BULL. GEOL. SOC. AM. VOL. 45, 1934, PL. Northeast of Rogers Dry Lake, Mohave Desert, California. GROUP OF SHARP YARDANG RIDGES SEPARATED BY TROUGHS Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/45/1/159/3430339/BUL45_1-0159.pdf by guest on 24 September 2021 BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 45. PP. 159-166. PLS. 1-7 FEBRUARY 28, 1934 YARDANGS * BY ELIOT BLACKWELDER (Read before the Gordüleran Section of the Society, April 7 ,19SS) CONTENTS Page Yardangs defined...................................................................................................... 159 Some examples......................................................................................................... 160 Formation of yardangs and troughs....................................................................... 161 Limiting conditions................................................................................................. 163 Relative efficiency of wind as anerosional agent................................................... 164 References................................................................................................................. 165 YARDANGS DEFINED In this paper it is my purpose to describe certain features of con­ siderable size which are distinctive and seem clearly to be made by wind erosion. Although they have long been known to explorers of deserts, they have not received from geomorphologists the attention they deserve. The erosive activity of the wind has two aspects—which are commonly, although not universally, distinguished as abrasion, or grinding, and deflation, or blowing away. Locally these two processes co-operate to bring about a joint result, but deflation also functions largely without the aid of abrasion by picking up loose dust and sand and sweeping them away. The condition is analogous to the relation between stream abrasion and transportation. Distinctive land forms carved out by the wind are apparently rare, for they have been reported by only a few observers. They consist essentially of round bottomed chutes, or troughs, separated by sharp ridges which range from a few inches to 25 feet or more in relief. Features of this kind were noted by the distinguished explorer of central Asia, Sven Hedin, and to him we owe the name yardang1 by which these features are known to the natives of Chinese Turkestan. Similar wind- * Manuscript received by the Secretary of the Society, May 15, 1933. 1 Sven Hedin: Central Asia and Thibet, vol. 1 (1903) p. 350. For the usage of this term, see Eliot Blackwelder: Yardangs and zastruga, Sci., vol. 72 (1930) pp. 396-397. XI—Bull. Geol. Soc. Am., Vol. 45, 1934 (159) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/45/1/159/3430339/BUL45_1-0159.pdf by guest on 24 September 2021 160 ELIOT BLACKWELDEK— YARDANGS carved ridges were also noted by Obruchev2 in central Asia, by Walther 3 and others in Egypt, by Stein4 in Turkestan and by Bosworth5 in Peru. Walther refers to them as “sphinx hills,” but the others had no special name for them. Bosworth has given the best description of them, but in a book which is likely to escape the notice of most geomorphologists. Similar forms in snow are familiar to explorers of the polar regions and are called by them zastrugi,e but it is apparent that this term is also used for other kinds of irregularities in the surface of the snow, due to melting and to the heaping of snow into ridges. Huntington 7 states that in Turkestan some of the yardangs “are enormous affairs a hundred feet high and hundreds of yards long. I have traveled for a whole day among yardangs of this kind where I had to zigzag back and forth because the slopes of clay were almost vertical and openings between the yardangs were far apart.” On a small scale they are developed almost everywhere that wind erosion is effective, as on damp sand along ocean beaches. SOME EXAMPLES Examples of yardangs thus far seen by the writer have been mostly in rather soft yet coherent deposits such as Tertiary lake beds, Pleistocene sand dunes now somewhat indurated, and sedimentary deposits from ore- separating mills. Such features have been found both east and west of Tonopah, north of Silver Peak, and northeast of Carson Sink, in Nevada. Some of the best examples may be seen along the northeast side of Rogers playa8 in the Mohave Desert of California. At that locality a broad belt of dune sand, which accumulated near the shore of a Pleistocene lake, has since been much eroded and no longer preserves the form of the original dunes. The material is coarse sand with which is mingled a plentiful, although variable, supply of small clay pellets swept up from the adjacent dry bed of the lake. The laminae which contain much clay are more coherent and hence more resistant to wind erosion than the layers of friable and almost uncemented quartz sand. a V. A. Obruchev, Leningrad. Personal letter, June 1930. 8 J. Walther: Das Gesetz der Wilstentildung, Leipzig (1924) p. 208. * M. A. Stein: Explorations in central Asia, Geog. Jour., vol. 34 (1909) p. 26. 5 T. 0. Bosworth: Geology and paleontology of northwest Peru, London (1922) pp. 297-300. 6More commonly spelled sastrugi (following the German style). 7 Ellsworth Huntington : Personal letter, 1930. »Except where otherwise stated, all photographs in this paper were taken there. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/45/1/159/3430339/BUL45_1-0159.pdf by guest on 24 September 2021 on 24 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/45/1/159/3430339/BUL45_1-0159.pdf BULL. GEOL. SOC. AM. VOL. 45, 1934, PL. YARDANG TROUGH Looking down one of the narrower wind-scoured troughs, with a sharp yardang (seven feet high) on the left and a higher one on the right. The latter has been torn by rills and strongly undercut by the sandblast. A thin sheet of rippled fine gravel occupying the middle of the trough repre­ sents the material used by the wind in its abrasion. on 24 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/45/1/159/3430339/BUL45_1-0159.pdf BULL. GEOL. SOC. AM. VOL. 45, 1934, PL. SIDE OF A LARGE YARDANG RIDGE WITH MINIATURE CANYONS CARVED BY OCCASIONAL RAIN RILLS At the base of this slope the sandblast has been so active that most of the small alluvial fans have been abraded away and the lower ends of the canyons left hanging. FORMATION OP YARDANGS AND TROUGHS 161 FORMATION OF YARDANGS AND TROUGHS Although the sharp crested ridges are the more conspicuous features and have, therefore, received the special name, yardangs, the attention of the geomorphologist should be concentrated upon the intervening troughs, for, as in the case of stream-made valleys, it is the excavation of the troughs that leaves the ridge outstanding. As yet there appears to be no technical term for these features and so in this paper they will be referred to as yardang troughs, or merely as troughs. The wind-eroded trough is characteristically concave-bottomed and steep-sided, being therefore U-shaped and somewhat like a glacial canyon in cross section (Plate 2). Many of them have been widened to such an extent that they have rather spacious and nearly flat bottoms. In some cases this is due to the fact that erosion has reached a more resistant stratum, such as a bed of tough clay. Since the wind works broadly rather than along narrow lines, there is no channel in the bottom of the trough, except in so far as occasional rills follow the depression. On the other hand, it is usually rippled with low transverse ridges of fine gravel, representing the coarsest material that the local winds have been able to sweep along (Plates 2 and 3). The beginning of each trough may reasonably be ascribed to some inequality either of material and structure or of surface. As suggested by Hedin, the initial irregularity which directed the sandblast was in many cases doubtless a gully formed by occasional rills of water. These small ravines afford the wind the slight advantage necessary to start the formation of troughs. In entering the mouth of the ravine the lower stratum of the wind is somewhat compressed, and its velocity is thereby increased. The wind drifts sand up along the ravine, and this stream of detritus grinds off the bottom, and particularly the sides, of the trough, undermining the adjacent ridges so that their slopes become progressively steeper. In many places they are vertical, or nearly so. The effectiveness of this abrasion is well shown in Plate 3, where tiny ravines, formed on the slopes of the yardangs by occasional rains, have been entirely erased at the bottom of the slope. In many cases (Plate 3) the small alluvial fans which are temporarily built at the mouths of such ravines can be found in all stages of destruction by the sandblast. As it is the coarsest detritus which does most of the abrasive work and as the coarse particles are only rolled or danced along the surface of the ground, the effects of abrasion are noticed chiefly within a few feet of the floor of the trough. Upward they rapidly decrease in intensity. Hence, the crests of the higher yardang ridges are left in a ragged Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/45/1/159/3430339/BUL45_1-0159.pdf by guest on 24 September 2021 162 ELIOT BLACKWELDER----YARDANGS condition (Plate 4) caused largely by the work of rills upon their slopes and partly by the collapse of the material along joints as the undercutting progresses. Where two stream-carved valleys join they enclose a spur, the crest of which gradually declines toward the junction. It is otherwise with a yardang between two troughs. The end of such a spur is sharply undercut by the sandblast and may be either vertical or distinctly over­ hanging (Plate 6).