Chapter 8: Wind Erosion at White Sands

Sections Introduction The Otero Deflation Basin The Alkali Flat Fossil dune terrains Yardangs Pedestal Dunes Big Dune Trail mini-basin Controls on eolian erosion Illustrations

The depositional features of White Sands, such as the dunes, are justly famous. On the other hand it is the extraordinary events of wind scour over wide regions that have defined dunefield setting and continue to play a large role in shaping the landscape. In this chapter I will discuss the major wind deflation terrains including (1) the basin carved from Lake Otero sediments by wind erosion, (2) wind scour on the Alkali Flats, (3) wind erosion of the fossil dune terrains, (4) yardangs, (5) pedestal dunes and blowouts (6) the Big Dune Trail mini-basin.

At some time in the past, gypsiferous sediments were precipitated from Lake Otero, probably in a very shallow lake or evaporitic sabkha regime. They extended more or less horizontally across the parts of the Tularosa Basin that were below 4000 feet above sea level. The evaporites may not have filled the basin to the 4000 foot level, but to a level somewhere lower, perhaps 30 feet above the present playa surface near the visitor's trail to Playa Lucero. Perhaps the surface looked something like the surface of Lake Trinity (See Chapter 2, Figure 2-13). Wind scour, however, has removed much of this sediment, leaving only gypsite-protected outcrops around the margins of the original sabkha, as shown in the photographs in Chapter 7. This deflation may have occurred either catastrophically, due to the unconsolidated nature of the Otero sediments, or perhaps occurred through down-cutting controlled by a slowly descending water table.

The Alkali Flat Scour Platform is mainly erosional in origin. In some places, wind is scouring gypsum crystals directly from sediments of the Lake Otero. Deflation surfaces on these sediments resemble horizons in the vertical outcrops illustrated in Chapter 7 (Figure 8-1). Along the downwind edge of the Alkali Flat, the scour surface climbs onto crossbeds left behind by the advancing barchanoid dune field (Figure 8-2). This area is shown on the maps and air photographs of Figure 2-17A and 2-17B. Along the western side of the basin, wind is removing fine gypsum and gypsum dust from among large selenite crystals that themselves are quite resistant to wind erosion (Figure 8-3).

There are quite a few areas of older, stabilized or partly-stabilized "fossil" dune terrains. These are illustrated on Figure 2-17A and Figure2-17B. Most of these terrains are partly lithified internally, and have developed gypsite crusts—due to long exposure to weathering and solution-re-deposition by percolating rainwater. However, wind is effective at eroding these terrains wherever the protective gypsite cover is broken, typically forming blowouts or slowly migrating parabolic dunes. There are extensive areas of fossil dunes around NE 30 and regions downwind of Playa Lucero. Some fossil dune terrains and underlying Otero sediments that are protected by them from wind erosion have been left standing as outliers on the western side of the active Holocene Lake Basins (Figure 2-17A). A large outcrop of this terrain is located on Playa Lucero south of Andrecito Creek (see also air photo Figure 7-14). Denuded remnants of this terrain can also be seen along the eastern shore of Playa Lucero (Figure 8-4)

Yardangs are streamlined, aerodynamically-eroded ridges that are commonly found in arid regions, although not many have been described in the United States (Whitney, 1985), (Figure 8-5). They occur widely in the hyper arid regions such as Peru, Iran, Egypt, and China. They can be carved from almost any material, but sandstone and lacustrine sediments are common substrates for yardangs (McCauley, et. Al. 1977; Ward, et. al. 1977; Blackwelder, 1934; Ward and Greeley, 1984) Yardangs occur extensively on Mars, and thus are of great interest to planetary scientists (McCauley, 1973). They have also been recognized in the Permian White Rim Sandstone in Canyonlands National Park (Tewes and Loope, 1992). Among the nearest well-studied sites for yardangs is the Walker Lake area of Nevada, long considered a classic locality since Blackwelder's classic paper (1934).

The yardangs at White Sands are very impressive in size, and the forms are quite fully-developed (Figure 8-6). Yardangs have formed at White Sands in the lightly-cemented dune and interdune sands at various places in the Barchanoid dune field, especially in upwind areas. They are also carved into the Lake Otero evaporite sediments on both sides along the eastern shore of south Playa Lucero and along the entire western shore of the Holocene Lake Basin (Figure 2-17). Some yardangs seem to be "emerging" from the cemented cores of large dunes as the dunes migrate forward (Figure 8-6D). The widespread occurrence of yardangs at White Sands is further proof of the great significance of wind erosion in the history of the dunefield. This is especially true in places where the yardangs are all that remains of sediments of the former Lake Otero. Interestingly, we did not see much in the way of yardangs in the fossil dune country, perhaps because cementation has become too great. Good yardang development at White Sands seems to work best in homogeneous, lightly-cemented material.

Some of the yardangs have very mature aerodynamic forms, with a rounded prow and wind-scoured trough around the base (Figure 8-5; Figure 8-6A). The tendency for yardang formation at White Sands, as opposed to other dunefields in the western United States, is probably due (1) to the persistent regime of wind scour, especially downwind of Playa Lucero, and (2) the cemented nature of the gypsum sands in some places (Schenk and Fryberger, 1988). The sands are cemented well enough to support these steep aerodynamic forms, but not so much that the wind is unable to erode them.

The type of sediment from which the yardangs at White Sands are formed does not seem to have much influence on their evolution—except perhaps to control surface texture. We have seen perfectly shaped yardangs carved from both cross bedded dune and flat bedded lake sediments. We have noted, however, that wind-resistant horizons within flat bedded playa sediments may form a cap rock on the yardang, causing it to develop a flat top, such as those along the eastern shore of south Playa Lucero.

Pedestal dunes may evolve into yardangs. As soon as pedestal dunes lose their protective vegetation, they commonly begin to assume the form of a yardang, although cemented blocks falling from lithified layers may disrupt the aerodynamics (Figure 8-7). Moreover, many pedestal dunes are in the active barchanoid dunefield, where there are weaker winds at the level at which yardangs develop, thus their evolution is impeded.

Yardangs can form at very small scales. Yardangs at White Sands range in size from small scours only a few centimeters high on the floors of interdunes all the way up to forms 10 or more feet in height (See Figure 8-6A). They are usually elongate from southwest to northeast, aligned with the prevailing wind.

Pedestal dunes are erosional landforms that owe their origins to temporary stabilization of dune sediments by trees or other deep-rooted plants. When the dune moves on, the "Pedestal" dune remains, commonly with the plant still in place and growing (Figure 8-8). There is also some evidence that pedestal dunes form due to stabilization of surficial dune-top sediments by campfires (either modern or ancient) that bake the gypsum sand to plaster of Paris. This process forms a hard, protective cap on the gypsum. Pedestal dunes are interesting forms, as they commonly harbor "island" communities of organisms living in the shelter of the tree or shrubs, occasionally even small mammals such as foxes.

The wind scour basin on the east side of the Heart of the Dunes drive, opposite the Big Dune Trail, has been noted earlier. It is a good small scale model of the evolution of the scour basin in the Otero sediments from which the vast majority of White Sands has been sourced. The surrounding scarps, the flat bottom controlled by water table, the wave-cut shorelines and scattered yardangs—all at a small scale—make this little basin a sand-table model for the much larger events that formed the present day scour basin in the Otero lake sediments (Figure 8-9). Sand scoured from this small basin has not formed migrating dune fields downwind, since there is not enough supply. Trains of eolian ripples can be seen at the northeastern (downwind) shore, atop low mounds of eolian sand scoured from the basin and piled up as lunette dunes. In a manner similar to Playa Lucero, the present evaporite sabkha contributes little new sand to the lunette dune on the eastern side. Fresh sand is brought onto the mini-basin by wind action and small rivulets that flow onto the basin floor and deposit fresh sand in a manner analogous to the Sheetflood fans growing onto Playa Lucero today.

Some of the controls on eolian erosion at White Sands have already been discussed, especially the importance of the water table and early cementation. One important remaining question is: Was the wind erosion of the gypsum beds of ancient Lake Otero sudden, or did the process occurred gradually? There is certainly much evidence to suggest that the process has been very rapid, and that the presently active barchanoid dune field is surely no older than a few thousand years. On the other hand very large inliers of uneroded lake sediments capped by partly lithified "older dune or fossil dune terrains" suggest that the process was not uniform in all places. It is interesting to compare the history of White Sands with that of nearby Lake Estancia and Lake Trinity. At Lake Trinity, the gypsiferous sediments are still in place and are gradually being covered by Holocene quartz dunes migrating eastward. At Lake Estancia, whatever dunes were generated in the past have become stabilized. White Sands, however, perhaps through sheer size, continues to be active, still supplied by sands fed from the slowly eroding remnants of Lake Otero exposed on the Alkali Flat, now scoured down almost to the water table.

It is possible that the water table in times past has fallen well below the surface of the Lake Otero sediments, perhaps during times of extended drought—there are several major droughts known to have occurred since the Altithermal about 7000–6000 years ago (See summary table of dates, Chapter 2). Or perhaps the altithermal drought lowered the water table such that surface disturbance by large animals or removal of protective vegetation by fire triggered a catastrophic erosional event? Further work with dating sediments around White Sands will enable us to decide whether White Sands dunefields represent a steady evolution from a drying lake, or a more sudden release of a vast volume of stored sediment of a perfect size for wind entrainment.

Another interesting question to ponder is: Why are the deepest scoured areas at North and South Playa Lucero located where they are? Is it possible that wind is funneled through the passes in the mountains to the west, increasing the force of the wind across those places due to vorticity or simple funneling effects? Helicopter pilots have reported that winds and turbulence are commonly stronger opposite the passes southwest of Playa Lucero than elsewhere around White Sands. Thus, it seems not entirely unreasonable to wonder if the structure of the San Andres Mountains may have affected the wind sufficiently to focus wind scour on the Playa Lucero area. This concept is not without merit, but would require detailed wind records to explore further. Moreover, it must be recalled that tectonically, Playa Lucero is a low spot in any event, and the deepest scour here may simply reflect that there were more evaporites available from the Otero beds for removal.

	Figure 8-1—The surface of the Alkali Flat, revealing twinned gypsum crystals being scoured from sediments of the former Lake Otero.
	Figure 8-2—The Alkali Flat Scour Platform climbs onto a surface consisting of cemented dune sands at the upwind edge of the main dune field. These scoured base-of-dune crossbeds are remnants of the present active dune field.
	Figure 8-3—Large selenite crystals in gypsum mud on the upwind margin of Playa Lucero. These crystals have been exposed by wind and water erosion. Although spectacular, they contribute little material to the dune field and, in fact, are one of the more wind-resistant terrains.
	Figure 8-4—The extensive wind scoured terrain on the downwind side of Playa Lucero; view to the west-northwest. This terrain suggests that only limited amounts of new sediment are being released from Playa Lucero at the present time. Prevailing wind is from the left to right.
	Figure 8-5—A yardang on the upwind side of the "fossil dune terrain" near the NE30 observation site. The prow, or upwind side of the yardang faces the camera. The yardang tapers downwind. This feature was carved by wind from a portion of a lightly cemented dune (note the crossbedding in the yardang).
A B C D	Figure 8-6—Further views of wind eroded terrains at White Sands. (A) A head-on view of a yardang, showing symmetrical shape. View is in same direction as the formative wind. (B) Yardangs with moats on the upwind side. Moats are the wind-scoured troughs at the front and sides of the yardangs. Wind from upper left of photograph. (C) A Yardang on the shore of south Playa Lucero, carved from flat-bedded sediments of Lake Otero. Formative wind from right to left. (D) Incipient yardangs emerging from the lightly cemented core of a barchan dune downwind of south Playa Lucero. Wind from right to left. Windward slope of the dune is in the background.
	Figure 8-7—A pedestal dune with case-hardened top. The affinity with yardangs is clear.
A             B	Figure 8-8—Example of pedestal dunes. (A) Pedestal dune with healthy vegetation on top. The original dune has migrated onward and left the plant and its case-hardened pedestal behind. The mechanism of case-hardening of the pillar supporting the plant is not known, but may involve processes associated with the presence of roots and rhizomes and the formation of rhizocretions as described by Klappa (1980). (B) A pedestal dune topped by a yucca plant, which has draped the sides of the pedestal and protected it from further erosion.
	Figure 8-9—The eolian deflation basin near the Big Dune Trail parking lot, with a yardang in the center. This small basin is in some ways a model for the evolution of the White Sands system, especially in the dominance of wind erosion processes in the formation of the landscape.