The Role of Vorticity in Developing Lineation by Wind Erosion

The role of vorticity in developing lineation by wind erosion

MARION I. WHITNEY Central Michigan University, Mount Pleasant, Michigan 48859

ABSTRACT This paper reports some of the results of a 15-yr investigation, the central objective of which has been to determine the precise de- Experiments and field observations indicate that both the gross tails of how wind erosion forms lineations. I considered earlier that shaping and the sculpturing of lineation and pit details on wind- wind might produce lineations by shifting sand along linear trends eroded surfaces may be accomplished by the impact of particles until grooves were worn and that it might erode pits by continu- which have been impelled aerodynamically by interfacial flow or by ously cycling sand. Both hypotheses were quickly found to be faulty vorticity. The vorticity operates along lines of positive, negative, when put to test. Particles did not travel in contact with groove and secondary flow over all surfaces of an object. floors or cycle in pits. Also, escape from both types of features was The axis of vorticity is typically normal to or at some high angle too rapid to determine the escape patterns. In the laboratory, with to the surface undergoing erosion. Therefore, it is not the so-called oriented specimens and very low velocity, the escape problem was roller vortex type which has been postulated by other investigators, resolved (Fig. 1). The lineation problem, however, required actual for one typical erosion pattern left abundantly by the vortex observation of the erosion processes and extensive experimenta- configuration is a round pit either helically or radically scored. tion. Often such pits occur in chains along the beds of the lineations. A Most previous investigators have not utilized the data of second type of erosion pattern common in channels is parallel aerodynamics to explain wind erosion. Higgins (1956) postulated transverse lineation, which this writer has seen in development in aerodynamic erosion by means of suspended particles in interfacial snow flutes under influence of normal-axis vortices. Such vortices currents, those currents at the boundary of rock and air. Wilson travel singly along lines of flow, pulling particles centripetally into (1972) suggested that interfacial currents might influence deposi- the vortex configurations. In snow flutes, for example, vortices be- tion and referred to such currents as a poorly understood sub- come visible due to suspended snow, and the secondary flow can be sidiary flow system. I believe, after nearly 15 years of testing flow delineated. Where general windflow is essentially unidirectional, patterns on hundreds of ventifacts in a wide range of shapes, after the erosional result of such vorticity is the creation of cross- extensive study of vorticity, and after conducting nearly 12 years of lineated, essentially symmetrical grooves. wind-blast studies and extensive analyses of electron micrographs of surfaces prior to and following wind-blasting, that I now have a INTRODUCTION good knowledge of these interfacial flow systems. In addition, the experimental data are corroborated by extensive field data on ven- This study is a continuation of that reported by Whitney and tifacts, sand ripples, sand flutes, and snow erosion. Dietrich (1973). In that report, it was shown that (1) aerodynamic Aerodynamic erosion may involve several mechanisms, such as processes, including negative as well as positive flow and vorticity interfacial flow, abrasion, vorticity, lift, wind cleavage, and possi- (cyclic activity with axis essentially normal to the surface on which it operates), may act simultaneously on all surfaces to shape, UI sculpture, and burnish ventifacts; and (2) suspended dust-sized particles moving along interfacial flow lines may act as the erosive tools. Subsequently, Dietrich (1977) illustrated dust erosion of nonsoluble minerals with hardnesses of up to 6 (Figs. 3A, 3B, and 3C). The original plan was to divide this work into seven papers, some of which were to be field studies and others laboratory studies. When the first four of these papers were submitted, the editor re- quested that they be combined into one paper. While there are obvious advantages to the reader in such a compilation, much of the experimental and supporting data had to be removed from the ac- WIND DIRECTION count. To the readers who feel the lack of these types of data, I can only say that the work could not be at once consolidated and carry Figure 1. Diagram of a simple interfacial flow relationship over a sharp amply detailed proofs. Such proofs were sought, however, for every crest. "W" refers to wind direction, "p" to positive flow, and "n" to nega- concept from some combination of the following: field observa- tive flow. This basic plan can apply either to a rock as a whole, or snow or a tions, comparative studies of thousands of ventifacts, microscopy sand dune, or to a hill or to minor irregularities on larger features. The line of wind-eroded surfaces, flow and vorncity tests, erosion of spec- of negative flow is in contact with the surface at the foot of the slope where the pressure is high and velocity is low but pulls outward to meet the over- imens by wind-blasting, and electron microscopy of the test spec- riding positive flow where velocity is higher and pressure is lower. Vorticity imens. occurs at the juncture of the flow lines at the crest.

Geological Society of America Bulletin, v. 89, p. 1-18, 9 figs., January 1978, Doc. no. 80101.

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bly electrostatic-electromagnetic phenomena. All of these have perimentation have been flow-testing and wind-blasting. Flow- been investigated by me except the last. Helene Brewer, who testing was done on more than 1,000 oriented ventifacts where the suggested wind-blasting and who built the wind tunnels used and impact zones could be determined. This is possible on Michigan monitored the first wind-blast tests, is currently experimenting with dolomite ventifacts with rilled margins because impact areas are the electrostatic-electromagnetic aspect. The main modes of ex- bald, that is, devoid or nearly devoid of rills (Fig. 2A). On other

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Figure 2. Ventifact forms, lineation, and pit features. Shaftless arrows indicate north orientation; "h" and "p" refer to high velocity and prevailing winds, respectively. Negative flow is indicated by an "n" beside an arrow. A. Top of dolomite ventifact with two principal rill sets or grooves, Big Sable Point, Michigan. B. Top of dolomite ventifact in which crestal and basal vortex pits met to form a perforation, Big Sable Point, Michigan. C. Top of dolomite ventifact with a large radially lineated vortex pit, Big Sable Point, Michigan. D. Top of keeled basalt ventifact, Shoshoni, Wyoming. E. Lift of the white grain from the pit occurs at extremely low velocity on negative flow. F. Lineated basalt boulder ventifact, Sleeping Bear moraine, Michigan. Approx- imately 1 m long. G. Profile of polished Pleistocene ventifact, showing undercut facet and basal concavity, gravel pit near Muir, Michigan (0.75x). H. Graywacke ventifact used in the experiment in Figure 2E, Big Sable Point, Michigan (0.75 x). I. Lateral view of chalcedony ventifact showing a number of wedge-shaped flutes, all of which have steep surfaces and vortex pits at their heads. Also, this specimen has a striking undercut facet beneath the high- velocity impact zone (0.75 x). J. Embossed vortex pit and helicoid scores, basal surface of sandstone ventifact, Big Sable Point, Michigan (2.5 x). K. Com- plexly eroded embossed pit on the top surface of the specimen in Figure J. This feature consists of an outer ring pit and a central boss which has undergone such profound erosion that a second ring pit has been eroded within the original boss, leaving only a very small ball-shaped boss within the inner ring (6x). L. Embossed vortex pit developed on top surface of wind-blasted halite block (about 50 x). M. Crescentic pit developed on left side of the wind-blasted margin of the same halite block as in "L" (about 30x).

ventifacts, it was also possible to identify characteristics associated the flute pit, though it may become somewhat misshapen in wind with impact. Actually, identification of probable impact areas on shifts where the dominant air motion through it is negative flow. nearly all ventifacts is now possible. As a consequence, concen- However, the overall dominance of air motion in such pits is cyclic. trated study could be made of the areas of ventifacts which, in na- Many vortex pits have radial or helical or bench-type wall sculptur- ture, were under strong negative and secondary flow, the areas ing. The flute pit may became modified to a vortex pit as it deepens; most likely to become lineated. Studies of vorticity within pits a vortex pit, subject to shifting wind directions, may become mod- yielded extensive new data on the high degree of complexity of the ified to a flute pit. Flute pits commonly have burnished floors. vortex configuration and the intimate association of different kinds Where vorticity dominates, floors may lack burnish and mineral of flow and vorticity. fragments may remain sharp. A pit chain, a series of adjacent pits, can consist of vortex pits or flute pits or both. Such chains coalesce DEFINITIONS into lineations. Embossed pits are pits with elevated centers (Figs. 2 J, 2K, 2L). A boss is a remnant of erosion left at the low pressure Physics terminology for the components of wind is used in this center of vorticity because the rising exhaust column carries the report. The downwind component is positive flow; backflow is tools of erosion away from the floor of the pit. A perforation is a negative flow. Low pressure along these flow lines creates trans- hole completely through a ventifact. A helical score is a general verse or secondary flow. Stagnation is deceleration at impact. term that I have used to refer to a small, shallow depression pro- Counterforce involves rebound of air from surfaces and refers to duced by vorticity which often bears both helical and radial linea- Newton's third law of motion. Lift refers to particles becoming air- tion. If the lineation is strictly radial, it can be termed a radial score; borne in response to differential pressure. Vortex configuration is if the lineation is distorted, it can be termed a helicoid score. A the form of a vortex, as in a flow line or in a pit. The lead line is the score is the inception of a vortex pit and, as such, may have a wide main helical intake to a vortex. Other intake lines join with this line variation in sculpture pattern, because vorticity is an extremely within a vortex pit, and where their direction reverses the exhaust complex process. column forms. The interface is the rock-air boundary. Features are defined as follows: A flute is a depression with fairly METHODS OF EXPERIMENTING regular outlines. It may vary from a short oval feature to a long symmetrical groove, commonly with rounded floor and flaring A number of types of tests were used in this study, but the two sides which may be parallel or convergent. Where the sides are con- most useful were the flow and vorticity tests, which yielded infor- vergent, the feature is called a wedge-shaped flute. The term flute mation on the interfacial flow patterns, and the wind-blast tests, groove is sometimes used herein to distinguish a long feature from which demonstrated that erosion occurred along the interface in the more pit-like type of flute. A flute pit may be rounded to oval the absence of sand grains. The flow tests were conducted in two with the lee side sometimes shallower than the windward side, due ways, using lung power in the one case and the compressor line as to the intake of negative flow. The dominant air motion in such a the air source in the other case. In the main, natural wind-eroded pit is across the pit from rim to rim. Also, lineation in a pit of this materials were used for testing because they already had channels type may converge along the negative flow lines from leeward to for the guiding of the interfacial flow. The air was delivered by windward. While flute grooves commonly occur in parallel or con- plastic tube, and the testing materials were of many sorts, shapes, vergent sets, many flute pits are more irregularly distributed. A flute and sizes. For low velocity and very delicate control, lung power pattern or rill pattern refers to the general configuration of linea- was used as the air source. For higher velocity or for work with tion on a rock. A flute set or rill set refers to a sequence of lineation, larger specimens, the compressor line was used. The lung power commonly in parallel or slightly convergent arrangement. The method was by far the more instructive, for it afforded a high de- terms rill and striation are nearly synonymous with flute. In a stri- gree of coordination between output of air, control of the test ation, the walls usually are not flared. For convenience in this pa- material, and visual perception of the behavior of test material. For per, the term "flute" is used to refer to grooves from 1 cm to several instance, by careful control of the output of air, test material on a metres in width; "rill," to refer to grooves about 1 to 10 mm in lee surface could be brought up to windward along this surface, width; and "striae," to refer to grooves less than 1 mm in width. In allowed to slip back leeward, reversed, and brought back wind- the ultra-microscopic range of electron micrography, lineations fall ward, just as one sometimes sees snow moving on and off snow into the range down to less than 0.1 wide. While these lineations drifts in the pulsing and waning of wind gusts. Also by careful fall into the size range of striae, occasionally some have flute form. handling, two directions of flow can be observed simultaneously. A A vortex pit is proportionately deeper and more often round than is sensitive probe can be introduced into a flow line to test the veloci-

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ty. Testing negative flow on a windward face is difficult, but if the wind source. We use very narrow wind tunnels (2.5 to 7.5 cm ventifact is cool, the condensate from warm, moist lung air shows diam) because the channeling effect is important in wind erosion. where the moist air of the impact begins to interfinger with the dry Most of the tests have been done using velocities from about 18 to air of the negative flow and finally ceases to make contact with the 28 kmph. When dust-blasting is done, the dust can be fed through sides of the ventifact. In some of these tests where moist air was the blower. directed onto the top surface of a ventifact, moisture instantly filled all the channels and concavities on the basal surface. This shows AERODYNAMIC EROSION OF VENTIFACTS the ease with which suspended particles can be circulated across bases to effect erosion in situ on these surfaces. Basal concavities Around Lake Michigan, there are two types of ventifact fields. are common on ventifacts and have aerodynamically meaningful One type lies within dune corridors near the Lake Michigan shore locations. where flattened, rounded former beach pebbles have been con- Secondary flow transverse to the main line of flow likewise is verted to ventifacts (Figs. 2A, 2B, 2C, and 2H). Perhaps the best of easily tested by lung power. This flow pattern is commonly gener- these areas is Big Sable Point in Mason County, Michigan. The ated in a channel by intake of air to vortices traveling in a line of other type of area consists of high morainal tops such as at Sleeping flow. Following impact, wind is deflected leeward, largely around Bear moraine, Leelenau County, Michigan, where (about 150 m bases and top margins of ventifacts. If the basal margin does not above the lake shore) a different and windier environment exists make ground contact, a submarginal flow with vorticity develops and where thick angular glacial pebbles (Fig. 21) and boulders (Fig. and pulls in secondary flow from the environment, from the pe- 2F) have been converted to ventifacts. Both areas contain many ripheral zone of the basal surface and down the flanks of the ven- ventifacts of the same kind of dolomite. However, while thousands tifact, often cross-lineating surfaces in all these zones. This activity of dolomite pebbles in the coastal dunes show exceptional rill pat- also pulls air down from above the ventifact, producing marginal terns, only a few of those of the high moraine are rilled. On the vortex pits on top surfaces of flat-topped ventifacts. The best test other hand, basalt and other hard rocks on the moraine are exten- material to use to test for falling air is a hand-held fiber such as a sively rilled and often exhibit coarse fluting, generally to the lee and hair, a thread, or a thin grass stem. During the period of in line with the high-velocity wind direction (Fig. 2F). Many mate- downsweep, the fiber will be held tightly against the top surface of rials of the coastal dunes are rilled. For example, man-made asphalt the stone. in the dunes has been shaped to ventifacts and often bears rills es- Sand grains can be used in testing vorticity, but because they are sentially identical to those on the different rocks. It was this discov- irregularly shaped, they may not give accurate information on the ery, in fact, that initiated the investigation of the origin of lineation details of the structure of a vortex. Perfect spheres such as those in on ventifacts. Previously the round-floored rills of the type found timed-release capsules maintain more regular courses than do sand on most of our Michigan ventifacts has been considered to have re- grains. Glass spheres would make good testing materials. sulted from solution. The asphalt ventifacts indicated that this as- Where ventifacts of sizes greater than 9 to 10 cm in length were sumption is highly unlikely. There are cases, particularly at Stur- used in the flow tests, the compressor line was used; in some cases, geon Bay, Emmet county, Michigan, where rills of dolomite ven- threads were taped to the specimen to demonstrate where flow lines tifacts have been modified by solution. Such ventifacts may exhibit created secondary flows, subbasal flows, and so on. the relatively simple rill sets of wind origin on one surface and the The wind-blast test is much simpler than the flow test, but to per- highly complex dendritic patterns of solution on the opposite sur- form this test with reliable results, preliminary photographs and face. Besides the active coastal ventifact fields, there are many areas micrographs should be taken of the specimen. A problem arises in where Pleistocene ventifacts can be found in Michigan, the best of this case as to where to concentrate the photographic effort, that is, which seems to be at the juncture of the Imlay Channel and the to anticipate where the best erosion will occur. The data gained by Maple River. However, the ventifact fields of the interior of Michi- flow testing are invaluable in this case. Once the specimen is photo- gan have been greatly disturbed by the activities of man, and few graphed, it should be glued to a glass slide which can be anchored remain in an active state of wind erosion. in some way in the wind tunnel. A recess on the tunnel floor is Vortex pits, abundant on both low coastal ventifacts and high sufficient to hold the slide in constant relationship to the wind morainal ventifacts, are particularly common on the broad tops source throughout the test. Either a compressor line or a blower and bases of the former beach pebbles. They are relatively uncom- that can run continuously for months or years can be used as the mon on lateral surfaces of the ventifacts except in some lee areas.

> Figure 3. Features caused by natural wind vorticity and flow and by experimental wind-blasting. Figures 3A, 3B, 3C, 31, 3J, and 3L are electron micrographs of test specimens. Figure 3K belongs to the same group of test minerals but is not an electron micrograph. All except Figure 3A were subjected to wind-blasting with no abrasive other than intake room dust. A. Pre-wind-blasted top surface of a cleavage block of periclase (1800X). B. The same surface after two years of wind-blasting at about 28 kmph (1800X). C. Pitted lee surface of the same block (146x). D. Vortex pit with raised center, sharp marginal groove, flared rim, and hundreds of small vortex pits, some bearing helical or radial scoring, Canyon de Chelly, Arizona (about 3 m diam). E. An embossed vortex pit in snow (about 1 m across). F. Dust-devil pan with helical scores and lineated sand deposit on the margin of a low cliff at the left. The rounded black spots at the bottom of the illustration outside the pan are juniper bushes, each one set in a ring pit, Canyon de Chelly, Arizona, about 10 m in diameter. G. Discs and knobs produced first in snow and then in underlying sand by wind vorticity, Big Sable Point, Michigan. Each knob is 2 to 3 cm in diameter. H. A hoodoo form in a ring pit near Ellsworth, Kansas. The concretion on top bears numerous hemispherical vortex pits and considerable burnish. These features, the taper of the soft pedestal, and the ring pit at its foot all denote wind influence. I. A lineation on the windward surface of a cleavage cube of halite formed as a coalesced pit chain. There are also a number of minute embossed features, some of which have ring pits around them (3500X). J. Vortex pit on the same halite face with bench-type rings and many associated lineations at the margin of a cleavage step (350x). K. Pitted and grooved lee surface of a sylvite specimen. Surface was roughly stepped before wind-blasting (5x). L. Basal surface of a fluorite specimen eroded for approximately five months. The cement by which this face was stuck to a glass slide is in the upper right corner. The cement raised the specimen about 2 mm off the slide. Hence, in the peripheral zone, interfacial currents brought dust and cleavage chips into the narrow space and eroded converging grooves around the cement (46x).

With a few exceptions, only very small pits are generally found on of principal impact). Beyond the reach of driving sands, tops and windward surfaces (a few boulder ventifacts on the moraine have basal surfaces (and sometimes lee surfaces, also) bear numerous large windward surface pits). Wedge-shaped flutes with pits at their large pits. heads (Fig. 21) are common on those parts of windward surfaces The rill form appeared to suggest fluid motion of the air along most subject to strong negative flow (usually peripheral to the locus the interface, and the pit forms appeared to suggest vorticity. With

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these as leads, experimentation began in the field but soon had to neath this impact zone, the largest and steepest undercut facet is be moved to the laboratory where lower velocities could be main- normally located (fig. 21). To the lee, the southeast, there is the long tained for better observation of test materials. The first major dis- plane, often bearing lineations in line with the high-velocity wind; covery was that negative flow (Fig. 1) governed the escape patterns that is, the crest shifts toward the high-velocity wind direction from shallow pits. This led to extensive studies of interfacial flow rather than away from the prevailing wind direction. This is ex- on oriented specimens. ceptionally well shown on the boulders at the north end of Sleeping During these studies, I noted that most of the specimens had con- Bear moraine (Fig. 2F). At the lower margin of the long plane, there cave bases. This observation led to more than 11 years of field is the lee undercut facet; adjacent to this facet on the basal surface, study of basal surfaces of ventifacts and numerous experiments the basal concavity begins. Thus the whole basic form of the ven- with in situ basal surface erosion. Figure 3L shows convergent tifact normally centers around the zone of the high-velocity wind lineations developed lee to the anchoring cement on a basal surface impact, even though some of the parts related to this wind direction during a wind-blast test. Not only do a large number of ventifacts are remote from the impact surface. Similar relationships may exist have concave bases, but the locations of these concavities have for the other impact zones, but when they do, they tend to be less specific relationships to impact patterns and to stage of develop- pronounced and frequently bear transecting relationships with the ment of the specimen. Concavities develop under influence of both primary pattern. Erosion is accelerated during high-velocity impact negative flow and vorticity, starting at or near the lee margin and but not necessarily so on the impact face, where stagnation, coun- migrating windward. Currently the cement beneath a specimen un- terforce, and cushioning effect by interfacial flow lines serve to de- dergoing wind-blasting is eroding thus under the lee margin. celerate and deflect particles and thus to retard erosion in the im- Hence, concavities on basal surfaces are not due primarily to solu- pact area. Low pressure in the impact zone causes negative flow to tion, chemical wedging, or to overturning, as have been suggested be initiated leeward and to move windward along the interface, by other investigators. During this study, tens of thousands of ven- thus bringing the sharp dust laths into close contact with the tifacts were examined in the field in Michigan and in a number of downwind surfaces. The impact faces remain convex, often pro- western states. Bases, in particular, were carefully studied. These truding (Fig. 2D); leeward in the zones of strong interfacial flow, surfaces are quite distinctive from all other surfaces. One asphalt faces tend to become flattened or concave. This is a pattern of ero- ventifact showed clearly the in situ origin of its basal concavity, for sion that is much more compatible with aerodynamics than that though it had not yet been severed from the large asphalt slab from which has been sought by geologists. which it had been wind-carved, its basal concavity had already de- A flat face may be the product of combined forces, particularly if veloped on the side lee to the high-velocity wind (the northwest one of these forces is the negative flow to the dominant wind and wind). another is an oblique impact from some other direction. Deflection Concurrent with the study of basal surfaces, several other obser- of the second wind up across the long plane will create a negative vations were made. One of these was that helical scoring is com- flow in the opposite direction. These flows have a planing effect. mon on wind-eroded surfaces. Another was that there is consistent Likewise they create lineations that transect the primary fluting on location of both basal concavities and marginal facets; this led to a the face. If these lineations include wedge-shaped flutes, the direc- general analysis of relationships among features of ventifacts and of tion of the secondary wind can be quite accurately determined, for all their features to wind directions. the heads of such flutes taper windward. Three or four sets of lin- In the dune corridors, where wind was channeled, consistent re- eation may occur on a single face, each denoting a particular wind lationships between a wind direction and the spatial distribution of pattern. Hence, it was a great mistake to relegate all lineation to the major parts and sculptural details of a ventifact were striking, solution, for in this study, lineation has proved to be one of the once a system was recognized. The problem that ventifacts present most precise indicators of the wind history of a rock. is that there will be as many of these systems of relationships as To test the obvious aerodynamic influence, grain-free wind-blast there are principal wind directions, and in the details the relation- experiments were conducted in wind tunnels, using velocities not ships are transecting (Fig. 2A). Along the Lake Michigan shores exceeding 28 kmph. One six-month test, however, was in the open where much of the field study was done, the high-velocity wind was and at 100 kmph. Atmospheric dust apparently served as the abra- from the northwestward and the prevailing wind was from the sives, for no abrasives were added except during one test conducted southwest. In some areas such as Sleeping Bear moraine, other by Dietrich (Dietrich, 1977). Specimens were anchored in place. wind directions are locally important because of topographic fea- Some wind-blast tests lasted two to three years. Various kinds of tures. In the coastal dunes, wind is deflected by shore contour and rocks and minerals were used as targets, and all became eroded. dune corridors. In these corridors where this study began, two wind When Dietrich joined the project, he set up two tests with differ- directions were dominant, varying with corridor orientation; at ent goals (Dietrich, 1977). The first goal was to see if the rate of Y-junctures of corridors, however, a third wind direction was erosion is directly related to bond strengths of the material being added and ventifact sculpture altered accordingly. Here, for in- wind-blasted; the second was to determine effects of different-sized stance, were located the driekanters, otherwise absent in the cor- and differently constituted projectiles. In the first test, he wind- ridors. The consistencies in long plane and crestal keel orientation blasted four blocks, all with mirror cleavages, for two years at to prevailing wind, which geologists have long sought, were never about 28 kmph; the only projectiles were the dust particles in the observed during this study. Only rarely was a long plane found to air. The four blocks were of similar bond type but of varying be facing the prevailing wind (southwest). Instead, most such hardnesses — two sylvites, hardness 2; one halite, hardness 2.5; planes were found to be lee to the regional or to a local high- one periclase, hardness 5.5 to 6. Because periclase is essentially in- velocity wind direction. The crestal keel would then be transverse soluble and has a relatively high hardness, its erosion under the to this wind direction. The regional high-velocity wind in this part existent conditions is highly significant. The erosion was much the of Michigan is the northwest wind. The highest and steepest part of same on all the blocks. Every face was eroded. Even peripheral most Michigan ventifacts is commonly the northwest corner. Be- areas of basal surfaces became eroded (Fig. 3L). The types of ero-

sion were lineation, vortex pits, flute pits, and wind cleavage (either role along top and basal margins by inducing secondary flow. If by small flake or by large sheets). It took about 50 days for the soft- there is a good channel between ground and basal margin, a venturi er minerals to become frosted and about 100 days for the periclase or line of low pressure is created which attracts secondary flow to become similarly frosted. Erosion increased leeward, some zones down the flanks of the ventifact. The secondary flow dominates the became more prominently lineated than others, and top and lee sur- erosion of the lateral surfaces of the ventifact, causing marginal rills faces developed a great many pits. In the second test, Dietrich used to develop in the early stages and a marginal bevel (Fig. 2C) in later two cleavage octahedra of fluorite: one was the control, subject to stages of ventifact development. One surface on the side of a speci- erosion only by the dust in the air, while the other had 25 hr of men in Dietrich's first group of test minerals developed striae with added calcite silt-sized particles followed by 25 hr of added barite rill form. Significantly, the basal margin of this face was beveled silt-sized particles. The duration of this test, however, was five somewhat like a basal margin on the rilled ventifacts. There is a months. The velocity was about 28 kmph. Although the period of considerable velocity increase in the channel formed between calcite and barite silt addition was short, it was very effective. Fig- ground and ventifact base. I have seen sand grains lifted and trans- ures 3A, 3B, 3C, 31, 3J, and 3K illustrate these tests. In these tests, ported in these flowlines when no other grains were being lifted Dietrich was able to demonstrate (1) that the rate of erosion is di- nearby. The secondary flow dominates the erosion of a ventifact rectly related to bond strength of the material being wind-blasted, with narrow lee end and with long axis parallel to the principal (2) the effectiveness of dust as a tool, and (3) that mass of the abra- wind direction. sive is more important than its hardness. Additionally, extensive Many factors enter into aerodynamic erosion, such as shape, electron micrography of these specimens shows a general size, orientation, hardness, surface texture, and characteristics of aerodynamic pattern in the erosion with distribution of pits and margins of the material under wind attack; kinds, sizes, hardness, lineation similar to that on ventifacts. mass, and sharpness of the projectiles; environmental factors such The general aerodynamic pattern just mentioned can best be re- as topography, neighboring stones, vegetation, supply of tools, ve- lated to the teardrop form where, after impact on the broad section locity of the wind in relationship to the size of the object, and so on. of the form, the air mass divides into leeward-moving streamlines, While all of these factors might appear to complicate the study of which again close lee to the form. Acceleration occurs at the curva- ventifacts, actually they clarify it. The progress of erosion of a ven- tures where some of the air escapes, thus lowering pressure and tifact is governed primarily by its shape and orientation to the wind creating a negative flow beneath the positive or downwind flow. directions. It will erode in any orientation; but if its high, steep side The more the shape of the object deviates from the aerodynamic is oriented to the highest velocity wind direction, there is then the form, the more irregular the flow pattern becomes, the more nega- maximum opportunity for the development of a strong system of tive and secondary flows develop, and the more erosion results. interfacial currents which increase the rate of erosion so long as an Also, the rougher a surface becomes, the more erosion occurs, optimal size-velocity relationship exists. While this means that either from flow patterns or from vorticity or both. Where sharp some very large boulders might not become shaped ventifacts in margins exist on the specimen, the positive flow leaves the interface their environments, still there is a marked tendency for the mag- abruptly and rapidly accelerates, causing strong negative flow nitude of forces to increase around large objects, for there is greater downwind from the margin, which results in increased rate of ero- updraft and larger vortices than around smaller objects. Thus sion. Negative flow forces the positive flow away from the inter- boulder ventifacts are more likely to have coarser fluting and larger face. Also, negative flow pulls away from the interface in joining vortex pits than do small ventifacts. the positive flow, and vorticity may take its place along the interface in the juncture of the flowlines (Fig. 1), or a much-reduced flow LIFT system may continue to the sharp margin, producing smaller lineations. In either case, events on the lee side of the margin are likely to Particles may undergo lift at very low velocities. This lift occurs influence events immediately adjacent to the margin on the wind- in all conditions conducive to the development of vorticity such as ward side, causing velocity increase and erosion. Hence, lineation is at the juncture of positive and negative flow lines, within pits, likely to develop on both sides of a sharp margin. If the lee side of around sand grains, at any sort of irregularity and to some extent the specimen is broad and relatively steep, closure of the stream- along most flow lines, especially where they carry vortices. How- lines may be far downwind from the specimen. Vorticity and nega- ever, in the pit (Figs. 2E and 2H), lift occurred more readily by tive flow develop lee to the specimen. The negative flow divides at negative flow across the pit than by vorticity. Tests in this pit were the lee base; some of it goes up the lee face, and some of it goes monitored by a sensitive velocity meter. Positive flow was excluded under the specimen to join the positive flow on the lower part of the from the pit by directing impact onto a lateral surface of the ven- windward face. This part of the negative flow can be traced by ex- tifact so that only negative flow passed through the pit. Wind- periments of various sorts, such as the moist-air tests. Negative fashioned pits with current-guiding striae and natural impact sur- flow can also occur locally at any surface irregularity along any line faces are best for these tests. An example: the white test particle in of flow. Hence, negative flow is not necessarily opposite to positive the pit (Fig. 2E) was a quartz grain 3 mm in diameter. Negative flow. Negative flow also constitutes one aspect of a vortex configu- flow created decreased pressure on the windward side of the grain ration. For ventifacts with broad lee faces, it dominates the erosive so that the grain started sliding to the windward. When pressure process. distribution changed so that relatively higher pressure existed be- On broad tops of ventifacts (Fig. 2A), negative and positive flow neath the grain, lift occurred. The grain slid about 4 mm before lift- may operate together in sculpturing the wide, flattened rills. Nega- ing slightly windward of the center of the pit to join the positive tive flow initiates them in the marginal zone where positive flow flow passing over the pit. This appears to be the normal lift pattern. leaves the interface, but positive flow returns to the surface and It is more difficult for lift to occur on positive flow because of a plays an active role in shaping these lineations as it moves constant tendency for pressure equalization above and below loose downwind along their beds. Positive flow also plays an important grains. Far higher velocities appear to be required to overcome this

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tendency on positive than on negative flow. On positive flow, lift of conducted with the use of natural wind-formed vortex pits. The a grain about 1/16 mm in diameter requires a velocity of about 12.8 advantage that vortex pits have over man-made depressions is that to 19.3 kmph (Kelley, 1962). The grain in the pit (Fig. 2E) has a the established lineation within the pits affords a strong influence diameter about 48 times that, but it lifted at the much lower veloc- on the interfacial flow. ity of 0.02 kmph. Even the velocity of the positive wind, measured In addition to the study of pit vorticity, I made field observations at the windward rim of the pit, was only 0.18 kmph. In vortex pits, of dust devils and of areas where small vortices were affecting rela- particles are lifted from rest through the central column of the vor- tively flat sand and snow surfaces. Most of the detailed studies in- tex after a comparable sliding phase. In nature, grains are lifted ex- volved pits and channels where confining walls somewhat modify tensively by these means. Velocity increases around irregularities to the pattern of activity, causing a downward influx to the center of create the necessary pressure differentials to lift minute particles. vorticity and perhaps imposing a greater degree of organization on Bagnold (1942, p. 90) had difficulty explaining the failure of mi- the vortex than is true of the dust devil. Also, the presence of pit nute uniform particles to lift. His problem appears to have been the walls restricts the central exhaust column, and so it differs from uniformity in the size of the particles. The amount of energy gen- that in the normal dust devil pattern. Hence, vorticity in a pit erated by a vortex bears relationship to its size. If all the particles should not be equated strictly with dust-devil activity, even though are equally small, then only minute vortices of uniform size will be the same phases may exist in both types of vorticity. The phases are developed under influence of a uniform flow. But when coarse par- more easily identified in a pit than in a dust devil; they consist of ticles are admixed with fine particles larger vortices form around intake, exhaust, negative flow, and satellite vorticity. The primary them, supplying the energy to lift the finer particles around their intake is helical. Low pressure at the center of vorticity and in the bases. satellite vortices around the pit rim causes secondary intake which During the lift tests, an important symmetry relationship was is radial. Both intake patterns influence erosion within and exterior discovered and later was observed in action on a small dune. The to pits. Negative flow and satellite vorticity form as in Figure 1. The ventifact used for testing (Fig. 2H) was nearly symmetrical on the familiar broad, centrifugally moving whirlwind of a dust devil is its end used for impact. When impact was directed along the axis of exhaust phase which follows and largely masks the true erosive this "symmetry," lift was effected at a lower velocity than when the phases of the form occurring on or very close to the ground. The current was directed lateral to the axis. The greater the lateral shift, exhaust of a dust devil is a tight spiral where, in order to gain the more the velocity required. When the impact was shifted to the altitude, the load makes many cycles around its central axis. In the zone 30° to 45° to the axis, negative flow gave way to vorticity, and confinement of a pit, the centripetally moving intake dominates the the grain cycled in the pit. vortex by nearly filling the pit. Thus the exhaust is restricted to a An observation along a beach only about 3 m wide at the base of narrow, loosely spiraling central column which escapes rapidly a bluff about 130 m high along Lake Michigan confirmed the exis- from the pit. In general, this exhaust column does not fling its load tence of such lift in nature. When a squall was approaching shore, I out against the pit walls unless the pit is funnel-shaped. Instead, it prepared to observe the first impact along the juncture of wet and pulls every loose particle away from the walls. During tests of vor- dry beach. When the clean wind from off the lake crossed that ticity, loose particles were stationed on walls and floor prior to juncture, the whole dry surface lifted, forming a dense sand cloud onset of vorticity. In general, particles stationed on walls were im- more than one-half metre high. mediately pulled laterally into the exhaust column. Pit shape makes some difference in this matter. In funnel-shaped pits, there may VORTICITY sometimes be a tendency toward outward fling because the exhaust tends to expand more than in a wide-floored pit. The diameter of Vorticity is one of the most common and most important wind the exhaust column decreases with increasing velocity, for acceler- motions responsible for erosion. It is involved with both abrasion ation in velocity increases the rate of intake and consequently alters and lift. It may involve electrostatic-electromagnetic activity as well a number of relationships within the pit. The narrower the column as air motion (Freier, 1960, p. 3504). Vorticity has been greatly becomes, the more nearly straight the course of an escaping particle neglected and poorly understood in most wind-erosion studies. is. For example, when the column occupies about one-fourth of the Possibly one reason for this is that much vorticity cannot be readily diameter of a 6-cm-deep pit, an escaping particle might make only seen. In many cases, no visible particles are in transit. In addition, one complete cycle in rising 18 cm. Also, while the movements in a many vortex configurations are miscrosopic or even ultra- dust devil are mostly horizontal and upward, the intake to a pit on microscopic and can be revealed only as erosion patterns (Fig. 3J). top of a ventifact is downward. This is a prime factor in altering the Another reason is that some minute pits formed by vorticity have form and behavior of a vortex operating in a pit. been misidentified as impact pits, the so-called percussion cones. If Two forms of vorticity were identified as to behavior and as to the same types of features can be produced by wind-blast tech- the character of pit and wall sculpturing which develop. These are niques where no sand grains are involved (Figs. 2L, 2M, 3J, and the normal-axis vortex and the oblique-axis vortex. The angle of 3K), then they are not grain-impact pits, and some other origin has wind impact upon the ventifact, the shape and orientation of the to be sought. Many vortex pits have distinctive characteristics in surface on which the vortex operates, and possibly other factors shape and fine sculptural detail which delineate the type of intake, influence the axial angle, which is simply the angle that the axis of the axial angle of the vortex, the events along the interface, and the vorticity makes with the surface on which the vortex operates. This location and character of the exhaust column. To interpret these axial angle influences the pattern of erosion. Thus we have sym- erosional patterns, one has to know how the different aspects or metrical and asymmetrical pits. The first of these may be hemi- phases of vorticity behave within a pit. Hence, along with the flow spherical, saucer-shaped, or funnel shaped. The asymmetrical pits tests, extensive experimentation was conducted with vorticity in are oblique bowl and funnel forms, helicoidal shapes, and crescen- pits. Since there are thousands of vortex pits of appreciable size on tic forms. I have no empirical or experimental data to support the the broad-topped Michigan ventifacts, much of this study could be existence of the so-called roller vorticity (Bagnold, 1953, p. 91).

The pit forms certainly would not support it. Possibly it represents to bring its tools closer to surfaces, it probably erodes more the exhaust of a vortex deflected by wind. All visible vortices I have efficiently than the normal-axis vortex does. Obliquity of the axis observed in lines of flow have been essentially normal-axis vortices. of vorticity is due either to the angle and location of impact or to A vortex may invert and exhaust downward when operating be- the shape of a surface on which the vortex is operating. The impact neath a falling air mass or beneath a solid object. The perforated surface may be far removed from a locus of vorticity, but its shape ventifact (Fig. 2B) was very useful in experiments with inverted and attitude govern flow patterns which affect vorticity. On some vortices. The comparison of pit vorticity to dust-devil activity is ventifacts, top and basal surfaces may have circular ring pits or em- based on the normal-axis vortex in which the axis of vorticity is bossed pits, but lateral surfaces have crescentic embossed pits. This essentially normal to the surface on which the vortex operates. difference is due to variation in the angle of impingement of the Such a vortex tends to produce a round pit, often with unbroken vortex upon the different surfaces. Thus, some undulating surfaces rim which may be raised above the surrounding area. The wall have variable shaped pits. It is possible during wind shifts to have sculpture of such a pit is more likely to be radial than helical, but first one and then another axial orientation within the same pit. may consist of both patterns of lineation. The oblique-axis vortex Where neither wind dominates, such pits may become misshapen. produces variously shaped pits, commonly with a broken rim adja- However, if one wind-shift does dominate, then a well-formed pit cent to a low-angled pit wall where intake is concentrated. The may develop, but its rim may have notches, and lineation at the rim exhaust is eccentric along a steep wall which may be opposite or or crossing the pit may indicate influence of other wind shifts. sometimes adjacent to the intake channel. Wall sculpture is more Negative flow crossing the pit as well as oblique-axis vorticity may helical than radial, due to the pattern of intake. Sometimes the heli- cause the pit axes of top surface pits to be inclined away from the cal lineation consists of a single groove, but multiple grooves in vertical, in which case, the rim outline may become oval. Vortex either parallel or anastomosing arrangements can occur. Where pits developed on the basal surfaces of ventifacts are more likely to several wind directions affect pit development, wall scoring disap- be vertically oriented. Where a perforation forms from the meeting pears. of top and basal surface pits, there commonly is an obtuse angle. The normal-axis vortex has a centrally located exhaust column Top surface pits which are the result of falling air usually have ver- where the lowest pressure exists. The location of the low pressure tical axes. Top surface pits resulting from impact on lateral surfaces affects the environment around the pit more or less uniformly so commonly have inclined axes. By tests and by lineation patterns, it that intake is in quite equally spaced bands which sometimes erode is sometimes possible to determine whether such pits were pro- radial grooves in the surface outside the pit rim. At the rim, conflict duced by vorticity or by negative flow or a combination of both. between the influx and the negative flow creates a ring of satellite One ventifact which was studied had a large crestal pit and five un- vortices (Fig. 1). The influx lifts over these, then sweeps downward dercut facets. Impact on two of these facets produced vorticity in into the pit. The primary intake is helical. It forms a lead line, the pit, but impact on the three undercut facets which were located which becomes the exhaust column when it reverses. The lead line at the north end of the specimen always produced negative flow may be only slightly curved in the upper wall area, but as it ap- across the pit, and three sets of lineation confirmed these flow pat- proaches the pit center, it bends sharply, and after about three- terns. Those lineations starting at the northwest undercut facet quarters of a turn begins to rise steeply. Part of the secondary or curved clockwise in passing through the pit, those starting at the radial intake appears to join the lead line, and each band bends northeast undercut facet curved counterclockwise, and those start- sharply at the base of the exhaust column. Part of the radial intake ing at the north undercut facet had no lateral curvature. In passing maintains contact with the wall and possibly constitutes the main through the pit, test materials, moving on either the positive or the eroding agent of a pit. Near the pit floor, the influx reverses direc- negative flow, followed the lineation pattern related to the specific tion and becomes the negative flow which moves back up the pit impact surface being used for the test. This pit also had an annu- walls under or between influx bands, possibly in some cases along lated wall grooving. Where oblique-axis vorticity develops from crests of ridges. To reach these ridge crests, it fractionates into only one direction, a spiral wall groove, or sometimes a bench, de- minor bands which move up the flanks of the ridges. Some of the velops (Fig. 3J). negative flow is swept into the intake and goes out the exhaust. If the axis is oblique, the intake is most likely to be concentrated Some of it goes onto the pit rim where, in conflict with the intake, it to a narrow sector of the pit rim and to take a helical course down forms satellite vortices which erode a ring of radial or helical scores the pit wall. This helical pattern may extend beyond the pit rim for around the inside of the pit rim. some considerable distance. The oblique-axis vortex has been ob- In a normal-axis pit test, material that is riding the top of the served to cut helical scores in snow and sand. Such a score starts at influx can come in on the lead line and go out through the exhaust the perimeter and develops inward. After a few cycles, the rim may without making contact with any surface. Therefore, the helical in- become circular. If the obliquity of the axis is slight, the pit rim may take often does not erode a score. Erosion, then, seems to be by the remain intact. However, in consolidated materials, the rim is usu- radial interfacial phases. Very small, shallow scores may exhibit ally breached where the lead line enters the pit. Where the axis of both patterns, and it was because of this that the origin of the radial vorticity is oblique, the exhaust is commonly eccentric, and the wall patterns was discovered. Scores of these sorts occur by the hun- along which it escapes is usually steep and burnished, or if the dreds on a single ventifact, but often are not isolated. In general, whole pit interior is burnished, this exhaust route may show a they form interlocking patterns because of complex factors such as smoother surface and a higher degree of burnish than elsewhere. Pit wind shifts, the alteration of surfaces through erosion, neighboring rims and bosses may also exhibit burnish, but bosses are less com- objects, and interference currents. In view of the fact that the entire mon in oblique-axis pits and flute pits than in vertical-axis pits. An surface of a ventifact may consist of these interlocking patterns, electron micrograph of burnish shows truncation of mineral grains vorticity must be the dominant mode of erosion. and very few sharp margins, whereas an electron micrograph of an The oblique-axis vortex creates some well-defined differences interior of a normal-axis vortex pit shows a high degree of angu- from those developed under normal-axis vorticity. Because it is able larity of mineral grains, very little truncation, and often extremely

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sharp margins. Hence, the character of a pit surface indicates a margins, and on the rims, bosses, and sometimes walls of vortex great deal about the history of the pit. pits. These areas are zones of low pressure during wind impact. There are a number of ways of experimenting with vorticity. To Burnish developed in some of the comparable areas on the wind- see what is happening, the velocity must be kept low. The confining blasted periclase specimen. These loci of low pressure are areas walls of the pit will sustain the vortex form at extremely low ve- where air moves outward, carrying the coarser tools away from locities. A wide variety of test materials were tried, but bits of pa- surfaces. Most of these surfaces are areas of small vortex centers per, sand grains, and minute spheres were among the most useful. where only minute particles might remain in the interfacial flows to The spheres have an advantage over the natural sand grains be- act as tools or generate the friction necessary to produce static elec- cause they do not catch on irregularities. If a particle is at rest on tricity. According to Rabinowicz (1968, no. 6, p. 96-97), burnish the pit floor at the onset of vorticity, it will slide a few millimetres results from the removal of exceedingly small particles (near and lift into the perimeter of the exhaust column and escape on a molecular size). In the zones of higher pressure where flow lines nearly straight upward course. If the particle is at rest on the pit bring their loads close to the interfaces, burnish is often lacking on wall at onset of vorticity, it is pulled directly into the central col- the Michigan ventifacts. Possibly the finer textured surfaces result umn, omitting the slide phase. If it is at rest outside the pit, it is predominantly from electrostatic effects in the zones of low pres- drawn over the pit rim but usually rises considerably above the rim sure and higher velocity and coarser ones from abrasion in the in crossing it. This is interpreted to be due to the presence of the zones of higher pressure and lower velocity, or perhaps the differ- satellite vortices just inside the rim which leave their scores and ence in surfacing is simply due to particle size. burnish as erosional evidences of their existence. Once past the rim, Many vortex features originate as embossed pits. On the wind- a test particle sweeps downward on a slightly curved course toward blast specimens, there are numerous examples no more than a mi- the pit center and floor, never quite reaching either one. At the bot- cron across. In multiple wind systems, the boss is often destroyed; tom of its ingress, it bends sharply around the perimeter of the in weak bedrock, however, the boss may outlast the ring pit, or at exhaust column and starts to rise on the long, loose spiral course. least dominate over it to become a hoodoo form (Fig. 3H). The fea- Negative flow forms along the pit wall and moves up to the satellite ture at the top in this illustration is a concretion on which wind has vortices where it is shifted into the intake and recycled. Suspended eroded hundreds of burnished vortex pits about 1 cm in diameter. dust may enter the negative flow. Negative flow erosion patterns The tapered stem and the ring pit at its base are further evidences of which can be identified in electron micrographs show clear rela- wind influence in the formation of this feature. Such ring pits are tionships to the erosion patterns of the satellite rim vortices. found around bases of many hoodoo forms. The development of A vortex pit is obviously a locus of accelerated erosion, yet the embossed pits (Fig. 3G) was observed during several winters when unique pattern of load shift makes the erosion of the pit difficult to the beach at Big Sable Point, Michigan, had a sheet of snow about 1 explain by abrasion alone. The vortex is a system of swift pressure cm thick covering its outer perimeter. There thousands of snow changes. In nearly all cases in this study, it has been noted that ero- discs, each about 6 cm in diameter, were formed when the north- sion is retarded in the zones of low pressure because tools are car- west wind impinged obliquely upon the foredune adjacent to the ried away from surfaces at these points, and no better example of beach. Air was deflected obliquely downward against the snow to this exists than the boss at the center of vorticity; yet just peripheral form inverted oblique-axis vortices with their highly efficient heli- to the boss, erosion is accelerated. It is difficult to tell whether this cal intake phases against the snow. Helical patterns which quickly means that the pressure is significantly higher in this zone than over become rings around central discs of snow were cut almost instan- the boss or whether it is simply that in the peripheral zone the in- taneously. Unlike a normal-axis vortex, these oblique vortices take is crowding to the pit floor and bringing particles into contact mainly cut helical grooves. The snow was breached within seconds. with it despite the lowered pressure. Another possibility is electro- Although sand grains were not involved in the primary cut, as soon static phenomena. Freier (1960) and others have shown large vor- as the sand was exposed it also was eroded, even though it was tex configurations to be electrical phenomena. It is quite possible damp. Within 30 minutes the ring was deepened to about 4 cm and that a vortex of any size generates static electricity. It is a well- the snow disc had gradually disappeared. After several days the known fart that static electricity is generated as air slips off sharp columns were narrow and shortened, and the rings were coalesced margins. Flow lines merge at these margins, and vortices form in into a generally lowered beach surface studded with thousands of the junctures (Fig. 1). In a vortex pit where mineral grains are small elevations and sporadic sand discs like those to the left of the sharp, as are the dust laths coming into the pit, static electricity may compass (Fig. 3G). As the vortex is a complex pressure system in possibly develop and play a role in the erosive process by elec- which the lowest pressure is in the central exhaust, and because this tromagnetic processes, or the presence of charged particles may at- is where the boss develops, this remnant of erosion appears to be tract other particles, causing stronger and more abundant impacts. left because of the outflow of tools along the exhaust column. Dietrich has shown mathematically that at the velocities we use Where hoodoo forms share an origin common to a boss, then for wind-blasting (around 28 kmph), there is sufficient kinetic perhaps differential pressure as well as differential hardness ac- energy available for erosion of a rock surface on collision of a dust counts for their preservation. particle 3 x 3 x 1 fx.3; density = 2.5. Therefore, he concluded that The boss may be a low dome, as in Figure 3D, or a high spire. It dust is a possible tool and that point or edge impacts of sharp laths is a common feature in snow (Fig. 3E) and also occurs in huge ring would be very likely. Under acceleration in vortices, such impact pits in ice in Antarctica (John Splettstoesser, 1970, personal com- might impose electrostatic and (or) electromagnetic effects. mun.). Individual plants, groups of plants, rocks, or other objects Electrostatic erosion may account for burnish. Among other as- can be centers around which cyclic motion occurs and ring erosion sets in the study of ventifacts which had been converted from beach develops. For example, dune forms of the spiral cone type with pebbles, there was very little burnish, and what was present had clumps of trees at their apices, as on the Beaver Islands in Lake distribution in specific loci. Hence, its limited occurrence was Michigan, can be considered a type of embossed feature. Some of meaningful. It occurred chiefly in wind impact areas where linea- the craters of Mars and other bodies in space bear a close re- tion was absent or nearly so, along ridge crests and other sharp semblance to embossed features. So much Martian landscape bears

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/1/1/3444012/i0016-7606-89-1-1.pdf by guest on 27 September 2021 Figure 4. Lineation caused by vorticity along lines of flow. A. Impact surface of wind-eroded glass, showing both lineation and helically modified impact pits, Big Sable Point, Michigan (25 x). B. The lee side of the same specimen, showing striae, some of which have regularly spaced vortex pits along them but no impact pits (50x). C. Intersecting sets of lineation on halite produced by wind-blast test (50x). D. Rills on dolomite ventifact, showing transverse lineation, Big Sable Point, Michigan (7x). E. Transversely lineated rills in limestone bedrock, Boquillas Canyon, Texas (8x). F. Truncation at the top of a complex lineation pattern, Silver Lake Dunes, Michigan (0.04x). G. The snow flute in which the development of transverse lineation was studied, Shepherd, Michigan (0.03X). H. Transversely lineated snow flute, Shepherd, Michigan (0.03X). I. Complexly fluted sand-draped bank where downward movement along flute grooves appeared to dominate over lateral movement, Henry Mountains, Utah (0.005 x). J. Enlarged detail of Figure 51, showing symmetrical cross-lineated flutes. K. Ripple marks, bearing minute striations. Three patterns of air movement combined to form and maintain the asymmetry. Y-junctures indicate points where air lifts away from the surface at centers of vorticity, Silver Lake Dunes, Michigan (0.25 x).

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striking resemblance to wind erosion features that I feel the probable cause of Martian erosion is wind.

ML THE ROLE OF VORTICITY IN FORMING iisssaseasiirf LINEATION AND RETICULAR SYSTEMS s JlHi- The best evidence for vorticity operating along a flow line was observed during snow studies. Actual vortices were outlined by snow, and the resulting erosion could be seen to occur in a regular pattern related to the passage of a vortex through a snow flute (Fig. 4 G). The flute was about 20 m long, 2 to 3 m wide and 1 to 1.5 m deep. It was closed and relatively narrow at its north end and open, wider, and deeper on its south end (Fig. 4G). The flute lasted four and one-half months and yielded a wealth of data. When first observed, it was blanketed with new-fallen snow. While I was observing it, a breeze crossed the north end and created a negative flow on the sloping closure. This caused the lift of considerable snow, which quickly joined the positive flow and slipped along the narrow section of the flute for about 5 m. Then where the flute wid- ened, the snow cloud became organized into a distinct vortex B which filled the channel. As the vortex moved down the channel, it cross-lineated the channel. More than 500 essentially straight, parallel lineations normal to the axis of the large flute were ripped out and downward along the walls of the large flute. Each lineation, or secondary flute, was formed in about 0.5 sec as the vortex moved downwind. The movement was centripetal, or toward the base of the vortex and main flute floor. The vortex was of the normal-axis type with a wide exhaust like that of a vortex operating in a pit at low velocity. The moderate velocity of the vortex and the proximity of the exhaust to the flute wall (thus forcing the intake against the wall) may have been important in establishing the lineation pattern on the wall. Numerous snowfalls obliterated old Figure 5. Diagrams of lineation patterns produced within a snow flute by vorticity, negative flow, and secondary flow, the latter resulting from the secondary flutes, making it possible to see the inception of new air intake to vortex configurations. "W" refers to wind direction, "n" to ones. The flutes always formed when vortices were present, but negative flow, and "s" to secondary flow. A. One large vortex in the line of never formed when essentially unidirectional wind was coursing positive flow, moving from left to right, creates transverse lineation where through the major flute. two bands of the intake to the vortex impinge upon the channel walls in their downward sweep to the base of the vortex. Satellite vortices run along While the intake pattern corresponded to that of a vortex operat- these bands and, in turn, cross-lineate them through their own intake pat- ing within a pit, the shape of a channel affords only slight contact terns, thus creating a reticulum. B. A wind shift causes the flow to cross the of vortex to wall at any one instant. It appeared as if only two channel wall and create a line of negative flow, moving from right to left, in bands of the intake to the vortex were causing simultaneous wall which a small vortex leaves lineations on the channel floor in the direction of its travel, one set of secondary and tertiary lineations on the channel wall erosion. The bands were oppositely disposed and passed through and a helical score where it escaped into the positive flow. C. Right-angled the midsection of the vortex, as shown at the right end in Figure impingement of the wind upon the flank of the snow flute creates both large 5A. Later, in observing a sand flute, I saw a similar event in which a and small lineations on the negative flow, the latter tending to develop number of grooves were ripped out oblique to the axes of smaller mainly in the upper rim and sometimes under influence of currents breaking flutes (Fig. 6A). In the snow flute, the resulting pattern was a long across the crest and coursing along its inner surface. D. Where currents have turned along the margin of the crest, undercutting results, causing some set of parallel secondary flutes, each about 75 cm long and 2 cm asymmetry in the normally symmetrical form of the flute. wide, along either side of the major flute. The first set was initiated

— — ; : :—• Figure 6. Lineation and secondary flow. A. Transversely Iineated, nearly symmetrical sand flutes about 25 cm wide at the steep lower lee margin of the dune shown in Figure 6B. The flutes shown in Figures 6A and 6D are on the same dune but to the right of the area shown in Figure 6B. Airborne sand moved along the directions of both the coarse and the fine arrows. The medium-sized arrow indicates the postulated negative flow (about 0.05 x). B. Ripple-marked and fluted dune near Molly's Castle, Utah. The symmetrical sand flutes in the lee zone with the arrow marked "n" show only slight tendency for transverse lineation and are about one-third as wide as those shown in Figure 6A. Their origin may have been largely the result of negative flow at much lower velocity than that of the positive wind. C. Transversely Iineated channels, complexly Iineated ridges and a few dunes in Seney National Wildlife Refuge, Michigan. Present prevailing wind from the northwest corresponds with the main lineation trend. In the barren period following the glacial retreat, the area was wind-eroded. Peat formation in the channels has helped to preserve the lineation pattern. Man-made dikes have recently caused the area to be partially reflooded. D. Transition from ripple marks to flutes on the dune shown in Figure 6B. E. A dust storm in western Nebraska. During gusts, the secondary flow of dust moved downward along the bounding hedges, as indicated by vertical arrows on the left, then out across the ground beneath and transverse to the main wind direction which was parallel to the furrows. At the time of photographing, the secondary flow was moving as indicated by the lines of arrows. Where the centripetally moving flow lines met, dust devils formed. F. Small longitudinal dunes at Padre Island, Texas. The Y-junctures of the transverse fluting indicate both upward and downward secondary flow along the flanks of the dune. Except for the rounded crests, these dunes resemble wedge-shaped snow drifts in which lateral fluting is caused by air movements up or down their flanks.

about 30 cm below the tops of the flute walls, leaving a line of trun- flutes was very striking. Figure 4G was taken when the flute was cation along their tops (Fig. 5A). Similar truncations occur on four months old and after several wind shifts had altered the nor- dunes (Fig. 4F). Later sets were initiated at or near the tops of the mally regular patterns. The floor of the flute was never cross- walls. The conditions causing the variations were not apparent. lineated, possibly because of its depth relative to the generally mod- The regularity and high degree of parallelism of the secondary erate winds that passed through it. Figure 4H illustrates a shallower

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flute in which the vorticity appears to have passed closer to the flute was a helical score, about 15 cm across, just beneath the point floor. where the vortex escaped into the positive flow. Numerous vortices It is significant that the vortices traveling through the flute were traveled this same course until the oblique lineations constituted a not paired in a Von Karman street, but instead passed singly, at ir- groove with a Y-juncture with the crest of the wall and pointing regular intervals. Several seconds following the departure of the windward. Such junctures are common both in rill patterns on rock first vortex, a second vortex formed in the same manner and posi- (Whitney and Dietrich, 1973) and in ripple marks (Fig. 4K). They tion as the first. This vortex pulled its intake down the flute walls, represent the loci of lift of vortices: when pointing upwind, vortices following the grooves formed by the first vortex. Indeed, the intake are on the negative flow; when pointing downwind, vortices are on to many vortices passed through the same secondary flutes before the positive flow. they were destroyed during the next snowfall. Only when the wind When wind impinged at nearly 90° to the flute (Fig. 5C), both direction or velocity altered appreciably was there any noteworthy long and short lineations were developed as a result of negative lateral shift or change in orientation. After each new snowfall, new flow. The short ones were closely spaced and small. Frequently secondary flutes were developed. Through several months, hun- these formed only in a narrow upper band about 10 cm high which dreds of vortices were seen to create new flutes or modify old ones. was commonly very steep or even undercut (Fig. 5D). Both the un- When the snow was wet, linear clumps of snow were pulled out, dercut groove and sometimes the small flutes formed when the wall leaving transverse grooves across the secondary flutes. Thus a re- crest was breached so that the wind crossed through and turned ticulum was formed from a single wind direction. The tertiary flutes along the upper surface of the wall. In this case, the small flutes were about 0.5 cm wide. The symmetry of the main flute was gov- were secondary flutes. erned by the shape of the large vortex and that of the lesser flutes by Frequently, very small vortices ran along the tops of the walls of small vortices traveling along intake routes into the larger vortex. the big flute and rounded and minutely cross-lineated the walls be- These observations indicate clearly that a flute may be formed cause part of the intake was from below the vortices. When a vor- parallel to the direction of flow, whether that flow is positive, tex paused momentarily, a radially lineated or sometimes helically negative, secondary, or tertiary. In this respect, as well as in sym- lined saddle was formed on the rim. Such features appear to corre- metry, the flute differs from a ripple mark. Whereas the flute is a spond to similar depressions along the ridge crests of rills on ven- product of erosion, a ripple mark is both depositional and erosional tifacts. in its origin. Once formed, the flute guides and deflects flow pat- Wind flow across secondary flutes may or may not convert flutes terns so that on rocks, for instance, semi-permanent interfacial flow to ripple marks. It depends upon the strength of vorticity along the systems become the primary means of erosion of entire surfaces. main channel. Although no such conversion ever took place within It should also be noted that secondary lineation is not always the big snow flute, it did occur on its outer flank. Also, when special transverse to main channels. In the study of snow-filled ditches dur- conditions were created where the flute was expected to form, there ing wind velocities around 90 kmph, lineation patterns were being was consequent ripple formation. An obstruction which was placed formed which ranged from narrow transverse flutes on the upper where the crest of the snow flute was expected to form during a walls to coarse grooves on the floors parallel to the ditch axes. Of subsequent winter created a large vortex pit about 1 m deep. Lee- special note was the influence of undulating surface configuration ward of the pit, the crestline dropped abruptly about 0.5 m and a in deflecting the orientations of small flutes on the upper walls. new flute formed lateral to the first one. Intake to the vortex created Such observations helped explain some of the questions which had irregularly radiating U-shaped grooves 1 cm wide by 15 to 45 cm arisen during extensive microscopic examination of minor lineation long on the higher segment of the flute wall. In a wind shift which within rills on ventifacts, as in Figure 4D. Furthermore, not all brought snow along the flute bed, the big vortex would pull the lineation within a flute is governed by the events that originate snow-laden air mass up the flute wall and out of the flute. As the within the flute. However, the quite regular cross-lineation, such as wind sheet crossed the small U-shaped grooves, however, some of shown in Figures 4D and 4E, probably does originate from the pas- its load was dropped on the lower side of each groove to create and sage of vortices along the flute bed, as in the snow flute. Hence, the then cross-lineate several asymmetric ripple forms. Although this common pattern of regular transverse flutes within larger linea- does not suggest that all ripple marks are formed in this way, it tions, whether on snow, sand, ventifacts, or bedrock, may be in- does suggest that ripple marks may be products of three-way pres- dicative of some combination of flow-vorticity-substrate relation- sure systems, possibly with remote controls. In this case, air flow ships. and vorticity along the pre-existing channels brought the snow After a new snowfall, a lateral wind-shift of about 30° caused the down, and the rival positive and negative flows shaped and lineated channeling of the wind through a clump of trees and the oblique it. There was in this system a remote control factor — that is, low crossing of the wall crest, both having influence on the subsequent pressure, created as the air from the channels escaped into the vor- events. As a result of the channeling, one long negative flow line tex pit. Most sand-ripple systems end in ridge crests or channels formed on the flute floor far downwind from the trees and was ap- which are conducive to velocity and pressure changes necessary to parent because of the formation of a visible vortex about 15 cm in establish and operate flow lines transverse to the wind direction. diameter. This vortex traveled upwind along the center of the floor The lowered pressure zone along a gusting band of wind may also for some distance, then shifted toward the wall (Fig. 5B). In the cen- create sufficient transverse movement to establish the necessary ter of the trough, the vortex made two or three erosion tracks more cross features over which the rival positive and negative flows may or less parallel to its line of travel, but as it reached the base of the build ripples. Although ripple patterns may form along the contour wall, it added the same sort of long, closely spaced, narrow second- in a dune corridor, commonly they extend from crest to floor. ary flute pattern of erosion as seen on the positive flow, but now on Some of the best examples of sand flutes that I have seen were on one wall only, much narrower and this time initiated at the crest of a dune which was transected by State Highway 24 near Goblin Val- the wall. As the vortex climbed obliquely up along the wall, the ley, Utah (Figs. 6A, 6B, and 6D). The dune had lee surface ripple lengths of these flutes regularly shortened. The final erosion form patterns along the contour (Figs. 6B and 6D). At the lower lee side,

the dune steepened and the pattern altered to coarse cross-lineated tainly many lineation-score-pit combinations exist (Fig. 31). I have flutes about 25 to 30 cm wide (Fig. 6D). Where the steepening was watched pit chains in sand develop into lineations. However, it is gradual, there was a transition zone of intersecting patterns (Fig. entirely possible that sometimes wind lineations may not be pre- 6D). Where it was abrupt, there was no transition zone, merely ceded by pit chains but simply develop where flow dominates over abrupt change from ripple to flute (Fig. 6B). The flutes in this latter vorticity. The lineation does not necessarily become pitted, but the case were apparently formed by the negative flow. They were about ultimate fate of a pit chain seems to be that of becoming a lineation. 8 to 10 cm wide, only faintly cross-lineated, and had smooth sur- Most lines of flow carry vorticity. As velocity increases and interfaces. Sand was moving upward through the flutes but downward ference develops, vortices increase in size and number. An irregu- in the larger flutes below the transition zone. In the areas shown in larity on the interface may cause a vortex to pause momentarily Figures 6A and 6D, positive flow dominated, and a great deal of and take a larger load than usual at this locus. If each passing vor- sand moved downslope parallel to the flute beds about 3 to 4 cm tex does the same, a depression soon forms at the locus where a above the centers of the troughs. When the wind gusted, complete stationary vortex center becomes established. Widening of this cen- secondary grooves were excavated centripetally both normal and ter may cause the joining of two centers. Thus the lineation starts oblique to the line of flow. Also, there was a strong breeze moving and progresses from center to center — a sort of pit piracy where along the dune corridor. That breeze appeared to be one of the fac- the stronger vortices extend their influence beyond the pit rims, tors responsible for the turning of the ends of the flutes. Plants, breach the rims, and lengthen in the direction of neighboring pits. however, were also local deflecting factors (Fig. 6A). The whole The wind-eroded glass in Figures 4A and 4B shows lineation on constituted what I consider to be a coordinated lineation system. both windward and leeward faces. The lee face is singularly lacking Flute orientation may be either in the direction of the wind or in impact fractures. It seems to have eroded largely by interfacial represent domination of one of the three interfacial current patterns flow in four intersecting negative flow sets — some vortex pit (positive, negative, or secondary flow). On one sand-draped hill chains, some flute pit chains, and some lineations. Each flute pit is (Fig. 41) while the wind was from the left, the dominant control ap- elongated in the direction of the negative flow trend under which it pears to have been directed downward along the lines of secondary formed and is dominantly lineated in that same direction, but car- flow, thus creating symmetry, sharp intervening crests, and fairly ries lesser lineation patterns in the other three negative flow direc- regular cross-lineation (Fig. 4J). tions. Along the major lineation patterns (Fig. 4B), rows of round The following are varied examples of the flow-vorticity-erosion pits occupy the beds of the grooves and often bear helical scores on relationship. Rising lines of low pressure along snow-crested ridges their floors. In some cases, regularly spaced cross-lineation is pres- of mountains may create wide bands of secondary flow which re- ent. In other cases, the ridge crests are scalloped. Pit and lineation sult in the formation of huge flutes converging into the ridges. In formation may have been simultaneous in this case. The flute pit Figure 6F, a small linear dune on Padre Island on the Texas coast, chains and the vortex pit chains seem to be in the process of becom- lines of low pressure along both ridge crests and valleys appear to ing lineation sets. While each set on this specimen can be related to have governed flute formation. Along a declining dune crest in the one of four directions of wind impingement, the transecting linea- Kelso Dunes, Mojave Desert, California, I saw a reticular system tion of Figure 4C developed under ten months of unidirectional develop where both crest and valley were simultaneously cross- wind-blasting. This pattern occurred near a lower lee margin where lineated. A fairly large area in Seney National Wildlife Refuge, the surface received the combined influence of currents moving on Schoolcraft County, Michigan, which now is underlain by lake-bed and off two margins. Pit chains also developed on this specimen. sands and studded with sand dunes (Fig. 6C), appears to have been The origin of wind lineation is not simple. There is considerable eroded in a similar way during early postglacial times. In a hedge- variation involved in the process. In some cases, there must also be lined field in Nebraska (Fig. 6E), flow came down along the hedges considerable interplay with other processes, but vorticity in the (left) and across the ground. Where two lines of flow met, dust dev- flow lines plays a dominant and complex role. ils formed. These secondary flows were normal to the main wind direction. A similar relationship occurred in Marble Canyon of the LINEATION ON THE LEE SURFACE OF Colorado River in Arizona, where the canyon wall is rather regu- A BARCHAN DUNE larly grooved; when the wind blew upriver, great sheets of dust swept off the tops of the bordering ridges toward the canyon floor, Plant pappus, the very lightweight parachutes of seeds, was ob- sometimes causing a blackout on the highway. This general rela- served cycling through a large arc to the lee of a barchan dune, ap- tionship also may be the pattern of the foehn wind. It may be a proximately 1.75 m high, 8 to 10 m long, and with a slip face about major factor in the sliding boulders of Death Valley, California. It 2 m wide, at Silver Lake Dunes, Oceana County, Michigan. When may also possibly play a role (along with waves) in the formation of the velocity of the wind was 24 to 30 kmph, the pappus would regularly spaced shore features such as beach cusps, and with solar make a complete cycle 4 to 5 m in height, starting in a lift pattern at radiation in the formation of ice cones. These examples show the the rim of the slip face, rising to a zenith about 3 m above the rim, possibility that such systems can operate on relatively large scales and falling to the lee of the slip face, to be picked up by the negative and may have a much broader role to play in erosion than just the flow and then swept back up along the central line to the rim of the formation of lineations on rock. This relationship of vorticity and dune (Fig. 7). There was much pappus in the air, and so the cycle flow may help to explain other phenomena which have been was seen many times during an afternoon. Updraft was considered difficult to explain in the past. Heretofore, transecting wind linea- to be an important factor because the barchan was located high on tion has been considered to form under wind shift. I agree that a dune complex about 20 m above Lake Michigan. To the lee of the some forms in this manner, but certainly some of it forms under slip face, a flat surface extended slightly beyond the horns of the unidirectional wind, and this is likely to be the more regular barchan (Fig. 7). As the pappus was swept along the slightly de- reticulum. pressed central line of the slip face, it dragged the ground until just Lineation may be preceded by score chains or pit chains. Cer- a few centimetres below the crest of the dune where the slip face

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surface, but contour and surface sculpturing also play roles. Although a small wind deflection did not destroy the flute pattern, a large deflection did. However, the flute pattern reformed instantly when the wind shifted back to the axial direction. Whereas the pappus was always shifted to the very central line of the slip face, sand, starting at the lee end of the flat, traveled windward within each flute. Along these flutes, the velocity appeared to be less than one-quarter of that of the positive wind on the windward side of the dune (south). The low velocity facilitated observation. It was possible to pick a reddish or a blackish sand grain for observation, to follow the grain to its escape at the rim and sometimes even to follow it for a few centimetres beyond the rim. In a narrow sector in the upper one-third of the slip face, sand which was falling from the rising arc sometimes knocked the grains out of the pappus. The cycle was complete. "W" refers to wind direction, "p" re- their flight paths. The falling grains did not alter the flute pattern; fers to positive flow, and "n" refers to negative flow. however, the vorticity within the flutes rapidly reshaped any minor surface flaws caused by the falling grains. Above the zone of falling grains, it was possible to select other grains and to follow thehi to steepened. As on the ventifacts, negative flow appeared to occur the rim. When the entire flute pattern was re-established after a only within the first few millimetres above the surface. wind shift, the event was so rapid that it was impossible to detect A remarkable aspect of the barchan's lee slope and adjacent flat much of the sequence. About one or two seconds after the pattern was the lineation of them (Fig. 7). There were about 1,000 small reappeared, sand began to move on the floors of the lee ends of the U-shaped flutes separated by sharp keels oriented nearly parallel, flutes and slid windward on the ground for about 1 m. The grains but very slightly convergent toward the axis of the dune and the began to lift smoothly and did not again touch the flute floors ex- wind direction. Each flute was about 5 m long and less than 1 cm cept as falling grains knocked them down. The grains stayed within wide over most of its length. The flutes were, however, only about 2 their channels until after crossing the abrupt change in surface to 5 mm wide in the uppermost 10 to 15 cm of the slip face where slope. While no strong cycling effect could be detected in the grain the surface was steeper. The break between the steeper upper mar- activity within the channels, grains could be seen to turn and gin of the slip face and the main part of the face was gradual at the vibrate in flight. Possibly when the velocity is low (4 to 5 kmph), axis of symmetry and progressively more abrupt around the horns. sand does not participate in a vortex configuration as readily as In the latter sectors, the flutes made sharp bends from the slip face snow does, and it is carried within the groove rather than escaping to the steep slope (Fig. 8). The lineation pattern was regular when from it. In addition, linear sliding of grains at the lee end of the flat the wind was approximately in line with the axis of the dune. This did not begin until after a flute pattern developed. Such pattern correlated well with the results in the experiment shown in Figure formation is due to aerodynamic causes or to vorticity operating in 2E in that both indicate that the combined influence of wind direc- streamlines more or less simultaneously over an entire slip face. tion and symmetry of an object govern the pattern of flow on its lee Sliding of the grains in the grooves altered the surfaces of the floors of the flutes for about 1 m, or up to the point where lift began. From the rim of the slip face to the lee end of the feature, all of the Figure 8. Detail of line- flutes were symmetrical and perfectly aligned. Their surfaces were ation in the steep upper smooth except for slight transverse lineation on the sharp interven- zone of the slip face of a ing crests and for the alterations caused at the lee ends by sliding barchan. The main air grains. Lift did not appear to be related to any impact of grains mass, as it moved up the upon other grains. A grain lifted when it attained a certain velocity. windward slope and rose at the crest line (as in Fig. 7), The point of lift was approximately the same in all the flutes, that exerted the principal is, about 1 m from the lee end. Vorticity was probably responsible influence on the negative for the shaping of the flutes and lift of particles. A highly symmetri- flow trends along the slip cal flute form would be indicative. face, making the long lineations. However, as the main When the wind shifted about 45° in either direction around the mass lifted over the dune dune axis, the flutes were quickly destroyed, and a new set of events crest, generating the long began with the establishment of negative flow obliquely across the negative flow trends of the slip face, some fractions of lateral rim of the dune crest (Fig. 9). The shift established a tempo- the air mass had to remain rary lee zone on the convex surface of the horn opposite the posi- at the interface both on tion of impact. Pappus, coming up this surface, behaved erratically windward and lee- upon arriving at the rim. When it finally settled on the slip face, the ward sides of the crests until they merged at the crestline. Those interfacial flows on the windward side exerted local influence on the interfacial nega- pappus and near-surface sand were thrown into a helical pattern tive flows at the top of the slip face, producing the short, narrow lineations. about 45 cm in diameter (Fig. 9). Neither left the ground. The pap- As each positive flow line escaped, a small vortex formed at the rim line of pus finally escaped the helical configuration, crossed the center line, the dune crest and attracted the interfacial negative flow. Either these vor- moved into a similar but smaller downwind vortex, and repeated tices or a flow line down the rim of a horn caused these short flutes to be- the behavior farther downwind several more times in progressively come progressively deflected away from the axis of symmetry of the dune. "W" refers to wind direction, "D" to deflection along die dune crest, and smaller vortices arranged on an arc, as though at the perimeter of a "n" to negative flow. larger vortex. The last of the smaller vortices was only about 10 cm

in diameter. From this vortex, the pappus escaped across the dune formity of flute pattern, and (2) they play a local role in the creation rim and went through an erratic flight back to windward for about of the small flutes in the steep upper zone (Fig. 8). During the tran- 0.5 m and then was swept to the leeward off the horn. In the ex- sit of the air mass over the dune, pressure changes in the escaping perimental work, conversion of negative flow to vorticity or vice air at the dune crest cause zonation of the flow into a rising sheet versa was a simple matter of changing the angle or position of im- and another sheet which adheres closely to the substrate. Both of pact. Hence, the observed changes on the small barchan dune corre- these zones exert influence on the air to lee of the crest, causing it to lated well with the experiments shown in Figures 2E and 2H and become zoned in similar manner. The upper zone on the lee side with similar experiments performed with wedge-shaped flutes. Fur- moves out from the interface to meet the rising positive flow, but thermore, while the dominant sculptural pattern within such flutes the lower zone of flow continues on to the crest line greatly at- is lineation converging into the flute head, some of this type of flute tenuated in its erosional ability; thus the upper slope steepens, and shows arcs of helical scores arranged like those on the slip face on the surface becomes lineated with short, narrow flutes. Two main the barchan. sizes in vortex configurations occur at or close to the crest line. The The behavior of the pappus was very different in the helical zones larger size occurs when the main positive and negative flows meet. where it clung to the slip face from that at the windward horn These extend their influence over the entire lee face, resulting in the where it underwent lift. Over the temporary lee side of the slip face, main lineation pattern. The smaller size occurs close to the crest there was a zone of falling air, indicated by the abrupt drop of thjfe line and exerts local influence in the upper 10 to 15 cm of the slip pappus onto the slip face in this zone. Possibly the vortices which face and to lesser extent on the windward side of the crest, creating formed beneath this falling air mass were inverted, for the pappus the smaller and differently oriented flutes in the crestal zone. was unable to get off the ground until it reached the horn on the There has been much speculation about air movement to the lee windward side. of dunes. Bagnold (1942, p. 209) and Sharp (1966, p. 1070-1071) Smooth crestal lines seem to have a double influence: (1) they tried to test lee-side currents with smoke. Bagnold noted a tendency play a role in keeping sizes of vortices uniform, contributing to uni- for reversal but considered it insignificant. Sharp, who was testing for the presence of a large lee-side vortex, found only a few small more or less randomly distributed vortices. It is my belief that their smoke candles and smudge pots probably emitted smoke too high above the Surface. Failure to understand the nature of negative flow appears to have caused them to more or less overlook it. In Sharp's area of investigation, the Kelso Dunes, the primary wind impingement upon the foredune is oblique to the dune front. Hence, the general symmetry relationships such as in Figure 7 were lacking. While negative flow must surely have existed as an interfacial flow, its activity is more likely to have been similar to that shown in Fig- ure 9. Sharp himself noted that his area of investigation might not be the best place for lee-side testing. An alternate location which seems promising, at least to demonstrate large-scale negative flow, is Grand Sable Dunes near Grand Marais, Alger County, Michigan. At that locality, dunes are perched on the edge of a high bluff at a promontory on the south shore of Lake Superior. William Marsh (1968, personal commun.) has noted that while he was mapping one of the highest of these dunes, a sudden squall struck the cliff and caused negative flow on the bare lee slope, and some of the suspended load attained a height of about 20 m above the dune. He estimated that the dune was about 120 m high and the bluff about 200 m high. In this area, the prevailing wind is from the northwest with several score kilometres of fetch over open water of Lake Superior. Strong negative flow under these circumstances is not surprising and probably even accounts for the presence of the perched dunes on the very edge of the cliff.

LARGE-SCALE WIND EROSION

Figure 9. Plan view of the barchan dune shown in Figures 7 and 8, with Because wind behaves much the same on all scales, the principles wind oblique to the axis of the dune. The streamlining of the negative flow governing the development of a lineation set on the lee surface of of Figure 7 is replaced by oblique-axis vortices beneath a mass of falling air. the barchan dune might be expected also to influence, if not con- The negative flow is deflected obliquely down the slip face, cycling the sand and plant pappus in large helical scores which diminish in size as the flow trol, the development of mega-systems of lineation in snow, sand, approaches the moving low-pressure zone of the current flowing dqwn the and loosely consolidated rock on Earth and on other planets. windward horn of the dune. The falling air mass possibly inverts the vor- While seif dune systems are rather generally considered to form tices; at least it does not permit either sand or the extremely lightweight with their axes parallel to the direction of the prevailing wind, it pappus to leave the ground surface more than the few millimetres necessary to get from one helical configuration to the next. The pappus becomes free seems just as likely that some might be formed in response to some to rise only at the horn. The letter "p" refers to positive flow, and "n" refers barrier such as a continental margin or valley or ridge. Such fea- to negative flow. tures could serve the same function as the dune crest in the barchan,

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that is, to create an updraft and a pattern of interfacial negative retards it at others; thus the high-centered, embossed pit is a nor- flow. This would result in the initiation of nearly parallel flow mal consequence of the pressure differentials within the vortex. trends which would influence both deposition and erosion. Once Another consequence of the pressure distribution within a vortex is established by negative flow, such trends could then guide the posi- the cushioning effect against grain impact. The exhaust shunts tive flow as it assumes the controlling role. grains laterally and forces them to enter the perimeter of a pit rather than its center. Vorticity and negative flow are both impor- SUGGESTIONS tant means of lift of particles. 5. The tools of erosion are suspensions, some of which may be Because wind apparently behaves much the same on all scales, derived from the eroding object. Dust, silt, and very minute parti- many questions m^y be resolved by scale-model studies. Bagnold cles from the eroding surface probably are the more important (1942, p. xvii—xixj recognized the potential of model studies and tools involved in the very close-running interfacial flows such as the possibility of aerodynamics as a shaping force. Such model negative flow and secondary flow and subsidiaries to these flows. studies might help resolve such wind problems as those relating to Sand involvement may be more often associated with the positive tornado activity, smog distribution, and dust blackouts along flow, especially where this flow is brought to the interface during highways. Most of the misconceptions geologists have had with re- off-sweep or where a channeled positive flow carries large vortices. gard to assessing wind erosion problems can be classed as Because the energy of a vortex varies with its size, occasional very academic. One serious one, however, strikes at the heart of soil large vortices confined within channels or valleys may move large conservation. The advice to leave fallow land in large clods or to objects. deep-plow fields may have been poor advice. Every test my col- 6. For all the variation in shapes of ventifacts, there still is a high leagues and I have made has shown increased erosion rate where degree of similarity in the placement of aerodynamically formed surfaces were rough, especially to leeward where negative flow and features on ventifacts, that is, lineation sets, vortex pits, undercut vorticity become such effective forces in dislodging and lifting away facets, concavo-convex relationships, basal surface characteristics, of microparticles. and so on. Some of the aerodynamic principles governing the shaping of ventifacts are applicable to larger features, such as dunes, CONCLUSIONS and even to landscapes as a whole.

Some of the more important discoveries made during this study ACKNOWLEDGMENTS are as follows: 1. A great deal of wind erosion is produced by vorticity and by I wish to express appreciation to M. M. Miller for suggesting aerodynamic flow patterns. parts of this study; to Helene B. Brewer for aid in the experiments; 2. The shift of air masses along the interface is governed by a to A. E. Kurie for aid in some of the field studies; to R. V. Dietrich system of pressure controls effected largely by pressure reduction at for use of his experimental material and for reading and criticizing air-escape routes and the consequent development of pressure gra- the original manuscript; and to LaV. L. Curry, M. H. Filson, J. dients leading into these loci of pressure reductions. However, at Hradel, L. A. McDermott, E. D. McKee, and A. Schouw for valu- any point or for any reason that air pressure is reduced, even very able suggestions and encouragement. slightly (such as at impact surfaces, centers of vorticity, or along flow lines), higher pressure air masses shift along the interface REFERENCES CITED toward the lower pressure centers. For instance, if a submarginal flow line beneath a lateral flank of a ventifact carries a vortex Bagnold, R. A., 1942, The physics of blown sand and desert dunes: New configuration, the low pressure at its center of vorticity initiates an York, William Morrow and Co., p. 90, 201, 209. 1953, The surface movement of blown sand in relation to meteorol- influx of higher pressure air down from the top of the flank toward ogy: Research Council of Israel, Spec. Pub. no. 2, p. 91. the vortex in the submarginal flow line. Commonly the orientation Dietrich, R. V., 1977, Impact abrasion of harder by softer materials: Jour. of this pressure gradient is normal to the submarginal flow line, but Geology, v. 85, p. 242-246. by the time the air mass in the gradient arrives in the near vicinity of Freier, J. D., 1960, The electric field of a large dust devil: Jour. Geophys. the submarginal flow line, the vortex has moved leeward. Hence, Research, v. 65, p. 3504. Higgins, C. G., Jr., 1956, Formation of small ventifacts: Jour. Geology, near the basal margin, the gradient may deflect sharply leeward. v. 64, p. 506-516. This shift in orientation may be sequentially repeated along the Kelley, R. W., 1962, Michigan's sand dunes — A geological sketch: Michi- flank so that it is reflected in sharp bends in the resulting erosion gan Dept. Conservation, p. 9. pattern. Whereas in the past geologists emphasized high velocity as Rabinowicz, Ernest, 1968, Polishing: Sci. American, v. 218, no. 6, p. 91 — 99. the important control factor in wind erosion, the pressure differen- Sharp, R. P., 1966, Kelso Dunes, Mojave Desert, California: Geol. Soc. tial control system is by far the more important. While it requires America Bull., v. 77, p. 1070-1071, PI. V. velocity change, it does not require high velocity to effect erosion. Whitney, Marion I., and Dietrich, R. V., 1973, Ventifact sculpture by What is essential is that tools be brought into contact with surfaces windblown dust: Geol. Soc. America Bull., v. 84, p. 2561-2582. by interfacial flow lines and vorticity. Wilson, Ian, 1972, Sand waves: New Scientist, (March 23, 1972), p. 634- 637. 3. Vorticity also plays a dominant and more direct role in shaping, sculpturing, lineating, pitting, and burnishing by its direct action upon surfaces either in association with flow lines or at isolated loci. MANUSCRIPT RECEIVED BY THE SOCIETY JUNE 1, 1976 4. A vortex is a complex pressure system which through its in- REVISED MANUSCRIPT RECEIVED APRIL 11, 1977 terfacial phases accelerates erosion in some parts of the vortex and MANUSCRIPT ACCEPTED MAY 3, 1977

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