a small but significant average grain). surface roughness exists, Once sand grains have accumulated into relatively large sandy patches, When surface unevenness occurs allowing for wind to pick small-scale geomorphic features and a small hollow is created, less particles.” will often result, of which surface saltation impacts will occur on the rippling is the most common. upwind side of the hollow than Rippling tends to develop on on the downwind slope (Figure 1 sandy surfaces that are in a state [page 6]). As a result, surface creep of relative equilibrium or slow along slope AB is considerably deposition. Surfaces experiencing greater than creep along slope CA, either marked erosion or vigorous as slope CA resides in a “saltation deposition generally do not display shadow.” Consequently, sand rippling (Sharp, 1963). is removed from point A and deposited at point B, creating a Even an apparently completely flat ripple. This, in turn, produces sand sheet is inherently unstable. a second hollow downwind of Due to variations in grain size the newly-created ripple and the a small but significant surface process repeats itself with nu- roughness exists, allowing for merous parallel ridges forming at wind to pick particles. Because right angles to the wind direction. larger grains saltate more slowly The coarser sand grains will tend than smaller grains, they tend to to collect at the crest of the ripples accumulate into “jams”, creating since they are not moved as easily more surface roughness. Also, by the wind and there is little chance unevenness on the sand surface creep down the lee side of surface will always be present the ripples (Bagnold,1941; Sharp, (Sharp, 1963). 1963).

Any unevenness, either random As a rule, the “wavelength” of or saltation-induced, will tend the ripples (the distance between to perpetuate itself due to the crests of successive ripples) sensitivity of saltating sand to slight increases with increasing wind variations in the angle at which speed and reflects the increasing grains impact the surface (angle of height of ripples and the resultant incidence). What Bagnold refers to lengthening saltation shadow as the “characteristic flight path” (Sharp, 1963). In extremely heavy of saltating sand grains is normally winds, however, ripples flatten out at a very low angle. Since, in most completely because all grain sizes natural sand surfaces, one grain are easily moved by the wind and

Geology of Sand Dunes Page 5 “As ripples increase in height, they move into levels of higher wind speeds, causing heavier grains to be blown from the ripple crests and into the troughs, filling them in.” the differential saltation and creep Sharp (1963; 1966) also found rates needed for ripple formation that ripples move downwind at decline (Bagnold, 1941; Sharp, relatively fast rates. At a threshold 1963). velocity of 18 kph, ripples advances downwind at a rate of .9 cm per The height of an individual ripple minute, with the rate increasing to 8 is a function of grain sorting. The cm per minute during the strongest more uniform the sand surface the winds. Consequently, Sharp shallower the ripples, because of concluded the “adjustment in size, the reduced amount of differential shape and spacing can presumably saltation and surface creep. Due to occur rapidly in response to the interference of wind speeds by differences in velocity” (1963, p. the growing ripples, a maximum 631). From an initially flat surface, height limit exists. As ripples ripples can form a complete increase in height, they move pattern in ten minutes (in a 48 kph into levels of higher wind speeds, wind) and can flatten out, reform causing heavier grains to be blown or change direction as quickly. This from the ripple crests and into the rapid formation and movement of troughs, filling them in (Cooke and sand ripples also contributes to a Warren, 1973). Bagnold (1941) large volume of sand movement. claims that the ripple height is At one test plot, Sharp discovered generally no more than one-tenth that, in one hour’s time, 48 kph the wavelength of the ripples. In winds could move 6000 pounds of the Kelso dune filed, Sharp (1963) sand across a 32-meter line. found a maximum wavelength of nineteen centimeters and a In areas where the sand surface maximum height of one centimeter, has a relatively large number of with an average wavelength/height coarse sand grains (greater than 1 ratio of 18. However, Sharp notes mm in diameter), a second type of that no satisfactory universal small-scale feature occurs, called a qualification of this height- “ridge” by Ragnold and a “granule wavelength relationship has been ripple” by Sharp . Granule ripples obtained. generally form in sands with a bimodal distribution of grain size In his studies of the Kelso dunes, - one fine and one coarse - where

Geology of Sand Dunes Page 6 “Individual granule winds are moderate-to-strong found that granule ripples indeed ripples can exist for but not strong enough to pick up adjusted very slowly to changing coarser grains (Bagnold, 1941; wind velocities and directions. decades and even Sharp, 1963). Sharp found granule ripples that centuries, allowing for were at least several months old much greater heights and Bagnold claimed that, like ripples, and measured up to 12.5 cms high wavelengths than can granule ripples resulted from finer and over two meters in wavelength. saltating grains pushing coarser (Bagnold reported granule ripples develop on ephemeral grains (via surface creep) into jams. in Africa sixty centimeters in height sand ripples.” Unlike sand rippling, however, and six meters in wavelength.) these concentrated ripples of Sharp also states that he found no coarser grains are rarely, if ever, gradations between sand ripples moved by direct wind pressure. and granule ripples; rather, he Consequently, they are more found them to be distinct features stable and can grow to larger even when occurring side by side, dimensions than sand ripples. “with a sharp line of demarcation” As more and more coarse grains (1963, p. 632). arrive from upwind, the granule ripple can grow quite high and the LARGE-SCALE SAND resultant saltation shadow prevents ACCUMULATION FEATURES movement of large grains from the crest into the leeside trough. In According to Bagnold (1941), contrast to sand ripples, growth certain conditions must be met and movement of granule ripples for flat sandy areas to build up is very slow, and individual granule into true dunes. The primary ripples can exist for decades and prerequisites are relatively strong even centuries, allowing for much winds and a fine-grained surface. greater heights and wavelengths Coarse sand grains and pebbles than can develop on ephemeral tend to stabilize a surface of sand sand ripples (Bagnold, 1941). by preventing light or moderate winds from moving the finer Sharp (1963) tested Bagnold’s grain sand mixed in the coarser theories in the Kelso dune field. material. Bagnold has observed Sharp found that granule ripples that large sand sheets (without were generally located in deflation dunes) are generally covered with hollows between dunes, where coarse sand or small pebbles and winds had removed the finer are generally devoid of ripples. material and produced a coarse- Only strong winds can remove the grained surface. The granule finer grains from amidst the coarse ripples at Kelso dune field were grains and carry them downwind more irregular than the sand where they can form dunes by ripples, often forming wavy chains self-accumulation. The rougher resembling miniature barchanoid the surface of a source of sand (the ridges. As Bagnold predicted, Sharp more coarse material mixed with

Geology of Sand Dunes Page 7 “Wind speeds are highest the finer sand) the stronger the saltating sand drops out at the crest and saltation drag is least winds needed for dune formation. and further steepens the leeward Also, sandy areas with periodic face until it reaches its angle of at the windward edge of a wet-season rainfall may also be repose (about 32 to 34 degrees for sandy patch.” prevented from developing dunes dry sand), at which time gravity by a light, intermittent vegetation. may pull sand from the crest down the leeward slope in the form Assuming a steady input of sand of either isolated slow-flowing from an upwind source, a patch avalanches or the shearing and of sand that meets the above slumping of whole blocks of sand. conditions and is four to six meters This sand movement by slippage long can develop into a true dune. rather than by saltation or creep A true dune generally refers to has earned the leeward face of an a self-accumulating mound of advancing dune the name of “slip- sand with a distinct slip-face, as face” (Bagnold, 1941). opposed to an obstacle formed pile of sand, such as a sand shadow, The slip-face is an effective sand which will not be discussed in trap. Bagnold’s field observations this paper. Saltation drag is a on barchans showed that even in primary factor contributing to strong winds a near-perfect wind dune formation. Wind speeds are shadow exists along the slip-face. highest and saltation drag is least Any sand driven up the windward at the windward edge of a sandy slope drops out as it hits the patch. The further downwind on stagnant air above the slip face, the sand, the greater the drag and resulting in renewed steepening of the slower the winds, resulting the slip-face near the crest of the in differential sand movement dune until the angle of repose is across the sandy surface. As the once again reached. Thus, entire movement of grains slows toward dunes slowly move downwind due the leeward side of the sandy area, to avalanching and slumping along accumulation of sand increases the slip-face. The rate of dune and sand begins to mound up on advance is directly related to the the leeward end (Bagnold, 1941) rate of sand movement over the (Figure 2 [page 9]). dune crest and inversely related to the height of the slip-face, i.e. as As the sand mound grows, the dunes grow in height their forward point of maximum sand deposition movement slows (Bagnold, 1941). on the leeward face moves closer to the summit, causing a steepening Sand dunes of many different of the leeward face relative to the types have developed, resulting in windward face. The steepening various classification schemes and and growing dune now forces wind descriptive terminology as well as a out over the top of the dune rather considerable amount of confusion. than down the leeward face. The Regardless of what terminology

Geology of Sand Dunes Page 8 “Any sand driven up the windward slope drops out as it hits the stagnant air above the slip face.” and classification scheme is used, of vegetation, topography, the most students of dune formation nature of surface material and the would agree with Bagnold’s dominant grain size of the sand are assertion that dune accumulation also contributing factors and must “is intimately connected with the be taken into consideration (Borsy, relative strength, duration and 1976; Fryberger and Ahlbrandt, direction of alternating periods of 1979). weak and strong winds” (1941, p. 184). Borrowing heavily from other studies, McKee (1979) has Bagnold felt that strong winds developed a modern descriptive tended to build up dunes in terms typology of dunes, although he also of height, while gentler winds includes a generic explanation of generally extend the length of the the different types and subtypes dunes at the expense of height. he describes. McKee presents Cooke and Warren (1973, p. 271) two related classification systems, explain this phenomenon as one dealing with the basic shape follows: “a wind capable of moving and structure of individual dunes sand over a sandy surface could not (based primarily on modifications move it over a pebbly surface, so of Bagnold’s classification) and a that a sand patch would be eroded second dealing with the complexity and extended downwind by such of pattern displayed by groups of a wind. A stronger wind on the dunes (based on field observations other hand might move more sand and remote sensing studies). over pebbles, because of the better rebound from the harder surface... McKee classifies individual and the sand patch grows.” dunes in terms of the number and position of slip-faces, under Because it is generally agreed the assumption that the number that the primary determinant of of slip-faces corresponds to dune forms in any given area is the number of dominant wind the nature of the wind regime of directions in the local wind that area, most dune classification regime. Most dunes that have a schemes are based primarily on the single slip-face are classified in the direction(s) and intensity of the “barchanoid” type, which consists winds carrying the sand, although of three intergrading subtypes: the other factors, such as abundance barchan, the barchanoid ridge, and of sand, presence or absence the transverse dune (Figure 3 [page

Geology of Sand Dunes Page 9 “The primary difference 11]). All are considered to have forms created by water currents. among the three axes perpendicular to a persistent, In laboratory experiments, Tyler undirectional wind regime. (1979) observed “dunes” on sandy barchanoid subtypes stream beds develop from barchan is the amount of sand McKee feels that the primary to barchanoid ridge to transverse available for dune difference among the three forms. formation.” barchanoid subtypes is the amount of sand available for dune An additional type of dune with formation. a single slip-face is the parabolic dune, with a crescentic form Barchans, despite their familiar opposite that of the barchan (i.e. crescentic shape, comprise only with horns pointed upwind). a small percentage of the world’s Parabolic dunes develop when dune areas and tend to develop vegetation begins to colonize the where limited amounts of sand are moister, less-mobile flanks of self- available. For example, individual accumulating dunes. Vegetation barchans have formed in the has a stabilizing effect on the dune, vicinity of the Salton Sea, with little retarding or completely stopping or no sand found in areas adjacent movement along the edges of the to the dunes (Shelton, Papson, and dune arms. The leading edge of Womer, 1978). the dune may continue to push forward and move ahead of the Maximum dimensions for arms, creating the typical reverse- individual barchans are crescent shape. Eventually, the approximately thirty meters in nose of the dune may be stabilized height and four hundred meters in by vegetation as well, or it may width and length (Bagnold, 1941). continue to progress, breaking Their relatively small size allows free of the arms and leaving two for rapid movement; individual vegetated linear ridges in its wake barchans in Peru advance as much (Cooke and Warren, 1973; McKee, as forty-seven meters annually 1979). (Cooke and Warren, 1973). McKee recognizes two dune As more sand becomes available, forms that display two slip- barchans merge into wave-like faces each, presumably a result barchanoid ridges; and, if sand of bimodal wind regimes. The continues to accumulate, the linear (or seif) dune is the most barchanoid ridges grade into common of all inland dunes and transverse dunes. These larger the most controversial in origin barchanoid subtypes can reach (Figure 4). These long, narrow heights of over two hundred dunes have sharp, steep crests meters and extend for kilometers due to the presence of a slip-face (Bagnold, 1941). This gradation on both sides. Bagnold recorded of forms is similar to that of sand linear dunes in Iran that were over

Geology of Sand Dunes Page 10 two hundred meters high, twelve hundred meters wide and one hundred kilometers long.

While it is generally agreed that the dual slip-face structure of the linear dune does represent the work of winds from two different directions, the exact mechanisms of the wind regime is uncertain. Most researchers feel that acute bimodal winds are responsible for the creation of linear dunes (Bagnold, 1941; McKee, 1979), while others feel that linear dunes are created by helicoidal or vortex currents within a unidirectional wind regime (Borsy, 1976). Bagnold (1941) suggests that some linear dunes are formed when barchan dunes migrate from an area of uni-modal winds into an area of bimodal winds, with one of the barchan’s arms being extended into a linear dune.

The great diversity of location, surface texture and wind regimes in areas of linear dune development suggests that more than one mechanism may be responsible for the formation and growth of these dunes, and/or that a combination of two or more mechanisms may be operating at the same time. For example, some areas of linear dunes may have persistent, gentle- to-moderate unidirectional winds most of the year, during which helicoidal flow may operate; while occasional intense storm winds from a different direction (often not manifest during the brief period researchers are in the field)

Geology of Sand Dunes Page 11 may also contribute to the building and lengthening of linear dunes.

A second type of dune with two slip-faces is the reversing dune (Figure 4) Better understood than linear dunes, reversing dunes are typically found in areas of bimodal winds from opposite directions. A generally persistent wind from one direction creates barchanoid-type dunes, while “reversing winds” create miniature dunes on the crest of the barchanoid dune with a slip-face in the opposite direction as the primary dune slip-face (McKee, 1979). In this respect, reversing dunes are generally ephemeral features, occurring only immediately after reversing winds. Resumption of normal wind flow from the opposite direction obliterates the smaller crestal dune and the main dune reverts to its original barchanoid form with a single slip-face.

Finally, McKee recognizes the star dune as a dune with three or more slip-faces. Star-dunes generally occur in the form of a central peak with three or more radiating arms, each with a slip- face in a different direction (Figure 4). Both star dunes and, to some extent, reversing dunes generally tend to grow vertically rather than laterally, as sand moves in and piles up from several directions (Ahlbrandt, 1979). Star dunes are sometimes found as the highest dunes amidst a field of transverse or linear dunes. The origin of star dunes in such situations is

Geology of Sand Dunes Page 12 “Reversing dunes are uncertain, but it is suspected that dunes forming within the arms of generally ephemeral when transverse or linear dunes larger parabolic dunes (McKee, grow large enough their size and 1979; Smith, 1968). features, occurring form may alter the local winds and only immediately after create multi-directional winds and Complex dunes are those in which reversing winds.” eddies persistent enough to build two different dune types coalesce a star dune (McKee, 1979). Sharp or overlap. Examples of complex (1978) noted a distinct alteration dunes include star dunes forming of ground-level wind direction on top of linear or transverse among the transverse dunes in the dunes, and barchans forming in Kelso dune field. As strong winds the hollows between linear dunes were blowing up the windward (McKee, 1979). face of the dunes, winds from various directions (up to 90 degrees In an aerial study of North African divergent) blew across the surface dunes, Smith (1968) found all types of the slip-face. of dune groupings, with complex dune patterns being the most A second type of dune common and most diverse. Smith classification discussed by McKee believes that complex dunes have (1979) and others before him is a complicated development over based upon the complexity of time, with climatic change and dune pattern: simple, compound resultant shifts in dominant wind and complex. Simple dunes are direction as major forcing factors. individual dune types of a unitary Cooke and Warren (1973) maintain nature, “not divisible into clearly that, on a global scale, complex defined component parts” (Smith, patterns are far more common than 1968, p. 14). Any dune field with either simple or compound forms. distinct individual dune types independent of each other would Recent Landsat studies of the fall into this category, such as a field world’s ergs have allowed for a of barchans or a field of parallel better understanding of large-scale linear dunes (McKee, 1979; Smith, dune-field structure and pattern. 1968). Breed and Grow (1979) found that dunes of similar type show Compound dunes consist of two the same patterns regardless of or more dunes of the same type location, and concluded that “the “combined by overlapping or being relationships of mean length, width superimposed” (McKee, 1979, p. and wavelength are similar among 13). Examples of compound dunes dunes of each type, regardless are coalescing barchanoid ridges, of differences in size, form or small linear dunes forming on the geographic location” (p. 257). top of larger linear dunes, small The authors also found a direct barchans climbing up the back of relationship between dune size and large barchans, and small parabolic complexity.

Geology of Sand Dunes Page 13 “Recent Landsat studies In another important study, note that all three subtypes are of the world’s ergs have Fryberger (1979) used Landsat frequently found together in the imagery and regional wind data same dune field, indicating that allowed for a better to show global relationships some other factor, presumably the understanding of large- between wind regimes and amount of sand, is responsible scale dune-field structure dune morphology. For each for subtype development. Not and pattern.” location under study, Fryberger surprisingly, Fryberger found developed a “sand rose” based that linear dune chains were on the traditional wind rose. The associated with wind regimes of a sand rose takes into account the greater directional variability than intensity and duration of wind as barchanoid dunes, ranging from well as wind direction, thereby wide uni-modal wind regimes providing a model of the amount (with a single peak direction but of sand a particular wind regime less than 90% of the sand drift is capable of moving in various within a forty-five degree arc of directions. the compass) to complex wind regimes (three or more directions Fryberger’s findings support of significant winds, or no clearly Bagnold’s assertions that both defined nodes). Like barchanoids, wind direction and strength affect linear dunes form in both high- dune morphology. He confirmed energy and low-energy wind the notion that, of all dune forms, regimes. The stronger the winds, barchanoid types are associated the less variable the wind direction with the least variability of needs to be and the fewer junctures wind direction. He found most (merging of two dunes) between barchanoid types to form in areas adjacent linear dunes. Fryberger with uni-modal wind direction, found that fields of star dunes although they also occasionally formed invariably in complex wind form in areas with acute bimodal regimes---areas with winds from winds (from two dominant several directions over the course directions less than ninety degrees of the year. Star dunes also form apart) where one wind is much in both high and low-energy wind stronger than the other. While regimes. barchanoid dunes appear to form in both high-energy wind regimes In recent years, desert and low-energy wind regimes, geomorphologists have the stronger the winds, the less recognized a higher order of sand- directional variability is required accumulation form -- large, widely- for barchanoid development. spaced mega-dunes, generally known as “draas.” Bagnold The regional wind data used recognized such features in North by Fryberger did not allow Africa, calling them “whalebacks,” discrimination for each barchanoid and recent aerial and satellite subtype; however, Fryberger does studies of ergs have revealed that

Geology of Sand Dunes Page 14 “Like barchanoids, linear draas are not restricted to the meters in height, were as wide as dunes form in both high- Sahara. While little study of draa three kilometers, and some ran forms has occurred, draas appear uninterrupted for 300 kilometers. energy and low-energy to be structurally analogous to Bagnold considered these mega- wind regimes.” dunes, but on a larger-scale and are dunes to be the accumulated considered to be slower moving coarse-grained bases (“plinths”) and older (Cooke and Warren, of former linear dunes. The 1973). whalebacks were topped with smaller linear dunes running at Draas usually form a base on an oblique angle to the direction which fields of smaller dunes have of the mega-dunes, suggesting the formed, producing a compound possibility of a major change in pattern. The whalebacks described wind regime. by Bagnold (1941) averaged fifty

Geology of Sand Dunes Page 15 REFERENCES

Ahlbrandt, Thomas S. 1979. Textural parameters in eolian deposits. In A Study of Global Sand Seas. E. McKee, ed., pp. 21-52. Washington: U.S. Geological Survey Paper 1052.

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