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 ki- netic 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/7/829/3444899/i0016-7606-95-7-829.pdf by guest on 26 September 2021 830 WARD AND GREELEY 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 appar- ently 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 down- stream end (Fox and McDonald, 1973; Hughes and Brighton, 1967). Most of the isolated yar- dangs at Rogers Lake have a ratio of about 1:4 (Fig. 5). The aerodynamic characteristics of three yar- dangs 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- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/7/829/3444899/i0016-7606-95-7-829.pdf by guest on 26 September 2021 EVOLUTION OF YARDANGS, CALIFORNIA 831 field results are only approximate, because mul- tiple synchronous wind data were not obtained over the yardangs. The velocity profiles along a yardang typi- cally showed a steady to accelerating air flow along the windward ends on the yardangs, a decrease in velocity just past their widest por- tion, and a reacceleration toward their down- wind 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 forma- tion are caused by changes in fluid pressure around the body, controlled by the form of the body. When the pressure decreases in the di- rection of flow, the decreasing gradient aids the downstream transport of particles.