The depositional features of White Sands are justly famous, however it is the extraordinary events of wind scour over wide regions that has defined the area, and continues 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, gypsum sediments were precipitated from Lake Otero, probably in a very shallow water sabkha regime and they extended more or less horizontally across the lower parts of the Tularosa Basin. Certainly the evaporites did not fill the basin to the 3995 foot level, but somewhere lower, 30 feet above the present playa surface near the visitor's trail to Playa Lucero . Perhaps the surface looked something like the surface of Trinity Lake (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 as they are preserved today, or perhaps occurred through steady 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 former lake Otero. Deflation surfaces on these sediments resemble in plan view the vertical outcrops illustrated in Chapter 7 (Figure 8-1). Along the downwind edge of the alkali flats, the scour surface climbs onto dune material 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 Figure 2-17B. Some of these terrains have become partly lithified internally as well as developing 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 area 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 are widely observed in hyper arid regions such as Peru, Iran 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 have been described 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 (1934) paper.
The yardangs at White sands are very impressive in size, and forms are quite well-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 2-17A, 2-17B). 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 persistent force of wind erosion in the history of the dune field, especially in places where the yardangs are all that remains of sediments of the former sabkha facies of 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, especially in the form of a tough surface gypsite crust. Good yardang development seems to work best in a 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 dune fields in the western USA 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 cemented well enough to support the 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 control on their evolution - except perhaps to control surface texture. We have seen perfectly shaped yardangs carved from both dunes and 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 (or cooked, in the case of campsites = plaster of Paris cement) may disrupt the aerodynamics (Figure 8-7). Moreover, many pedestal dunes are in the active barchanoid dune field, 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 yardang-shaped scours on the floors of interdunes (See Figure 8-6A) all the way up to forms 10 or more feet in height. 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 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 large mammals such as foxes.
Big Dune Trail mini-basin.- The wind scour basin on the east side of the Heart of the Dunes drive, near the Big Dune Trail, has been noted earlier. It is remarkable in that is almost a perfect model of the evolution of the scour basin in the Otero sediments from which the vast majority of the White Sands has been sourced. The surrounding scarps, the flat bottom controlled by water table, wave-cut shorelines and scattered yardangs - all at a small scale, make this little basin a sand-table model for the much larger scour events that formed the present day scour basin in the Otero lake sediments (evaporite phase) (Figure 8-9). Sand scoured from this small basin has not formed dune fields, since there is not enough of it, but trains of eolian ripples can be seen at the eastern (downwind end) along with mounds of eolian sand scoured from the basin and piled up as lunette forms. In a manner similar to Playa Lucero, the present evaporite sabkha contributes little new sand to the dunes 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 alluvial fans flowing onto Playa Lucero today from the San Andres mountains (Figure 8-10).
Some of the controls on eolian erosion have already been discussed; especially the water table and early cementation. Among remaining questions are, how sudden was the breakdown of the old sabkha surface of ancient lake Otero, or has 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 that 3000 years. 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 Lakes 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 have become stabilized. White Sands, however, perhaps through sheer size, continues to be active, still supplied by sands fed from the slowly eroding remnants available from lake Otero, 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 at time of severe drought - there are several 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 the White Sands will enable us to decide whether the White Sands represent a steady evolution from a drying lake, or (as I suspect), a catastrophic release (in a few hundred years) of a vast volume of stored sediment of a perfect size for wind entrainment.
Another interesting subject 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 funnelled through the passes in the mountains to the west, increasing the force of the wind across those places due to vorticity or simple funnelling effects? Helicopter pilots have reported that winds and turbulence are commonly stronger opposite the passes west of Playa Lucero than elsewhere around the 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. 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 for removal.
Figure 8-1.- Wind scoured surface of 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 barchanoid dune field. These scoured base-of-dune crossbeds are remnants of the present active dune field.
Figure 8-3.- The large selenite crystals on the upwind edge of Playa Lucero. These crystals have been exposed by wind a 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.- Wind scoured terrain on the downwind side of Playa Lucero; view to the west-northwest. This terrain is widespread, suggesting that only limited amounts of new sediment are being released from Playa Lucero at the present time.
Figure 8-5.- A wind-formed yardang on the upwind side of the “fossil dune terrain” near the NE30 observation site. Prow, or upwind side of the yardang faces the camera. The yardang tapers downwind. This feature was carved by wind from a lightly cemented dune (note crossbedding in yardang).
Figure 8-6.- Further views of wind eroded terrains at White Sands. (A) Direct view of a yardang, showing symmetrical shape. View is in same direction as 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) Yardang on the shore of southeast 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 in background.
Figure 8-7.- Pedestal dune with case-hardened top. The affinity wind yardangs is clear. This feature was formed from portions of a dune.
Figure 8-8.- Pedestal dunes. (A) Pedestal dunes with healthy vegetation on top. Dune has migrated onward and left to plant and it's case-hardened support behind. 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) Pedestal dune topped by a yucca plant, which has draped the sides of the pedestal and protected it somewhat from further erosion.
Figure 8-9.- 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.
Figure 8-10. - 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 alluvial fans flowing onto Playa Lucero today from the San Andres mountains