Evolution of the Yardangs at Rogers Lake, California

Evolution of the yardangs at Rogers Lake, California

A. W. WARD U.S. Geological Survey, Flagstaff, Arizona 86001 RONALD GREELEY Department of Geology and Center for Meteorite Studies, Arizona State University, Tempe, Arizona 85287

ABSTRACT others (1977) estimated the changes in yardangs 1960; Feth, 1964; Snyder and others, 1964). through time, detailed geomorphic studies and These playas are within Edwards Air Force Base Yardangs are streamlined, wind-eroded the application of aerodynamic principles and and have been used for many years as natural hills common to most deserts. Yardangs at techniques have not been used previously to runways for landing experimental aircraft and Rogers Lake, Mojave Desert, California, analyze their formation. In this study, theoret- spacecraft. The present environment is charac- have streamlined forms characteristic of ob- ical and experimental approaches (including terized by precipitation of less than 10 cm/yr jects eroded by moving fluids, a teardrop wind-tunnel simulations) are used to explain the (mainly between November and February) and shape that approaches an ideal 1:4 width- evolution of yardangs at the Rogers Lake year-round strong, dry, westerly winds (U.S. Air to-length ratio. In wind-tunnel simulations, locality. Force, unpub. data) (Fig. 2). miniature forms of various shapes changed The yardangs are a series of northeast- sequentially by (1) erosion of the windward ROGERS LAKE YARDANGS southwest-oriented streamlined hills carved in corners, (2) erosion of the windward slope, moderately consolidated nearshore and shore- (3) erosion of the leeward corners and Rogers Lake (a playa) (Fig. 1) and neighbor- line deposits on the northeast side of the playa flanks, and (4) erosion of the leeward slope. ing Buckhorn Lake and Rosamond Lake (also (Figs. 3, 4). The Rogers Lake deposits contain Prominent mechanisms in yardang evolu- playas) formerly were covered by Pleistocene beds of fine gravel, sand, silt, and clay. The sand tion apparently are abrasion at the wind- Lake Thompson (Thompson, 1929; Dibblee, and gravel are predominantly quartz and alkali ward end and deflation and reverse air flow near the middle and at the downstream end. Width-to-length ratios of yardangs are grossly similar to those of some fluvial and glacial streamlined landforms. The low kinetic energy of wind relative to ice and water, the erosional resistance to wind of most rocks, the rarity of long-term, unidi- rectional winds, and the presence of run- ning water, topographic roughness, and vegetation all limit the abundance of yardangs.

INTRODUCTION AND PURPOSE

Yardangs (streamlined, wind-eroded ridges) are little studied and poorly understood desert landforms. They were originally described and named by Hedin (1903) in Chinese Turkestan and were likened to inverted boat hulls by Bos- worth (1922). These streamlined ridges are found in most of the major deserts of the world and typically occur in great fields. Yardangs are commonly cut into moderately consolidated rocks of Pleistocene and Holocene age but are also found in Tertiary sandstones, and rarely in older indurated rocks. Yardangs in China, Peru, and Iran are tens of kilometres long and more than 100 m high (Mainguet, 1972; McCauley and others, 1977; El-Baz and others, 1979). The best-known yardang location in the United States is at Rogers Lake, California, originally described by Blackwelder (1934). Al- though, in their global study, McCauley and Figure 1. Map showing location of Rogers Lake, California.

Geological Society of America Bulletin, v. 95, p. 829-837, 12 figs., July 1984.

829

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height, 50 m in length, and 10 m in width. All trend from 248° to 274°, aligned with the re- gional winds. The windward ends of many large yardangs consist of steep surfaces. Downwind, the flanks of these yardangs are incised by small gullies. Locally, the surfaces are covered by wind-blown sand. The leeward ends of many of W the yardangs grade into the rolling hills of the W shoreline deposits. These junctions consist of gentle, ungullied slopes mantled by as much as 3 m of eolian sediments, consisting of grains derived from the yardangs and transported downwind. The largest gullies on the yardangs occur im- mediately downwind of the widest part of a streamlined ridge. At the leeward end of the Figure 2. Summary of wind data collected at Edwards Air Force Base by U.S. Air Force yardang, gullies either are covered by sediment from 1947 to 1977. Wind roses show that (a) the most frequent winds (as percentage of ¡ill or are prevented from forming by the loose, winds) and (b) the strongest winds (in knots) are from the southwest. porous mantle. The absence of gullies at the windward end of the yardangs is probably due to frequent intense abrasion. Gullies thus are feldspar grains. Diastems are common, as are action, dried and . . . blown into [a] dune in a preserved in only the more stable midsections of cross-beds of both wedge and planar type. The manner similar to the formation of coastal yardangs. induration of the deposits results partly from the fore-dunes." Bowler (1968) considered ordinary Individual beds are etched in bas-relief on the desiccation of clays and partly from the diage- lunettes (Hills, 1968) to result from deflated clay surfaces of many yardangs by differential ero- netic precipitation of calcium carbonate. aggregates from a dry playa. Sandy lunettes are sion. The grain-size distribution varies from bed Thompson (1929, p. 303) considered these considered to represent beach deposits that peri- to bed, and sandier beds are eroded more easily hills to represent beach ridges deposited in stand- odically may be inundated, producing alternat- than are silt- and clay-rich beds. In coarser- ing water, whereas Blackwelder (1934) consid- ing beds of sand or gravel and clay. The deposits grained beds, grains can be detached from a yar- ered them to be dunes. Bowler (1971, p. 53) at Rogers Lake have such alternating beds. dang by the touch of a finger; this friability described similar features that he called sandy At Rogers Lake, about 50 yardangs occur in 2 suggests little resistance to wind gusts or rain. lunette dunes. He believed that these deposits are clusters. Together, the clusters cover about 0.5 Etched layering is present only on the more sta- formed by "sands, thrown on the beach by wave km2. The largest yardangs average 5 m in ble part of the yardang, such as the tapering part downwind of the beam, the widest part of the yardang. The delicate, etched texture is not found at the "bow." Differential erosion apparently occurs only where the rate of abrasion is relatively low. Lengths and widths of many of the Rogers Lake yardangs were measured in the field and on aerial photographs. According to principles of fluid mechanics, the equilibrium form for a body immersed in a moving fluid has a width- to-length ratio of 1:4, with a tapered downstream end (Fox and McDonald, 1973; Hughes and Brighton, 1967). Most of the isolated yardangs at Rogers Lake have a ratio of about 1:4 (Fig. 5). The aerodynamic characteristics of three yardangs were investigated by measuring air llow in the field during windstorms in Februaiy and March 1977 using a hot-wire anemometer. The probe was placed on a bamboo pole about 3 m long, so that the operator would not interfere with the flow field. Readings were taken by walking near the yardang and placing the probe along the crest, at the middle of the side, on the Figure 3. Low-angle oblique view looking to northeast and showing one yardang cluster at trough floor, and 3 m above the trough floor at Rogers Lake. Note transverse granule ripples and braided-stream channels. selected points on each yardang. These flow-

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field results are only approximate, because mul- tiple synchronous wind data were not obtained over the yardangs. The velocity profiles along a yardang typically showed a steady to accelerating air flow along the windward ends on the yardangs, a decrease in velocity just past their widest portion, and a reacceleration toward their downwind ends. These data suggest that the yardangs studied are aerodynamic landforms. Flow accel- eration occurs along the crest and flanks of the yardangs, except where deep gullies have disfig- ured the surface (especially at the beams) or where the yardangs merge into massive (un- eroded) hills at their leeward ends.

AERODYNAMIC CONSIDERATIONS

Bosworth (1922) stated that, by either erosion or deposition, wind will adjust a landform to an equilibrium form—the shape of least resistance A to (or creating the least disturbance in) the air flow. A model for the creation of yardangs can be developed from principles of fluid mechanics. Three flow regions surround a body immersed in a moving fluid (Fig. 6): (1) the "ideal" (undis- turbed) flow region upstream from the body; (2) the boundary layer, which is the frictional shear layer near the body in which the horizon- tal component of the velocity is <99% of the free-stream velocity; and (3) the wake, which is the turbulent region downstream from the body caused by separation of the boundary layer. Boundary-layer separation and wake formation are caused by changes in fluid pressure around the body, controlled by the form of the body. When the pressure decreases in the direction of flow, the decreasing gradient aids the downstream transport of particles. Conversely, B increasing pressure in the direction of flow op- poses downstream transport. If the form of the Figure 4. Two views of typical 15-m-long yardang at Rogers Lake. Note tapered bow body changes, the pressure gradient can reverse, (b) and stern (s), sharp crest (c), and smooth flanks (f). which causes boundary layer separation and wake formation (Fig. 7). At the separation point, the momentum of air particles (hypotheti- cal, infinitely small fluid masses) near the surface decreases and the particles come to rest against 3 u- the surface. These particles then deflect subse- 0) quent air particles out and away from the sur- E 2 3 face, causing the actual "separation." Down- z I stream of this point, a local reversal of the pressure gradient occurs and flow of air masses within the boundary layer is reversed. This reverse flow also has been termed "backflow" and "negative" flow (Hughes and Brighton, 1967, Width / Length p. 76; Fox and McDonald, 1973, p. 396; Figure 5. Width-to-length ratios (W/L) for 27 of the Roger Lake yardangs. Isolated (ma- Whitney, 1978). ture) yardangs are represented on the left portion of the chart; yardangs still attached to hills By definition, the body form of least resis- are on the right. tance has the least total drag (Fig. 8). Total drag

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Boundary layer separation point Ideal flow region Figure 6. Flow regions around (a) a cylinder and (b) a streamlined form. Open arrows indicate wind direction. (After Hughes and Brighton, 1967, p. 7) 0: experiments also produced forms, the final shapes of which can be compared to those of natural yardangs. Boundary layer with no separation The wind tunnel used is located at the Plane- tary Geology Laboratory at Arizona State Uni- versity, Tempe. The tunnel is an open-circuit atmospheric boundary-layer wind tunnel in which air is drawn through the test section to provide smooth, uniform flow across the mod- 0 2 els. The tunnel is 15 m long and 1 m in cross section and is capable of producing velocities as great as 45 m/sec. Initial experiments using natural rock samples from a Rogers Lake yardang in a wind tunnel indicated that, even with sand-sized abrasives in the flow field, moderate to full erosion and Pressure decreasing Pressure increasing in flow direction in flow direction streamlining require several hundred to probably several thousand hours. Such a duration made the semiquantitative investigation of the flow fields around several types of nonstreamlined forms impractical for this study. Instead, synthetic samples composed of materials with properties similar to those of natural yardangs carved in soft sediments were used, so that yardang formation could be studied in a reasonable amount of time. Of many materials testec., a mix of 33% fine and medium silica sand, 33% hom- iny grits, and 33% moist coffee grounds was found to be satisfactory. This sample medium could be molded into any desired shape to produce coherent samples with textures similar to those of the Rogers Lake yardangs, but with loose grains that eroded faster by an order of magnitude than those in natural, cohesive materials. Initial experiments also showed tha': sand- sized abrasives destroyed the synthetic samples Enlarged view of yardang crest within several seconds to a few minutes. Instead, neutrally buoyant soap bubbles 2-3 mm in di- Figure 7. Pressure variations along a surface of varied height. Dark arrows represent flow ameter were used to trace the flow field, using a direction. (After Fox and McDonald, 1973, p. 396) procedure similar to that of McCauley a:.id others (1979). The bubbles simulate the action of (the sum of skin-friction drag and pressure or elongate forms would be more subject to erosion wind-borne particulate matter, but they are "body-form" drag) is at a minimum when the in the longitudinal direction and would be nondestructive, so that the relative importance width-to-length ratio of an object is 1:4 (Fox trimmed at the front and rear. of abrasion, deflation, and reverse flow can be and McDonald, 1973). Any object immersed in estimated. a moving fluid tends to be modified by erosion EXPERIMENTAL SIMULATION OF Initial and final shapes and width-tc-length or by deposition to this ratio. For example, YARDANG DEVELOPMENT ratios of samples that showed measurable forms with high width-to-length ratios (that is, change are presented in Figures 9a-9e. For all those nearly circular in shape) would tend to As natural yardangs develop slowly or under samples tested, the final width-to-length ratios have material deposited by reverse flow in the wind conditions unfavorable for direct obser/a- were about 1:4. Furthermore, most samples lee, thus increasing the effective body length. tion, wind-tunnel studies were performed to tended to change sequentially by (1) erosion of These forms could also be eroded at the flanks, simulate their evolution, especially with respect windward corners, (2) erosion of the windward effectively decreasing body width. Conversely, to where and how erosion takes place. These slope, (3) erosion of the leeward corners and

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flanks by negative flow, and (4) erosion of the 0 Width upper surface downwind of the windward slope, also by reverse flow (Fig. 10). Length Figure 8. Graph show- Two other forms were tested but did not un- ing coefficient of drag on dergo any significant streamlining: an elongate 0.10 a streamlined form as a ridge (initial width-to-length ratio of 1:10) and a function of the thickness low mound (initial ratio of 1:1). Apparently, ratio, with contributions these shapes did not generate enough turbulence of skin-friction drag and (at the low wind speeds used) to create signifi- o 0.05 pressure drag to a total cant negative flow. At higher speeds, however, o drag. Open arrow indi- the models were destroyed in a matter of several Q cates wind direction. (Af- seconds to a few minutes by the primary air ter Fox and McDonald, flow, with grains leaving the model completely 0.1 0.2 1973, p. 412) and with no significant negative flow around the model. Thickness ratio . . I length I In summary, the wind-tunnel experiments (Circularity of form increases demonstrated (1) the stability of the 1:4 width- to-length shape, (2) the sequence that most forms followed to achieve that shape, (3) the

a Figure 9. Sequences of forms generated in wind-tunnel tests. Numbers above the forms indicate hours of exposure to wind, numbers below forms indicate width-to-length ratios. Wind velocity, lOm/sec. Arrow shows wind direction, a. 4-cm-high, kidney-shaped sample. Initial width-to-length ratio of 0.53 was changed to 0.31. b. 6-cm-high, rectangular-shaped sample. After 116 hr, sides taper at an angle of 18°, which approximates a shape ratio of 0.30. c. 10-cm-high pyramid-shaped sample with a base ratio of 1:1. After 4 hr, the new streamlined hill had ratios of 0.29 at its base and 0.42 at its peak. d. Oblong sample. The initial 0.35 ratio decreased to 0.17 before the form stabilized at ratios between 0.26 and 0.30. e. The forms from 9d at 0,9, and 222 hr are superposed for comparison. Initial, pseudo-streamlined form (1) of 0.35 width-to-length ratio undergoes primarily a decrease in width to an intermediate state (2) with a ratio of 0.24. The new, nearly perfect form shrinks from front to back (3) and the ratio changes to 0.29. Generally, the tail was fixed, and little downwind erosion occurred. This form was modified by headward and side erosion.

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for most of the initial streamlining of the ridge, whereas deflation and abrasion maintain the form. Field evidence confirming this model of the evolution of the yardangs is minimal, because of limited historical information. The earliest known photographs are those of Blackwelder (1934), probably taken in 1932. After relocating his stations and comparing yardang morpholo- gies (Fig. 11), we estimate that the average rate of headward erosion (primarily by abrasion) since 1932 was 2 cm/yr, and the average rate of lateral erosion (by abrasion and deflation) was 0.5 cm/yr.

SUMMARY AND CONCLUSIONS

The Yardang Form

1. The Rogers Lake yardangs are carved into moderately consolidated sediments of late Pleis- tocene age. They are small (a few tens of metres long) in contrast to the large (tens of kilometres long) yardangs that are common in the great deserts and that are carved into a variety cf rock types (McCauley and others, 1977). The Rogers Lake yardangs are moderately gullied. They are less streamlined than yardangs of the hyperarid coastal desert of Peru but are more streamlined than others, for example, in the semiarid Lut Desert of Iran (McCauley and others, 1977). Few of the yardangs at Rogers Lake are isolated landforms (Figs. 3, 5). The attachment of many yardangs to the shoreline deposits suggests that these ridges are still forming and eventually will become isolated forms. Their immaturity may be a function of (1) the young age and limited exposure time of the sediments, or (2) the relative paucity of strong winds compared with other deserts where mature yardangs are found. 2. Most of the isolated (mature) yardangs at air-flow patterns around the evolving forms that son. Subsequently, strong southwesterly winds Rogers Lake have a width-to-length ratio of can be inferred from these sequential changes, blew parallel to these channels and widened and about 1:4, as do some yardangs in Arizona, and (4) the relative ease of wind in streamlining deepened them by abrasion and deflation (as Peru, Iran, and South Africa (Fig. 12). How- forms with moderate width-to-length ratios (be- hypothesized by Hedin, 1903, for the Asian yar- ever, many streamlined fluvial features from the tween 1:1 and 1:10), but the apparently great dangs, and by Blackwelder, 1934). Simultane- Channeled Scabland of Washington and glacial difficulty of wind alone in creating an ideal form ously, the ridges were streamlined, primarily by landforms (drumlins) from several areas in the from stubby or highly elongate forms. abrasion. Abrasion was greatest at the windward United States have similar form ratios. Baker end and at all projecting points of the ridges and (1978, 1979) found that populations of stream- A MODEL FOR THE EVOLUTION was also effective at the sheltered downwind lined uplands in the Scabland have width-to- OF THE YARDANGS part of the ridge through the process of reverse length ratios ranging from 1:1 to 1:8. Mills flow. Reverse air flow was reduced and the de- (1980) found ratios in drumlin populations of Combining aerodynamic principles, field ob- flation of loosened grains became more impor- 1:1.8 to 1:12. Streamlined uplands, drumlins, servations, and wind-tunnel simulation results, tant as the streamlined form emerged. Abrasion and yardangs have both interfield and intrafield the following evolution is suggested for the Rog- was still effective near the front of the forms, but variations in their form ratios, depending on the ers Lake yardangs. Local runoff eroded small deflation aided in shaping in the low-pressure degree of local erosion. Baker (1978, 1979), channels in the sandy lunettes around the time of regions (especially near the middle) when re- however, believed that a ratio of 1:3 is typical of the disappearance of Pleistocene Lake Thomp- verse flow diminished. Abrasion thus accounts uplands; P. D. Komar (1981, personal com-

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Figure 10. Sequential evolution of blocky, nonstreamlined form to streamlined form proceeds as (a) erosion of windward corners, (b) erosion of front slope and crestline, (c) erosion of lee corners, and (d) erosion of lee slope. X represents final sample height, 2X is final beam width, and 8X is sample length of a perfect form. Arrow shows wind direction.

Figure 9. (Continued).

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sharp crestlines and concave-upward flanks, whereas, for example, many highly streamlined yardangs in the extremely arid coastal region of Peru have broad crestlines and convex-upward flanks. In the less dry deserts of the United States, perhaps sheetwash and gullying modify yardang flanks too frequently for wind lo restore the convex-upward form. 5. The streamlined (teardrop) form of yardangs is independent of scale. Under equilibrium conditions (that is, "uniform" fluid flew), that form probably will be maintained until individual elements that introduce roughness (boulders, grains, inhomogeneities, gullies) become large in relation to the over-all size of the streamlined form. These elements might disrupt the smooth flow field, and residual stubby knobs and other aberrant shapes might result instead of streamlined forms.

The Wind Streamlining Process

1. Grains are eroded from these yardangs by wind and water. Those grains eroded by wind can be moved downwind to form wind ripples or an eolian mantle on the yardangs. Grains eroded by water are moved downslope in shal- low stream channels, but they can be reworked by wind and moved upslope to form ripples or to contribute to the eolian mantle. Wind erosion destroys all gullies at the windward end of a yardang, and porous, mantling surface sediments may prevent gullies from forming at the leeward end. Gullies tend to be preserved in the tapered midsection of a yardang, where deposition is negligible and the rate of wind erosion is low. 2. Wind abrasion must be the most impor- tant process in the shaping of the Rogers Lake yardangs, because it probably dominated during trough formation and initial sculpting of the ridges. Thereafter, deflation increased in importance as it combined with abrasion to maintain the streamlined form. B 3. To produce streamlined forms, the moving Figure 11. A. Yardang (outlined) photographed in 1932 (Blackwelder, 1934). Arrow indi- fluid alters the length and width of the original, cates automobile roof. B. Same yardang (outlined) photographed in 1975. Note undercut nonstreamlined form. If the original form is (arrow) in large yardang, the absence of fluvial features near the undercut, and smoothing of stubby and has a width-to-length ratio that is several gullies visible in 1932. Wind from right in both photographs. greater than about 1:4 (Fig. 8), the dominant streamlining effect will be to decrease the width of the form by erosion; however, a depositional mun., 1983) believed that 1:2 is the most com- 3. Yardangs found in other deserts and eroded tail may also form, effectively optimizing the mon ratio for drumlins. We believe that the in other rock types may be different in detail width-to-length ratio. If the original, nonstream- ideal (mature) ratio is 1:4. We consider, how- from the Rogers Lake yardangs. Factors impor- lined form is elongated (having a width-to- ever, that simple form ratios are not always ge- tant in this morphologic variation probably in- length ratio less than 1:4), the dominant netically diagnostic for different kinds of stream- clude: (1) rock lithology, (2) wind strength, streamlining effect will decrease the length of the lined features. Other field evidence (fluvial frequency, and directionality, (3) rainfall and form. channels and bars, glacial striae, moraines), vegetation, and (4) the amounts of locally avail- 4. In side view, the yardang is a teardrop however, allows easy determination of the dom- able abrasive sediments. form sliced in half longitudinally, the upper half inant sculpting agent. 4. Many of the Rogers Lake yardangs have then being placed on the ground with the blunt

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100 km Figure 12. Log-log plot of length versus width for yardangs, drumlins, and streamlined features from the Channeled Scabland of Washington. Data compiled from field measurements, topo- 10 graphic maps, and aerial photographs (Ward, 1978; McCauley km and others, 1977). A, yardangs in northeastern Arizona; L, yardangs in the Lut Desert, Iran; N, yardangs in the Namib Desert, South-West Africa; P, yardangs in the lea Valley, Peru; R, yar- I dangs at Rogers Lake, California; G, drumlins near Green Bay, km Wisconsin; S, streamlined uplands in the Channeled Scabland, .C T3 Washington. Not all landforms are perfectly streamlined, owing Î to secondary processes such as gullying, undercutting, and mass 100 wasting. However, a single line (representing a shape of —1:5 m width-to-length ratio) describes the shape of most of these landforms.

10 m

Im 10m 100m Ikm 10 km 100 km length

end into the wind. The windward slope and Administration-Ames Research Center Grant York, Wiley, 630 p. Hedin, S., 1903, Central Asia and Tibet: New York, Scribners, 608 p. broad beam all undergo normal air flow, where- NSG-2284 through the Planetary Geology Pro- Hills, E. S., 1968, The lunette, new landform of eolian origin: Australian Geographer, v. 10, p. 15-21. as the leeward slope and flanks are all modified gram. We also thank D. R. Scott and W. P. Hughes, W. F„ and Brighton, J. A., 1967, Fluid dynamics: New York, Schaum, by reverse air flow. Albrecht for their help at Edwards Air Force 265 p. Komar, P. D., 1983, The shapes of streamlined islands on Eanh and Mars: 5. Owing to the low kinetic energy of wind Base, and I. Lucchitta, R. F. Madole, C. A. Wal- Experiments and analyses of the minimum-drag form: Geology, v. 11, p. 651-654. (on account of its low density), streamlined lace, S. Coleman, D. L. Schmidt, V. 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Wind-tunnel experi- Journal of Geophysical Research, v. 84, p. 8205-8221. ments were conducted at Arizona State Univer- Feth, J. A., 1964, Review and bibliography of ancient lake deposits: U.S. MANUSCRIPT RECEIVED BY THE SOCIETY MARCH 14, 1983 Geological Survey Bulletin 1080, 119 p. REVISED MANUSCRIPT RECEIVED AUGUST 1,1983 sity, under National Aeronautics and Space Fox, R. W., and McDonald, A. T., 1973, Introduction of fluid mechanics: New MANUSCRIPT ACCEPTED AUGUST 15, 1983

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