Chapter 4: Dunes at White Sands

Sections Introduction: Dunes, Interdunes, Sand Sheets,     and Eolian Sabkhas Sand movement Dune Growth and movement Origin and nature of the gypsum sand at White Sands Dune types at the Monument Sedimentary features of the dunes Rate of dune advance Illustrations

White Sands has many features in common with other dunefields around the world. In addition to a similarity of process and product due to the importance of wind in shaping the landscape, it also consists of a number of terrain types common to all eolian systems (Figure 4-1). The dune sediments encompass all the myriad types of dunes found around White Sands. The interdune sediments are deposited in the low areas between dunes. Interdune sediments have very different sedimentary features than dunes. This is because process regimes are so different from those of nearby dunes. Interdunes have thus been treated separately by geologists; even though they evolve interdependently with surrounding dunes. The third important eolian sediment group is sand sheets. These are widespread, flat-bedded deposits that are commonly found at the margins of many dunefields. The fourth sediment group, which is also found at White Sands is the eolian sabkha. Eolian sabkha deposits form when dry sand is blown across damp surfaces near the water table, particularly in evaporitic settings such as White Sands. Eolian sabkha deposits are common on the Alkali Flat and on the margins of Lake Lucero, and are also found in some open places within the dunefield.

In addition there are important terrain types at the White Sands that are not formed by wind—for example the playa sediments of Lake Lucero and fluviatile sediments of streams that empty onto the playa and into the dune field. Additionally, there are striking erosional terrains around the monument that testify to the forward movement of dunes or even the removal of significant portions of the landscape by wind scour.

Our method in the next series of chapters is to discuss these basic terrain types one by one, after reviewing certain basic aspects of sand and dune movement, and the gypsum sand that typifies the monument. We hope to provide some basic facts about the origins and growth, internal architecture, sedimentary features and current process regimes of the various terrains of White Sands.

Sand movement by wind is a complex process involving several styles of grain movement by wind that occur more or less simultaneously (Bagnold, 1941). The process most easily observed is saltation, the bouncing of sand grains near the sand surface, sometimes in streamers (Figure 4-2). A second component of the sand drift process is surface creep. Surface creep is the jerky forward movement of larger grains that are too heavy to be lofted by the wind, but are jolted forward when struck by smaller flying grains. The third manner in which sand moves is by suspension. Suspended grains are so small that they are carried along without returning to the ground once they are thrown into the air by saltating grains or direct wind scour. Some of the suspension population is merely dust, which is carried into the atmosphere and far away from the dunes. One of the reasons dune sand is so well sorted is the narrow size range of sand that wind can move under most conditions—usually grains up to .5 mm or so in size. Larger grains are too heavy to be moved by wind and are soon left behind, while the silt and clay size fractions are either removed to the atmosphere or settle into sheltered places such interdunes.

Dune growth and movement is the result of sand flow on and around a dune during periods when the wind is strong enough to move sand (for dry sand this threshold is about 15 mph). Dunes are constantly changing shape in response to changes in wind velocity or direction. Dunes grow when more sand drifts onto them from surrounding areas than is removed downwind. Figure 4-3A shows a small barchan (horseshoe-shaped) dune near the Heart of the Dunes loop road. Figure 4-3B illustrates the terminology used to describe dune and other types of deposits in dunefields. This drawing will help the reader unfamiliar with some sedimentological terms to follow this discussion. The small dune shown in Figure 4-3A has grown by trapping sand drifting from the right side of the picture toward the left. During storms, sand flows over all parts of the dune. Sand that flows over the center parts of the dune settles on the upper part of the slipface as grainfall deposits. When the sand accumulates to a certain thickness and angle (the angle of repose: about 32 degrees) it becomes unstable and slides down the slipface. This process, known as avalanching is the basic mechanism of forward advance of most of the bedforms at the Monument. Note also the arms of the dune in Figure 4-3A. It is clear that some sand that drifts onto the dune from upwind can move past this dune and not become trapped in the slipface. Thus, this dune lives in a continual balance between sand loss at the arms and sand entrapment on the slipface.

One curious aspect of dune growth that can be seen in Figure 4-3A concerns the relationship of the slipface to the windward slope of the dune. It can be seen in the photograph that the highest point on the dune is not at the top of the slipface, but upwind, on the dune crest. Clearly, in this bedform, the dune crest deposits, which consist of ripple strata have grown higher than the slipface. Thus, part of the key to the upward growth of this bedform may lie in the ability of the ripples on the top of this dune to trap oncoming sand, as well as in the ability of the slipface to store it.

White Sands is extraordinary in that most of the eolian deposits are composed almost entirely of gypsum sand. The geologic origins of this sand are discussed in Chapters 1 and 2, but it is useful to briefly review here the physical nature of the sand and its proximal sources.

Generally speaking, average sand size becomes finer from upwind (near Lake Lucero) to downwind across the Monument (Figure 4-4). Figure 4-4 shows some coarse sand recovered from the Alkali Flat, as well as samples of finer sand from the dunefield. The fining of sand downwind reflects the breakdown of the gypsum crystals through weathering as well as rounding and breakage to smaller sizes through saltation impact.

Gypsum, with a specific gravity of 2.32 g/cm3 is slightly less dense than quartz, which has a specific gravity of 2.65 g/cm3. Despite this difference, which makes gypsum slightly easier than quartz for the wind to move, we could find no major difference between the behavior of gypsum and quartz either in habit of transport by wind, or in the way in which dunes are formed. One significant difference does become evident after sand is deposited, however. Because gypsum is much more soluble in water than quartz, early cementation of the dune and other sands at White Sands is widespread. This occurs in two main ways: (1) solution by rainfall, followed by drying (light meniscus cement between grains) or (2) precipitation due to evaporation at the top of the capillary fringe (heavy, pervasive cement) (Schenk and Fryberger, 1988). This may slow dune migration rates, or perhaps change the shapes of dunes slightly due to resistance to scour of windward slopes. It may also affect rates of eolian down cutting of source areas to feed new sand to the dunefield.

The similarity between the eolian deposits and processes at White Sands and other dunefields formed mainly from quartz is quite striking, in the author's experience, while differences are subtle.

The present study, as well as those of Almendinger (1971) and Almendinger and Titus (1973), indicates that the primary source of sand for the dunefield both in past and present is the recycling of gypsum crystals from deposits of Pleistocene Lake Otero. Secondary sources include recycling of sand from older dunes and much smaller quantities of gypsum freshly precipitated from the shallow groundwater table.

Most of the freely moving dunes at the Monument are of the barchanoid type that develops a major slipface transverse to a single dominant wind direction and moves in that direction—which is from the southwest at White Sands. Barchanoid dunes are one of the several major classes of dune morphology.

Two other important types—known as linear dunes (elongate dunes that form in parallel rows) and star dunes (star shaped in plan view)—are not known to exist at White Sands. Linear dunes develop in bimodal wind regimes, and star dunes in complex, multidirectional wind regimes (Please see Schenk, 1990, for a very readable summary of dune forms and wind regime from a worldwide perspective, and Chapter 10 for summary of dune forms and wind regime). Although the wind regime at White Sands is not perfectly unimodal, the wind is sufficiently dominated by the single southwest mode that barchanoid forms dominate the landscape.

Figures 2-17A and 2-17B show sand roses that summarize effective wind directions through the year for White Sands, based on wind data from Holloman Air Force Base located at the eastern boundary of the Monument. These roses illustrate that the strongest and most common winds are from the southwest, although there are significant flows from the north-northwest and southeast, as well, at various seasons.

There are a number of subtypes of the barchanoid family present at White Sands including barchans, barchanoid ridge and transverse ridge dunes (McKee, 1966). Barchan dunes have curved slipfaces and two horns extending downwind, with proportions, in plan view, much like a horseshoe (Figure 4-3A, and Figure 4-5). Barchanoid ridge dunes have a longer slipfaces that are sinusoidal in plan view, thus forming a more laterally continuous bedform. Transverse ridge dunes have slipfaces that are relatively straight and continuous. All these types migrate downwind through the erosion of the windward slope deposits and deposition on avalanche faces and lateral horns or extensions (Figures 4-5, 4-6).

Another type of dune which has transverse affinities is the dome dune. Dome dunes, however, have no slipfaces most of the time. They have long been considered embryonic forms, that evolve downwind into barchanoid types with slipfaces; and indeed are found most commonly at the upwind or lateral margins of dune fields.

In addition to freely moving dunes, White Sands also has many tracts of dunes partially anchored by vegetation. Parabolic dunes have an actively migrating central mass and long arms that extend upwind, as opposed to shorter arms of the barchan that extend downwind (Figures 4-5, 4-6) Also, there are much smaller dunes that do not move called coppice dunes that form when sand accumulates within and around small shrubs or grass. Usually, when the plant dies, the sand blows away and may or may not survive as a dune. Another unusual dune type at White Sands is the lunette dune, so named because of its shape. Lunette dunes form in the lee of lakes, and assume the shape of the shoreline, which is the immediate source of sand for construction of this immobile bedform. Of course, if the shoreline is not roughly circular in shape lunette dunes can grow quite elongate; however, the shape is quite distinctive. They often form in a semiarid setting and are commonly partially vegetated. Most of the lunettes at White Sands appear to be older than the present active dune field and have been somewhat eroded. They are, nevertheless, easily visible on aerial photographs (Figure 4-7). These dunes seem to be non-migratory, perhaps due to stabilization by vegetation.

Distribution of the major dune types at White Sands follows the broad patterns mapped in Figure 2-17A. Small numbers of each of the dune types, however can be seen almost anywhere as is visible on Figure 2-17B, the larger air photo mosaic included with this study.

The principal small-scale sedimentary features of the dunes include various kinds of primary and secondary laminations and bedding, as well as an internal structure reflecting growth, then partial erosion followed by renewed growth. The most common small-scale features are known as primary stratification. The two most common primary eolian stratification types are avalanche and ripple strata (Figure 4-9). Avalanche strata are formed when sand slides down the slipface of the dune after accumulating at the top and over-steepening past the natural angle of repose for dry sand. These strata are often an inch or more in thickness and rather massive, with drag structures that give evidence of shearing (Figure 4-10). Sometimes they are inversely graded due to rise of finer grains through the turbulent mass of sand that is sliding downhill. If the sand is damp at the time of avalanching, blocks of cohesive damp sand may be seen in trenches or on the slipface. Ripple strata are formed as one ripple migrates over another, preserving part of the ripple in front of it. Ripple strata are nearly always expressed as fine, thin laminations that are rather straight. Each thin ripple stratum is separated from the next by a thin layer of fines that accumulated in the trough of the ripple. These thin strata are known as pin-stripe laminations and are quite distinctive of eolian deposits (Figure 4-10).

The internal stratification of the dunes at White Sands was studied by McKee (1966) in a classic study that has been used worldwide by geologists. The light cementation typical of the gypsum dunes in the main dune mass made it possible for the U.S. Army, which helped McKee on this project, to bulldoze clean, flat cuts in several directions that revealed, in detail, the internal structure of the major dune types at White Sands (Figure 4-11 A, B). Figure 4-12 shows the long, steep crossbeds that typify the internal structure of barchanoid dunes with large slipfaces. Crossbedding and main bounding surfaces present in the main and cross trenches for barchan and transverse ridge dunes are shown as Figures 4-12, 4-13, and 4-14.

Rates of dune advance at White Sands were measured by McKee and Douglass (1971) who measured dune advance rates from 1962 to 1968, as summarized in Table 4-1.

Table 4-1

Rates of movement determined by measurements of distances on aerial photographs

The data in Table 4-1 show that the dome dunes at the upwind end of the field are the fastest moving bedforms. As the field is crossed from upwind to downwind, rates of dune migration slow, partly because bedforms become larger and vegetation becomes more abundant. Moreover, McKee (1966) felt that some the force of the wind diminished from southwest to northeast due to interference by the dunes themselves. Crabaugh, et. al (in press) also measured rates of barchanoid dune advance along two transects along the Alkali Flat Trail. Average rate of dune advance on the transect at the edge of the Alkali Flat was 7 feet per year (average of 3 dunes). Along the second transect, located about ½ mile into the field, dune advance rates were 4–5 feet per year (average of 8 dunes).

McKee and Douglass (1971) also documented the pace of sediment accumulation at the base of a barchan dune (Figure 4-15). Figure 4-15 illustrates the episodic nature of eolian accumulation, with single avalanches intertonguing at weekly intervals with ripple deposits deposited more slowly by crosswinds.

	Figure 4-1—A schematic drawing illustrating the four basic types of eolian sediment (facies). These include dunes, interdunes, sand sheets and sabkhas. Although White Sands is well known for its dunes, it is important to recognize the existence and role of the other basic sediment groups at White Sands. Each of these groups is the subject of a chapter in this volume.
	Figure 4-2—The three basic modes of sand movement by wind. Grains in saltation bounce across the desert. Surface creep is the jerky forward movement of the larger grains impacted by saltating grains. The finest grains of silt and dust travel in suspension and are commonly removed from a basin over time, unless trapped in sheltered zones within the dunefield and buried by dune advance or other processes.
A B	Figure 4-3—A small barchan dune at White Sands and some terminology useful to describe it. (A) A Barchan dune with horns on either side that extend downwind. Note the slipface on the center, downwind side of the dune. Note also that the bedform is almost entirely covered with small ripples, which also play a role in its growth. Direction of advance is toward the left. (B) A schematic drawing of a cross section of a dune similar to that in (A), migration also from right to left as in (A). This diagram is worth a little study, because it provides most of the terms necessary to work with the next four chapters in this volume. Note that dune sediments can be described in several ways. For example, with reference to original position on the dune ("slipface deposits") or with reference to the way the strata formed ("avalanche deposits") or even the geometry of the deposits, should enough data be available ("tabular-planar cross bed set").
	Figure 4-4—The gypsum sand from which the dunes and other eolian deposits are built. (A) coarse sand from the crest of a 20 foot high dune on the Alkali Flat, about 2 miles northwest of site NE 30. (B) A sample of sand from the base of the slipface of a dune ½ mile from the start of the Alkali Flat Trail, about 1 foot above the interdune. Note how much finer the sand has become by the time it reaches the main dunefield.
	Figure 4-5—Barchanoid dunes at White Sands. (A) the upwind end of the active dune field, showing rapid growth of small dunes into larger ones as the wind moves them from left to right. Most of the dunes in this view are barchanoid ridges. (B) air view of the Loop Drive showing barchans and barchanoid ridge dunes; view toward the south-southwest (obliquely upwind). Dunes are moving toward viewer in this photograph. (C) crosswind view from the Heart of the Dunes toward the visitor center showing barchan and barchanoid ridge dunes. A couple of transverse ridge dunes (straight slipfaces) are visible on right side of photograph.
	Figure 4-6—Parabolic dunes at White Sands. (A) Close aerial view of a parabolic dune. Note the central prograding lobe of active sand and the much less active, vegetated arms. Wind is from the lower left in this view, arms are "dragging" behind the advancing slipface. (B) Air view of the main dune field near Lost River, with rounded parabolic dunes with vegetated arms in the foreground. On the far side of the creek are more parabolic dunes, beyond which lie the barchanoid dunes of the main dune mass. (C) Air view of what an area looks like after parabolic dunes have passed through (from upper right to lower left). Long sandy ridges are the elongate arms of parabolic dunes that have migrated toward the lower left. On the right side of the photograph, a new set of parabolic dunes approaches. View to the south.
	Figure 4-7—Air photograph showing lunette dunes associated with ephemeral lake shorelines just south of White Sands.
	Figure 4-8—Air photograph showing parabolic and barchanoid dunes near the Visitor Center and west of the road to the Heart of the Dunes. Note that each dune type is dominant over relatively wide areas.
	Figure 4-9—Diagram illustrating the formation of the four basic types of eolian primary strata (after Fryberger, et. al, 1983). (A) By means of avalanching. (B) By fall of grains from flight into sheltered areas. (C) By lateral migration of ripples, with climb of one upon another. (D) By adhesion, when dry sand is blown onto wet sand and sticks due to capillary forces.
	Figure 4-10—Photograph of a trench on the slipface of a barchanoid dune. In this view, the two types of primary strata, avalanche and ripple strata, are distinctive. The ripple strata form the finely laminated layers that in places weather out in positive relief. The avalanche strata weather in recess, and are massive to gently wavy in this view. Near the bottom of this trench are irregular, blocky laminations caused by slumping (zone of slumping ends just opposite the lower right corner of the image).
	Figure 4-11—The now-famous trenching of the White Sands dunes with a bulldozer in the 1960's. (A) Extremely brave people, including Eddie McKee, standing in the trench studying the geology. (B) bulldozer cutting a trench in a transverse ridge dune.
	Figure 4-12—The view of the trench wall in the transverse ridge dune whose cross section is shown in figure 4-14. Note the long foresets typical of avalanche strata (slipface deposits). View is toward the southwest, into the wind.
	Figure 4-13—Cross stratification in McKee's trench in a barchanoid dune. Note the many erosional bounding surfaces (heavy lines) that subdivide packages of primary strata. These probably are formed during crosswinds from the north or southeast (see annual and monthly sand roses Figure 2-17). These surfaces thus may mark seasonal or at least storm events in the dunefield. Trench in A is oriented parallel to the dune advance direction, trench B perpendicular to it. After McKee, (1966).
	Figure 4-14—Trenches in a transverse ridge dune that is somewhat larger than the barchan of the preceding figure. Note the long, straight foresets of avalanche strata formed as the slipface advanced. (A) Trench parallel to the direction of dune advance. (B) Trench perpendicular to the direction of dune advance. After McKee, (1966).
	Figure 4-15—Drawing of a trench at the base of the slipface of a barchan dune, showing the episodic (once a week, roughly) pace of accumulation of individual avalanche strata during April and May (windy season). Sand flow toes at the base of each avalanche are separated by layers of ripple strata deposited by crosswinds. After McKee and Douglass, (1971).