Sliding stones, Racetrack Playa, California

ROBERT P. SHARP Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125 DWIGHT L. CAREY Department of Geology, University of California at Los Angeles, Los Angeles, California 90024

ABSTRACT 1952a, 1952b; R. E. Kirk, 1953; Shelton, any other playa. Some immutable law of 1953; Stanley, 1955; Schumm, 1956; Clem- nature probably prescribes that movements Twenty-eight of 30 monitored stones on ents, 1958; W. E. Sharp, 1960; Creutz, occur in the darkness of stormy moonless the southern part of Racetrack Playa moved 1962; Bradley, 1963; Findley, 1970). This nights, so that even a resident observer within a seven-yr interval, leaving distinct playa is not unique, as stone tracks have would see newly made tracks only in the tracks. Movements occurred principally been reliably reported on at least eight dawn of a new day. Tracks made by objects during the winters of 1968-1969, 1972- other dry in southeastern California moved by other mechanisms in different 1973, and 1973-1974. Some stones moved and adjacent Nevada (Table 1). However, geological settings contribute little to the in all three episodes, some only in one or Racetrack Playa is distinguished by the playa-track problem (Sainsbury, 1956; Di two, and a few on other occasions. Move- great number and large size of stones and Cesare and Pratelli, 1967; Hattersley- ment is clearly related to wet stormy an abundance of tracks. Smith, 1969, p. 45; Dionne, 1969). The weather. We realized in April of 1968 that no sys- moving stones and their tracks are hardly Greatest cumulative movement, 262 m, tematic observation of stone movements matters of greatest scientific import, but and greatest single-episode movement, 201 was being maintained, and so permission broad interest in this curious phenomenon m, were by a small, 250-g stone. Other was obtained from then incumbent Monu- motivated the investigation. monitored stones weighing as much as 25 ment Superintendent, John W. Stratton, to kg moved cumulative distances of 60 to 219 establish a monitoring program. The objec- PHYSICAL SETTING m. Net direction of movement was north- tives were to determine the season, fre- AND CONDITIONS northeasterly with deviations to east and quency, conditions, direction, and mag- southeast on occasions by some stones. nitude of stone movements, their relation- General Movement most likely occurs within one to ships to meteorological conditions, the pos- several days after playa wetting, and ve- sible role of ice, and to confirm or challenge The playa lies at the southern end of locities on the order of 0.5 to 1 m/sec are in- the generally prevailing opinion that wind topographically closed Racetrack Valley, ferred from track characteristics. is the motivating force. Observations from nestling between two northward-projecting Thin sheets of ice form in winter on this May 1968 to May 1975 were made mostly prongs of the (Fig. 1). The playa, and eyewitness accounts of ice during weekend visits on 16 occasions. elliptically shaped floor, 1,130-m ele- sheets, some with infrozen stones, being No authenticated record has been dicov- vation, is 4.5 km north-south and 2.1 km driven by wind across other southern ered of anyone seeing a stone actually make wide. Closely bordering bedrock ridges rise California playas indicate that stone tracks a track by natural means on Racetrack or above 1,830-m elevation, and Ubehebe may be made in this manner, as earlier ad- vocated. However, movement of stones out TABLE 1. PLAYAS IN SOUTHEASTERN CALIFORNIA AND of an encirclement of iron stakes, large ADJACENT NEVADA DISPLAYING STONE TRACKS changes in neighboring stone separation during movement, disproportionate corre- Playa Location Source of information (lat long) sponding reaches within contemporaneous tracks of neighboring stones, and other re- Racetrack Calif. (36°41'N, 117°34'W) McAllister and Agnew lationships strongly suggest that monitored (1943) stone movements occurred without the aid Little Bonnie Claire Nevada (37°10'N, 117°10'W) Clements of extensive ice sheets. (1952) Wind acting directly on the individual Nevada (35°51'N, 114°57'W) Stanley stones is considered the prime moving Nelson Dry Lake (1955) force. A critical element promoting move- ment may be deposition of a thin layer of Rogers Calif. (34°55'N, 117°47'W) Motts fine slippery clay, the material that last set- (1969) tles from suspension after playa flooding. Rosamond Calif. (34°50'N, 118°5'W) Motts (1969) INTRODUCTION North Panamint Calif. (36°20'N, 117°23'W) Motts (1969) The sliding stones of Racetrack Playa in Drinkwater Calif. (35°30'N, 116°33'W) Harold Ericsson National Monument, (unpub. data) California, and the tracks they leave on this (34°53'N, 117°46'W) Win. Frazier smooth, normally dry lake bed have at- Small playa, Edwards Calif. Air Force Base (unpub. data) tracted much public and considerable scien- tific attention (McAllister and Agnew, Unnamed playa north Calif. (35°05'N, 116°26'W) Lyle Hoag 1948; Anonymous, 1952; L. G. Kirk, of Afton (unpub. data)

Geological Society of America Bulletin, v. 87, p. 1704-1717, 21 figs., December 1976, Doc. no. 61205.

1704

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3S°40'

Figure 1. Map showing access routes to Racetrack Playa.

Peak at 1,730 m lies just 1.37 km off the west play a edge (Fig. 2). The surrounding bedrock is principally Paleozoic carbonates and Mesozoic intrusives (McAllister, • 1956). Racetrack Playa is bordered mostly by Figure 2. Physical bouldery alluvial fans backed by steep bed- setting map, elevations in metres; feet in pa- rock slopes (Fig. 3), but at its south end, a rentheses. large ridge of Cambrian dolomite rises abruptly from the playa margin (Fig. 4). Near its northern end, there are two bed- rock islands; the larger, , is of coarse porphyritic quartz monzonite, the Cottonwood Mountains (Fig. 1). Usual stricted by prohibitive signs and a ditch and the smaller is a knob of Paleozoic access is by way of a rough but maintained bulldozed in 1969 along the west margin. limestone. The proximate bouldery fans dirt road extending 42 km south from Monitored stones closest to the west playa and steep rock slopes provide the abundant . Shorter access from the edge have suffered some tourist disturb- rock fragments resting on the playa surface. south can be gained, in proper season, by ance, but stones farther east have not been Cambrian dolomite comes from the south- the secondary Hunter Mountain road via molested. Total data losses are less than 10 shore rock ridge, syenite from coarse fans Jackass Spring, Harris Hill, Hidden Valley, percent. along the southeast and southwest margins, Lost Burro Gap, and Tea Kettle Junction This area receives rain and snow in and quartz monzonite from east- and west- (Fig. 1), generally transversable by a sturdy winter and occasional thunder showers in side fans farther north. Porphyritic quartz 2-wheel-drive vehicle. summer for an estimated annual precipita- monzonite is supplied by The Grandstand Tourists' visits to the playa are numer- tion of 7 to 10 cm. As much as 30 cm of add by northernmost west-side fans. ous, owing to National Monument public- snow has been seen on the playa surface Racetrack Playa is at the western border ity and keen interest in the sliding stones. (D. W. Carney, unpub. data), and freezing of Death Valley National Monument and is The playa surface was formerly disturbed temperatures are frequently attained at separated from northern Death Valley by by vehicular traffic, but that is now re- night in winter and spring. Winds are

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Figure 3. Looking north, local wet area and standing water along east shore with large projecting fan beyond. Dark bedrock island, The Grandstand, at arrow in distance, May 18, 1973.

strong and reasonably frequent. Local ficial 2 cm contains the following per- scabby surface (Fig. 5). The clay chips pile topographic configurations may enhance centages: 24 fine sand, 41 silt, and 35 clay. up around bushes at the playa margin to velocity by channeling winds, particularly An inference that the northern end is 5 cm form small clay mounds. Thus, a significant from the south-southwest quadrant. The higher (Stanley, 1955, p. 1333) was fraction of the finest clay accumulates Ubehebe Peak topographic quadrangle confirmed by complete flooding in early around the playa margin. The clays of this (1:62,500; 1951) covers the area. Its geol- October 1974 that, under windless condi- playa are lower in montmorillonite and ogy has been mapped by McAllister (1956). tions, resulted in 5.5 to 7 cm of water near higher in illite than the average Mojave Although stones have moved on essen- the south end and only 0.5 to 1 cm over the Desert playa (Droeste, 1961). Algal depos- tially all parts of the playa, this study fo- northern part. Much of the playa surface its laid down from water standing a week cused on the southern sector, where stones displays a regular pattern of polygonal or more on the playa are also at least partly and tracks are most abundant. This was a mudcracks (Fig. 5) mostly 5 to 10 cm removed by the wind when dry. Material small-scale operation, personally funded, across, which survives wetting as new cracks exposed after removal of the top clay layer and resources to establish instruments for develop, largely along traces of older contains significant silt and fine sand. Ex- continuous recording of precipitation and cracks. Tracks of sliding stones cross this perimental measurements of friction and of wind were simply not available. polygonal pattern without deviation the force required to move stones may have (Fig. 6). been skewed by being made on such silty Nature of the Playa Surface The dried playa surface is darker brown materials rather than on the fine-clay layer. and more shiny immediately after flooding Although Racetrack Playa is occasionally The Racetrack qualifies as a dry playa than at other times because of the deposi- completely flooded, the more common wet- (Thompson, 1929, p. 125), that is, its sur- tion of a new layer of fine clay, a fraction of ted condition is fractional coverage by a face, when not temporarily wetted, is dry, a millimetre thick, representing the last ma- water sheet about 1 to 2 cm deep (Fig. 3). smooth, and firm. Motts (1969, p. 14) terial to settle from suspension. Upon dry- would classify it as fine grained. The sur- ing, this clay layer cracks and curls into small chips about 1 cm across which are rapidly removed by wind, as observed elsewhere (Blackwelder, 1946), leaving a

Figure 4. Cambrian dolomite ridge at south shore viewed from northern end of 250 m track Figure 6. Looking north along part of 440 m (Table 4) made on or about December 2, 1970, Figure 5. Mud-cracked playa surface; scabby track near southwest playa margin as seen April by stone R (Hortense), as photographed January appearance caused by partial removal of upper- 19,1969. Sharp turn at arrow extends track past 23, 1971. most thin finest clay layer. dark glasses to A.

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Winds can move such local sheets to dif- ferent parts of the playa and by concentrat- ing water against a shore can create depths as great as 10 cm (Bradley, 1963). A wind- driven water sheet advancing across a dry playa is accompanied by a sizzling noise made by air bubbling out of the dry depos- its (D. W. Carney, unpub. data). The possi- bility of stone movement resulting from wave action within wind-driven water sheets is discounted, here at least, by the predominant offshore movement of stones at and near the south playa edge.

MONITORING ARRANGEMENT

In May of 1968, 25 stones on the south- ern part of Racetrack Playa were labeled and their positions fixed with respect to stakes of 1.6-cm structural steel (Fig. 7). By the conclusion of this study in May 1975, the number of stones had been increased to 30. Stones were identified by letter and name; A (Mary Ann), B (Ruth) . . . , and the reference stake, set 3.1 m due west, was identified by the same letter. Each time a stone moved, its new position was marked by a similar stake identified by letter and subscript, A1( A2 . . . . Special arrangements included two groups of small stones (G and C of Table 2), a closely associated pair of larger stones (D, Table 2), and some stones enclosed by 7 stakes set 64 to 82 cm apart in a 168-cm circle, known as the corral (Fig. 8). The ob- jective of this arrangement was to test the possible role of ice in moving stones (Stan- ley, 1955). A sheet of ice from the outside should have trouble getting to the stones, and any in situ ice incorporating the stones would be anchored down by the stakes or would become too badly fragmented to hold the stones if moved. The corral ini- tially enclosed a single stone that soon moved out and was replaced by two arti- ficially introduced stones, one of which subsequently moved out. Stones of a variety of sizes, shapes, and locations were selected for monitoring, and all had previously made prominent tracks. Except for one unusually large stone, their largest linear dimension ranged from 6.3 to 36 cm, and weights were 170 g to 25.4 kg. The large stone, J (Karen), was a rectangu- lar block of dark Cambrian dolomite measuring 74 x 48 X 51 cm with a calcu- Figure 8. Experimental arrangement, the "corral," to test possible role of ice sheets in stone lated weight of 320 kg. It lay at the north movement. Farther stone subsequently slid out of the corral but nearer stone remained in place; photographed January 23, 1970. end of a nearly straight track bearing N20°W and extending 174 m directly out- and surface configuration, and syenites to 472 m (Fig. 10), and cumulative ward from a point within 25 ft of the south have the smoothest flattest surfaces, clearly movements in excess of 3.2 km are implied shore (Fig. 9). About 75 percent of the joint controlled. by the distance between stones of distinc- marked stones were dolomite; the remain- tive lithology and likely sources. Most der were syenite and quartz monzonite. CHARACTERISTICS OF tracks exceeding a length of 300 m have Sizes, shapes, and weights are recorded in STONE TRACKS probably involved more than one episode Table 2. Most stones are fragments little of movement. Among the 30 stones modified by weathering or transport. Stone tracks have been traced with monitored, the greatest seven-yr cumulative Dolomites are the most irregular in shape confidence for continuous distances up movement was 265 m and the largest

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single-episode movement was 202 m. Track curve may produce a still larger lateral ac- abrupt bends or sharp angular turns (Fig. widths are 1 cm to 39 cm, and the width of cumulation. Differences in depth of seg- 6). Sinuosity is common, and some tracks a single track may vary by as much as 40 ments of a single track made at different are multi-cusped. Abrupt changes in direc- percent, because stone rotation or changes times indicate that playa condition, as well tion often occur at lateral offsets of 5 to 10 in direction while sliding place different as stone weight, is a factor in track depth. cm or at a local depression bordered by a basal dimensions orthogonal to the direc- Thus, it appears that as stones of foreridge of mud, called a "sitz mark," tion of movement. Many tracks, particu- medium-to-large size move across the where a stone came to rest. Lateral offsets larly those made by dolomites, have linear playa, they push mud aside, leaving an in- suggest sidewise lurching of the stone dur- striations and grooves created by uneven- dented track bordered by levees. Such ing movement or following a pause. Track nesses on the stone base (Figs. 10, 16). The stones seemingly also generate a compres- segments on opposite sides of many sitz spacing and number of these markings sion collar of mud at the leading edge, and marks are of a distinctly different age, in- change with track width. Some stones have most, upon stopping, partly overrun this dicating a pause of a significant duration. prominent basal ridges or keels, and their collar, coming to rest in a gently tilted posi- Complex track patterns involving sharp tracks are planimetrically more regular tion with the leading edge above the playa offsets, abrupt changes, and even reversals than those of smooth-bottomed stones surface and the tailing edge below it. in direction with crossings (see L. G. Kirk, which tend to wander, like an inadequately Dark-brown lobate stains extending as 1952a, Fig. 1), are probably the product of keeled sailing vessel. The deeper tracks in- much as 50 cm forward of some stones more than one episode of movement. Most dent the playa surface about 1 mm and suggest that a thin residual film of water, tracks made during a single episode display have rounded bordering levees (Fig. 4) typi- rich in fine brownish clay, was shoved some degree of geometrical similarity, or cally about 0.5 mm high, although Stanley along by the stone and that it surged ahead what Stanley (1955) terms similar "signa- (1955, p. 1333) records a maximum track when the stone stopped. tures." For example, 8 of the 23 stones that relief, including levees, of 3 mm. The levee Planimetrically, most tracks are not moved in the winter of 1973—1974 (Table on one side may be larger than on the other, strictly linear (Fig. 11). Straight or gently 3) proceeded in a north-northwesterly di- and tobogganing on the outside of a sharp curving reaches are usually separated by rection for about 30 m along an irregular

TABLE 2. CHARACTERISTICS OF MONITORED STONES

Stone Shape Dimensions5 Weight Lithology When Comments identification (cm) (kg) marked

A (Mary Ann) Rectangular 20 x 16.5 x 10 6.6 Syenite May 1968 AD 59 engraved on base f (Ruth-af) Tabular 13 x 10 x 3.8 0.85 Dark-gray May 1968 Lost B dolomite [ (Ruth-fc) Irregularly 18 x 16.5 x 11 6 Dark Jan. 1973 Replacement angular dolomite ' C! (Sue) Angular 15 x 6.3 x 5 0.65 Gray May 1968 dolomite O. 3 C2 (Sally) Irregular 12.7 diameter 1.0 Light May 1968 2 carbonate Pyramidal 7.5 x 5 x 2.5 0.18 Dark-gray May 1968 u C3 (Val) dolomite G, (Marion) Irregularly 15 x 7.5 x 5 0.8 Dark-gray May 1968 angular dolomite D, (Ev) Tabular 25 x 18 x 9 5.9 Gray May 1968 dolomite

D2 (Irene) Slab 33 x 15 x 5 2.8 Gray May 1968 dolomite

f (Peggy-a) Rhombohedral 18 x 10 x 10 4.55 Dark-gray May 1968 Lost, recovered, E dolomite lost [ (Peggy-fc) Roughly 23 x 15 x 10 3.6 Dark-gray Jan. 1971 rhombohedral dolomite Í (Jean^a) Irregular 23 x 18 x 9 4 Dark-gray May 1968 Lost and F dolomite recovered [ (Jean-b) Rectangular 23 x 13 x 10 4.5 Gray Nov. 1971 Replacement dolomite lost

' G, (Bep) Tabular 23 x 14 x 7.5 3.7 Light-gray May 1968 carbonate G2 (Milly) Roughly 14 x 10 x 5 1.25 Light-gray May 1968 tabular dolomite d. 3 G3 (Gerry) Tabular 12 x 7.5 x 5 0.7 Buff May 1968 2 1 bJD dolomite a G4 (Jane) Irregularly 7.5 x 2.5 x 2.5 0.22 Buff-gray May 1968 tabular dolomite

G5 (Carmen) Angular 12 x 9 x 9 1.4 Dark-gray May 1968 dolomite

G6 (Margie) Irregularly 6.3 x 5 x 5 0.34 Discolored May 1968 angular gray dolomite

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course composed of linked cusps and arcs unmarked stones. Clods of burro droppings such movement, sometimes by as much as 5 before straightening into a longer and more sometimes record movements seemingly to 10 m. Furthermore, the dimensions of regular course bearing north-northeasterly not experienced by stones, perhaps because corresponding reaches within tracks of (Fig. 12). they are light, but in the instance of stones similar appearance are often not the same The motive force does not always act in of roughly comparable size, weight is obvi- (Fig. 14b), resulting in a distortion of the exactly the same direction over all parts of ously not the controlling factor. Probably, similarity. Tracks made by reasonably large the playa. Tracks in the area lying close to just a fortuitous combination of circum- stones, up to 20 kg, separated by distances the south-shore bedrock hill have occasion- stances is involved, including local condi- as great as 1 km may also display a degree ally had a more easterly, or even a south- tion of the playa surface, a possibly un- of geometrical similarity, but not identity, easterly, component than contemporaneous stable position at a sitz mark, and a local in contemporaneous segments. tracks elsewhere in this southern sector. wind gust of unusual force that causes an Some small stones have alternately slid Furthermore, single stones sometimes re- individual stone to move while others and tumbled (Fig. 15), and a few have spond to a phase of an episode of move- nearby remain in place. Sometimes, during moved solely by tumbling for distances as ment not recorded by other nearby stones. quiet winters, only one or two stones may great as 17 m. Tumbling can be induced in For example, in the winter of 1973-1974, move, but tracks made on those occasions a sliding stone upon impact with another 23 out of 30 monitored stones moved in a may extend 100 m and more, suggesting stone (Fig. 16), but impact is not required. net northeasterly direction, but stone I that once a stone starts to move, it keeps Small size, 2.5 to 7 cm and rounded shape (Kristy) after moving northerly 50 m, going rather easily. favor tumbling. traveled back south-southwestward (Fig. Stanley (1955) describes in detail a great 13) 55 m, ending up only 20 m west of its similarity in tracks made by some groups of TRACKS MADE BY starting point after a total journey of 107.5 small closely associated stones. This charac- OTHER OBJECTS m. The only other southward tracks that teristic has been seen repeatedly during this were observed and considered attributable study, but it was also noted that generally Racetrack Playa displays marks made by to this same episode were made by 3 small the spacing between stones changed during burro droppings, twigs, brush, and what

TABLE 2. (Continued)

Stone Shape Dimensions' Weight Lithology When Comments identification (cm) (kg) marked

H (Nancy) Irregular 7.5 diameter 0.25 Syenite May 1968 cobble

I (Kristy) Rectangular 14 x 9 x 6.3 1.85 Dark-gray May 1968 dolomite

J (Karen) Rectangular 74 x 48 x 51 320 Dark-gray May 1968 calculated dolomite

(Daphne-a) Tabular 10 x 6.3 x 3.8 0.6 Dark-gray May 1968 Lost K < (Daphne-fe) Triangular 10 x 10 x 9.5 1.7 Syenite Jan. 1973 Replacement pyramid

L (Helen) Rectangular 28 x 23 x 10 14.2 Dark-gray May 1968 dolomite

M (Clem) Irregularly 28 x 23 x 14 12.7 Dark-gray May 1968 angular dolomite N (Alice) Rectangular 28 x 20 x 14 17.8 Syenite May 1968

(Grace-a) Angular 7.5 x 6.3 x 2.5 0.28 Dark-gray May 1968 Lost o dolomite [ (Grace-b) Triangular 7.5 x 3.8 x 3.8 0.17 Syenite Jan. 1973 Replacement R (Hortense) Irregular 28 x 20 x 19 13.2 Dark-gray Jan. 1971 dolomite

S (Dottie) Angular 36 x 13 x 18 24.3 Dark-gray Jan. 1973 dolomite

T (Enid) Rhombohedral 18 x 15 x 15 5.9 Dark-gray May 1974 dolomite

X Rectangular 7.5 x 7 x 5 0.43 Dark-gray May 1968 dolomite

XX Rectangular 18 x 10 x 6.3 2.8 Syenite Jan. 1971 XXX Irregular 23 x 10 x 15 3.2 Syenite Jan. 1971

* The last figure is the vertical dimension of the stone. t Designations a and b are used when a lost stone (a) is replaced by another stone (b).

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Figure 10. Looking north along part of a 472-m track (left), with stone B (Ruth) and its track on right, photo- graphed April 19, 1969.

15 meters A' (64.5 m)

Scale N' (21.6m) 0' (15.2m)

Silz mark

Figure 9. View south over 320 kg Cambrian Di (5.9 kg) ^ dolomite boulder. Only the deeper part of the

greater than 7-year-old, 174-m-long track is D2(2.8kgí DÍ (4.3m preserved. Canteen in mid-distance.

are presumed to have been sheets of ice. The hoof prints and droppings of wild bur- 0 (0.17 kg) ros are abundant, and piles of burro drop- E'(1.8m) K' (12.4m) pings or individual clods had moved freely 3» across the wet playa surface, making shal- E (3.6kg) N (17.8 kg) low, short-lived tracks (Fig. 17). The low X'(8.5m) bulk density of this material probably pro- motes easy movement, and some may have been frozen when making tracks. Marks F' (40.9m) left by twigs and brush are distinctive o-o enough to be easily identified, even though K(1.7kg) X (0.43 kg) the object is usually gone. Broad striated swath marks (Fig. 18) up to 60 m wide and many tens of metres long H'(18.3m) are seen after episodes of winter activity, but, being shallow, they are not long endur- ing. Some swaths have irregular ridges of playa mud as much as 1 cm high along their leading edge. Along the playa shore, cur- vilinear accumulations of pebbles as- sociated with scrape marks are found. All of these marks and associated ridges are at- tributed to grounding of moving sheets of ice. The mud curls mentioned by the Kirks A (6.6 kg) F (4.5kg) (L. G. Kirk, 1952a, p. 177; R. E. Kirk, 1953, p. 323) may also be the product of Figure 11. Plots of tracks made by monitored stones between January 14 and March 17, 1969, scraping ice (Stanley, 1955, p. 1344). showing individual behaviors within general north-northeasterly trend.

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Figure 13. Abnormal track of stone I (Kristy) in winter of 1973-1974.

N13W \

r Jog \JP

N7W Figure 12. Geometrically similar tracks made by five selected stones in winter of 1973-1974. Stone weights in ' Jog parentheses, 1.7 to 17.8 kg. Track lengths 84.5 to 168.2 m. Single-barb arrow initial direc- S85W Start End tion and double-barb arrow net direction of movement. Maxi- mum separation on playa -î— —I 1 about 600 m. 10 16 20 meters

DURATION OF TRACKS

Most shallow tracks made by light ob- N4W. jects and small stones last less than a year, for they indent only the uppermost thin clay layer which is removed by wind after dry- ing, cracking, and curling. Larger monitored stones, to 18 kg, make tracks that survived 2 to 4 yr but seldom as long as 7 yr. The most deeply engraved part of the track made by a 320-kg stone is known to be more than 7 yr old and could sig- nificantly exceed a decade. Track duration reflects the condition of the playa surface when the track was made, and the weight Figure 14. Diagramma- and configuration of the object as well, as tic sketches of field relation- indicated by older segments within a track ships of small-stone con- which are more apparent than younger temporaneous tracks: (a) identical signatures in segments. which separation of stones remains fixed; (b) similar RESULTS OF THE signatures in which separa- OBSERVATIONAL PROGRAM tion of stones changes; (c) crossing tracks of stones Magnitude of Movement which could not have been frozen into a single ice Three principal episodes of movement sheet; and (d) tracks of three small stones showing occurred during the seven-yr observation, similarities, differences, and specifically in the winters of 1968-1969, changes in separation. - 1972-1973, and 1973-1974. Twenty-eight out of 30 marked stones moved, six in all three episodes. A few marked stones and some unmarked stones moved on two or three other occasions. Greatest single-episode movement, 201 m, and greatest cumulative movement, 262 m, were by stone H (Nancy), a small, 250-g, irregular cobble of syenite. Other stones, some considerably larger, had cumulative movements (Table 3) in excess of 150 m; examples are stone K (Daphne), a 1.7-kg pyramidal chunk of syenite with Approximate Scale a smooth triangular base, which moved

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episodes, with a cumulative distance of 152 m and a greatest single-episode movement of 99.5 m. Stone S (Dottie), an angular 24.3-kg block of dolomite moved 61 m in 1972-1973 before being marked, but thereafter it remained inactive. A nearby unmarked dolomite block of estimated 36 kg weight moved 27.5 m within the obser- vational interval. Most syenite stones have Figure 15. Tracks made by moved greater distances than many dolo- small pebbles in early 1969. Left pebble tumbled, slid, tum- mite stones. This could be due to the joint- bled; right pebble only tum- determined, smooth, flat bottoms of syenite bled. Pencil and notebook to blocks. However, stone R (Hortense), an ir- left of tracks. regular 13.2-kg chunk of dolomite, moved at least 250 m during the winter of 1970- 1971 before being marked. Individual stone movements occurring on occasions other than the major episodes in- clude that made by stone E (Peggy), which moved 45 m in the winter of 1970-1971, and the lengthy trip of then unmarked stone R (Hortense) that same winter. On occa- sions, some marked stones appear to have a total of 219 m, and stone A (Mary Ann), been jostled and rotated without moving a 6.5-kg rectangular syenite block, a con- out of their sitz sites, and a few others have sistent performer that covered 202 m dur- moved only 2 to 5 cm. This behavior may ing the three episodes. When first labeled, it locally reflect disturbance by tourists, but in lay at the north end of a 402-m track that remote locations, it appears to be natural. started near the south playa edge. Dates of movement can be determined The heaviest monitored stone to move only approximately by reference to storms was N (Alice), a 17.8-kg rectangular chunk and playa visits. Observations by Donald of syenite. It was active during all three W. Carney (unpub. data) fix movements during the winter of 1968-1969 within the following limits. Prior to the first significant winter rainstorm on January 14, 1969, the stones were in their established positions. When Carney visited Racetrack on Feb- ruary 28, several stones had moved. More rain fell on March 8, and when Carney re- visited the playa on March 17, he saw that these same stones had moved again. This two-stage movement is confirmed by sitz marks, changes in direction, and differences in degree of preservation of track segments. Sometimes a combination of storm dates and playa visitations permits a closer de- termination; the movement of (E) Peggy and (R) Hortense in 1970-1971 very prob- ably occurred on or about December 2, 1970, three days after a heavy rain on November 29. Movements in the winter of 1972-1973 seemingly occurred twice, pos- sibly in about mid-February and again in mid-March to late March. Extensive movements in 1973-1974 probably oc- curred in early mid-January and possibly again in early mid-March. Middle to late winter and earliest spring appear the most favored seasons for move- ment. During this investigation, no stone movement is known to have occurred fol- lowing summer thunder showers, but Her- Figure 16. Track made in early 1969 by 5-cm Figure 17. Striated shallow track made by bert R. Gercke (unpub. data), Ranger at sliding cobble which tumbled after impacting pile of burro droppings (near top) in early 1969. Grapevine Station from 1963 to 1966, re- smaller stone. Brunton on left. Dark glasses in track. ports summer movements. Stones do not

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move every time Racetrack Playa is wet. This was amply confirmed in October 1974 when heavy rains flooded the entire playa Stone Winter of Winter of Winter of Cumulative to a depth of more than 5 cm. Two weeks identification 1968-1969 1972-1973 1973-1974 (m) later, when largely dry, no evidence of stone (m) (m) (m) movement could be found. Weather during A (Mary Ann) 64.5 28.4 109.3 202.2 the interval between wetting and drying B (Ruth) 0 1.1 53.4 54.5 was calm and mild. Likewise, repeated wet- tings of the playa during the winter of C group 1974—1975 produced no movement of C, (Sue) 0 0.5 24.1 24.6 monitored stones. C2 (Sally) 0 0 23.5 23.5 C3 (Val) 0 0 15.9 15.9 Direction of Movement C4 (Marion) 0 8.9 7.6 16.5 D, (Ev) 4.3 0 0 4.3 Direction of stone movement is in- D2 (Irene) 4.9 0 0 4.9 fluenced by location on the playa, vector of 1970- •1971 the motive force, local conditions of the playa surface, and stone size and configura- E (Peggy) 1.8 45.1 0 Missing 46.9 tion. The bearing of a line connecting the F (Jean) 40.9 0.3 89 130.2 beginning and ending points of a track (A, G group Fig. 11) created during an episode of G, (Bep) 0 0 5.8 5.8 movement is called "net direction" and is a G2 (Milly) 0 0.6 20.6 21.2 useful parameter, for the actual track may G3 (Gerry) 0 0.9 25.3 26.2 consist of numerous linked segments of var- G4 (Jane) 0 0 20.4 20.4 ious bearings (Table 4). Cumulative net di- G5 (Carmen) 0 8.1 3.4 11.5 rection for all episodes of movement of a G6 (Margie) 0 0.9 32.6 33.5 monitored stone is easily determined from H (Nancy) 18.3 43 201 262.3 the first and last stakes set for that stone I (Kristy) 0 3.4 107.5 110.9 (Table 5). Since net direction is the product of J (Karen) 0 0 0 0 length as well as direction of individual K (Daphne) 12.4 38 168.2 218.6 L (Helen) 0 25.3 84.5 109.8 segments, it is more meaningful than the M (Clem) 0 27.4 17.7 45.1 mean of the various directions traveled. Net N (Alice) 21.6 31.2 99.5 152.3 direction for stone R (Hortense) in 1970- O (Grace) 15.2 3.9 72.3 91.4 1971 (Table 4) is N20°E, but individual reaches of the track depart as much as 63° 1970- 1971 from this bearing, and the mean is N22°E. R (Hortense) 250 0 0 250 Most monitored stones moved north- easterly during the seven-yr observation 1971- •1972 period (Table 5), which is consistent with S (Dottie) 61 0 61 earlier data (L. G. Kirk, 1952a, p. 175; T (Enid) 86.5 86.5 Stanley, 1955, p. 1336). Significant depar- X 8.5 0.6 4.6 13.7 tures from this habit were, on occasions, XX 0 3.7 3.7 XXX 0 0 0 displayed by stones lying just north of the

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TABLE 4. TABLE 5. NET DIRECTION OF MOVEMENTS, 1968-1974 EXAMPLE OF THE SEGMENTS WITHIN A STONE TRACK MADE BY STONE R Stone 1968-1969 1972-1973 1973-1974 Cumulative (HORTENSE) in 1970-1971. net direction

Direction Distance Remarks A N21°E N62°E N17°E N22°E (m) B n.m. N30°E N34°E N31°E Finish N10°W 1.8 C group n.m. c, S 5°W N23°W N77°E 2.3 n.m. C2 n.m. N 6°W

N23°W 7.6 n.m. C3 n.m. N 8°W N32°W 22.9 n.m. c4 S45°E N 6°W N25°W 12.2 N82°E n.m. n.m. N82°E DPAIR{D. N25°E 18.3 N80°E n.m. n.m. N80°E N45°E 10.7 N83°E 9.2 1970-1971 N25°E 5.5 N80°E 16.7 Convex south E N 5°E S70°E n.m. Missing N30°E 35.1 Convex east N18°E 13.1 F N 3°E N70°E N12°E N 7°E N38°E 33.6 G group n.m. G, n.m. N 9°E N23°E 21.3 n.m. G2 N52°E N 6°E N 2°E 16.8 n.m. G3 S71°E N 6°E N 7°W 22.9 Small offset n.m. G4 n.m. N1.5°W Fresh track 250 n.m. G5 S61°E N14°E G6 S72°E N21°E Fainter and probably older track H N26°E S58°E N25°E N36°E N17°W 9.2 N 9°W 33.6 I n.m. S63°W S81°W (loop) S80°W N 0°W 36.6 J n.m. n.m. n.m. N 4°E 30.5 Sitz mark N10°E 33.6 Very- faint, K N10°E S43°E N23°E N32°E 0 probably L n.m. S44°E N15°E N30°E still older M n.m. S53°E N 8°W N78°E Starting point N N21°E S58°E N19°E N35°E 7.6 South edge of O N11°E S40°E N25°E N24°E playa 1970-1971

south-shore bedrock ridge, as shown by the R N20°E n.m. n.m. N20°E two D stones in 1968—1969, stones in the G group in 1972-1973, and stones in the C 1971-1972 group in 1973—1974. Stones farther out on the playa and nearer the eastern shore, K, L, S N30°E n.m. n.m. N30°E M, N. and O (Fig. 7), experienced an un- T N 9°E N 9°E usual southeasterly movement in 1972- X N20°E S10°E N40°E N25°E 1973, which possibly also reflects a pertur- bation of the wind pattern by the south- XX n.m. N39°E shore bedrock ridge. XXX n.m. n.m. Individual episodes of movement usually Note: Net direction is the bearing of the straight line connecting the starting and stopping points differ somewhat in net direction. The on a track created during the interval specified, 1972—1973 event had a greater easterly, n.m. = no movement. and locally southeasterly, component than did the events of 1968-1969 and 1973- 1974. Neighboring stones often show a monzonite and white carbonate detritus, an influence on movement direction for small divergence in behavior, differences of the dark Cambrian dolomite fragments are stones lying in its lee. Scattered observa- 10° to 20° in net direction being com- clearly exotic. Dolomite fragments along tions suggest that net direction of stone monplace. This may be due partly to stone this shore are being buried in playa mud; movement on more northerly parts of the configuration, especially the basal part in burial depths of 4 to 11 cm have been playa may be more to the east and south- contact with the playa. noted. Stones farther out on the playa east. If fragments of Cambrian dolomite that (> 100 m) show no signs of burial and were derived from the south-shore bedrock presumably are still moving toward shore. Velocity of Movement ridge have moved predominantly north- Complete burial of stones that have ceased northeasterly, they should have accumu- moving may explain why Cambrian dolo- Some impression of velocity can be lated along the south side of a large fan mite fragments are not even more abundant gained from the following relationships. projecting from the east shore which inter- here. Brownish stains on the playa surface for- cepts such a path (Figs. 3, 7), Such stones Most stones on the southern part of the ward of traveled stones are attributed to a are indeed found there in modest abun- playa have moved northeasterly on most little excessively muddy water that gathered dance, the largest measuring 39 x 36 x 25 occasions, and local bedrock topography at at the leading edge of the stone and surged cm. As the projecting fan consists of quartz the south playa edge has seemingly exerted a short distance forward after the stone

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stopped. Low-compression collars at lead- ing edges of stones and unusual accumula- tions of playa mud on the outside of sharp curves are also seen. Stones with keels carve straighter tracks than keelless stones. Small sliding stones have impacted stones of simi- lar size and set them in motion without being stopped, and some small stones have flipped over upon stopping. These features suggest that the stones were moving at something more than a slow creep. As a guess, velocities may have attained 0.5 to 1 m/sec, so that a track 100 m long might have been created in a few minutes of con- tinuous movement. Irregular tracks made during an episode appear to reflect distinct changes in direc- tion of the motive force. Such movements could have occurred as a series of spurts separated by pauses long enough for a Figure 19. Ice crystal molds in playa mud. change in motive direction to occur. By contrast, most long linear or gently curving while gripped by wind-driven sheets of ice, track segments gave an impression of unin- and they may make tracks in the underlying terrupted stone movement. mud, but are all stone tracks on playa sur- faces necessarily made in this manner? The THE ROLE OF ICE following observations from the present in- vestigation suggest not. Figure 20. Cuspate ice-shove features on Winter temperatures at Racetrack Playa 1. On two occasions, stone X in 1968 - south shore, photographed April 21, 1969. often drop below 0°C. Molds of ice crystals 1969 and stone XX in 1973 - 1974 (Table in wet playa mud are abundant (Fig. 19), 3), a stone moved out of the corral, a circle and ice has been observed on parts of the of seven iron stakes, and off across the of essentially parallel contemporary tracks, playa with maximum local thickness of 7.5 playa, leaving a clear track. When stone XX all of different lengths, were made by to 10 cm (Stanley, 1955, p. 134; David moved, another stone of similar size (XXX) closely bunched stones which could not Stanwood, unpub. data). Most ice sheets of remained undisturbed within the corral. have been continuously gripped in a single any extent are probably thinner, as water These behaviors are not consistent with an sheet of ice. on the playa is seldom more than 2 to 5 cm ice sheet moving into the corral from the 4. Many contemporary tracks inscribed deep except where locally concentrated by outside or forming in situ and moving by small neighboring stones are indeed wind. away. Even if the force moving the sheet geometrically similar, as maintained by Stanley (1955, p. 1344-1345) described was great enough to drive it past the stakes, Stanley (1955), but most of those observed ice-shove features along the south shore, it would have been shattered and incapable in this study displayed one or more reaches and similar forms have been seen during of holding stones. of matching trends but disproportionate this investigation (Fig. 20). Wide scraped 2. Near the corral, there are two stones lengths (Fig. 14, b, d). Such deviations swaths on the playa surface are attributed of moderate size, D! (Ev) and D2 (Irene), would not be possible for stones firmly fro- to grounded sheets of ice, and occasional which initially were 117 cm apart. After zen into the same sheet of ice. Each stone blank segments within otherwise clearly moving approximately 4.5 m N80°E in would have to have its own sheet with continuous stone tracks may mark places 1968-1969, they came to rest in juxtapo- planimetric dimensions of only a metre or where a stone locally slid across ice. sition. This behavior and the crossing paths two. Stanley (1955) marshalled data and ar- inscribed (Fig. 21) would be unlikely if not 5. Often, only one or two stones within guments supporting the hypothesis that impossible for stones frozen into one or a closely associated group of five or ten of some tracks on Racetrack Playa were made more ice sheets of even modest extent. similar size have moved, leaving the others by stones gripped by wind-drven sheets of 3. In many instances, small stones, up to undisturbed (Fig. 15). ice with planimetric dimensions as great as 7.5 cm in diameter and initially close to- 6. Tracks made by stones frozen into a 90 by 300 m. David Stanwood (unpub. gether, have inscribed similar paths but end sheet of ice can cross, if the sheet pivots or data) has photographed a stone frozen in up with a different separation (Fig. 14, b, rotates (Stanley, 1955), but crossings with the ice on Racetrack Playa, but he did not d). The separation change was commonly the configurations sketched (Fig. 14, c, d) see it being moved or making a track. as much as 5 to 10 m after only 30 to 50 m are not possible. These tracks were clearly Charles F. Robinson (unpub. data) saw of travel; in one instance, it was 19 m. Sets contemporaneous. sheets of ice, perhaps 1 cm thick and 1 to 2 acres in extent, being driven by wind across Goldstone Playa in north-central Mojave Desert in the spring of 1942. Ralph Smythe (unpub. data) reported ice sheets, perhaps 2 to 4 cm thick with small infrozen cobbles, being moved across this same playa by wind, but he saw no tracks being made in the underlying playa mud. Stones can be moved across playa surfaces

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7. Some small stones have alternately occur within an hour or two after standing of some of the discrepancy between the re- slid and tumbled (Fig. 15). This would water forms on the playa; however, essen- sults of experiments and natural occurren- perhaps be possible for stones not tightly tially all fresh stone tracks are inscribed ces. Shelton's (1953) work with backwash gripped by the ice, loose in the socket so to into the layer of finest clay that settles from from an airplane propellor was very likely speak, but it is much easier if no ice is in- suspension considerably later. Tests made conducted on a playa surface from which volved. with fine clay collected at the edge of Race- the fine clay layer had earlier been removed 8. Changes in track width show that track Playa indicate a settlement time in by flaking and deflation. Coefficients of some stones rotate around a steep axis calm water on the order of 12 hr for sig- friction used in some calculations (W. E. while sliding. These stones were not firmly nificant clay deposition. Continuous rain or Sharp, 1960) may be too high because such gripped by ice. snow in this region usually occurs for 24 to determinations were made on material pos- 9. Contemporary tracks of stones weigh- 48 hr at most and is often followed by sibly not comparable to this ephemeral ing as much as 18 kg and separated by 0.6 strong winds, water does not usually stand fine-clay layer. Considering its geological km have similar configurations (Fig. 12), on Racetrack Playa for more than a few setting, Racetrack Playa should have a but the size of the ice sheet required to en- days to two weeks unless renewed, and the low-chloride, high-carbonate environment case them would far exceed anything postu- surface seems more slippery when the water which favors slower settling of clay and bet- lated by Stanley (1955). Differences in cover is just a thin residual film. These rela- ter alignment of particles, producing a more length, proportions, and other characteris- tionships suggest that stone movements are slippery clay layer (Langer and Kerr, 1966, tics of these tracks, and the lack of similar most likely to occur not much sooner than p. 293-294). configurations in tracks of intervening 24 hr and not much later than a week or The greater wind velocity required to ini- stones indicate that no such all-en- two after wetting is initiated. tiate movement than to sustain it is a matter compassing ice sheet was involved. of concern. Since markings show that in 10. Jogs in tracks (Fig. 12), probably THE CAUSE AND CONDITIONS situ stones experience slight displacement caused by sidewise lurching, display con- OF STONE MOVEMENT and rotation while occupying a sitz posi- siderable individuality and are not consis- tion, and since abrupt offsets of stone tent with the behavior of stones firmly fro- The conclusion drawn from this monitor- tracks at many sitz marks suggest that zen into ice. ing program and related observations is stones lurch out of the sitting position, 11. Detailed characteristics of individual that stones are individually skidded and rocking of sitting stones by abnormally tracks appear to be influenced by the basal rolled across the wetted surface of Race- powerful wind gusts is suggested as a means configuration of the inscribing stone. How- track Playa by wind, in spite of calculations by which they are set in motion. The partial ever, such configuration would seem inade- (W. E. Sharp, 1960) and comparisons perching of stones upon their own com- quate to influence the movement of sheets (Schumm, 1956) suggesting that this is un- pression collars of playa material many of ice. likely because of the unreasonably high facilitate this behavior. 12. Stone movements reported by H. R. wind velocities seemingly required. Wind- Once underway, the stones, judging from Gercke (unpub. data) to have occurred in driven sheets of playa ice are capable of their tracks, literally sail across the playa summer could not have involved ice. moving stones, but the relationships de- surface. The wind may exert something of a Stanley (1955) might attribute some of scribed indicate that significant stone lifting force on stones of larger sizes, which the above relationships to splitting of ice movements can occur without aid from ice may partly explain why experimental at- sheets and to temporary loosening and sub- sheets. tempts to push or drag stones across a wet- sequent regripping of individual stones, or Significant stone movements during three ted playa surface give unreasonably high to combinations of these effects. Ice sheets out of seven winters show that this is is not force values. Pushed or dragged stones tend can certainly split, but it is less clear that a a rare phenomenon. A direct relationship of to dig into the playa surface to depths stone once broken free can be reincorpo- movement episodes to wet stormy seasons greater than natural stone tracks. Wind- rated into the ice during the same episode of and years is clear. Movment occurs only tunnel experiments are more realistic movement. The behaviors described above when conditions are just right, and this ap- (Grove and Sparks, 1952), although they require such a complex manipulation of ice pears to involve the following. Wetting of yield results not particularly supportive of sheet splitting, loosening, and reincorpora- the playa surface is required to the extent the wind-propulsion hypothesis (Schumm, tion of stones as to lead to the conclusion that a thin, slimy, water-saturated mud 1956). The more easterly and southeasterly that the stones making the tracks described layer overlies a still firm base. This condi- movement of stones lying close to the were not frozen into sheets of ice when they tion is attained within an hour or two after south-shore bedrock hill, a lee-side effect, moved. The possibility that each stone had water gathers on the surface. Stained areas and the individualistic behavior of some its own little floe of ice or that the playa forward of some moved stones suggest that stones, seemingly related to their bottom surface was covered by a flexible mush of only a thin film of water lay on the playa configuration and perhaps their entire shattered ice fragments can hardly account surface at the time those stones moved. This shape, are consistent with the individual for all of the relationships itemized above. is consistent with observations that the propulsion of stones by wind. It is con- slipperiest condition observed occurs when cluded that wind moves the stones when The Time of Movement water remains only in cracks and as a thin conditions are just right, that this normally film on the surface. A drier surface or one happens at least every one to three yr on Stones clearly move when the playa is covered by 1 to 2 cm of water seems less Racetrack Playa, and that ice sheets are not wet, but at what period following wetting is slippery, at least to a person on foot testing required. movement most likely to occur? The playa sliding distances. surface becomes soft and fluid to a depth of Most stone movements take place long ACKNOWLEDGMENTS 1 cm within an hour, if a layer of standing enough after flooding for much of the finest water is maintained on it. Thickness of the clay to have settled from suspension. This The following persons have aided this in- soft muddy layer increased to 2.5 cm after fine clay layer may be an important factor vestigation: Donald W. Carney, Ranger, 24 hr of soaking. Thus, movement might facilitating movement and may be a source Grapevine Station; Dwight T. Warren and

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Peter G. Sanchez, Death Valley National Tunisia: Amsterdam, Holland, Holland Motts, W. S., 1969, Geology and hydrology of Monument Headquarters; and James R. Breumelhof, N. V., 293 p. selected playas in western United States: Lawrence and Michael G. Foley, students at Dionne, Jean-Claude, 1969, Tidal flat erosion by Amherst, Univ. Massachusetts Geol. Dept., California Institute of Technology. Useful ice at La Pocatiere, St. Lawrence Estuary: AFCRL-69-0214, 286 p. Jour. Sed. Petrology, v. 39, p. 1174-1181. Sainsbury, C. S., 1956, Wind induced stone data on Racetrack and other southwestern Droeste, J. B., 1961, Clay minerals in the playa tracks, Prince of Wales Island, Alaska: desert playas have been provided by David sediments of the Mojave Desert, California: Geol. Soc. America Bull., v. 67, p. 1659- Stanwood, John Cronin, Harold Ericsson, California Div. Mines and Geology, Spec. 1660. Ralph Smythe, Charles F. Robinson, Rept. 69, 19 p. Schümm, S. A., 1956, The movement of rocks by Harold D. Palmer, Herbert R. Gercke, Findley, Rowe, 1970, Death Valley: Natl. Geog. wind: Jour. Sed. Petrology, v. 26, p. 284— Major William Frazier, and Lyle Hoag. Mag., v. 137, p. 69-103. 286. Benefit was derived from constructive criti- Grove, A., and Sparks, P. W., 1952, Le déplace- Sharp, W. E., 1960, The movement of playa cal manuscript reading by Donald O. ment des galets par le vent sur la glace: Rev. scrapers by wind: Jour. Geology, v. 68, Doehring and William B. Bull. Géomorphologie Dynam., v. 3, p. 37-39. p. 567-572. Hattersley-Smith, G., 1969, Glacial features of Shelton, J. S., 1953, Can wind move rocks on Tanquary Fiord and adjoining areas of Racetrack Playa?: Science, v. 117, p. 438 — REFERENCES CITED northern Ellesmere Island, N.W.T.: Jour. 439. Glaciology, v. 8, p. 23-50. Stanley, G. M., 1955, Origin of playa stone Anonymous, 1952, The case of the skating Kirk, L. G., 1952a, Trails and rocks observed on tracks, Racetrack Playa, Inyo County, stones: Life Magazine, v. 32, no. 10, a playa in Death Valley National Monu- California: Geol. Soc. America Bull., v. 66, p. 53-54. ment, California: Jour. Sed. Petrology, p. 1329-1350. Blackwelder, Eliot, 1946, Evolution of desert v. 22, p. 173-181. Thompson, D. G., 1929, The Mojave Desert re- playas [abs.]: Geol. Soc. America Bull., -1952b, The Racetrack mystery: Westways, gion, California: U.S. Geol. Survey Water- v. 57, p. 1179. v. 44, p. 24-25. Supply Paper 578, 759 p. Bradley, H. C., 1963, Race track theories: Pacific Kirk, R. E., 1953, The moving rocks of Death Discovery, v. 16, no. 2, p. 24-26. Valley, Natural History, v. 62, p. 320-323. Clements, Thomas, 1952, Wind blown rocks and Langer, A. M., and Kerr, P. F., 1966, Mojave trails on Little Bonnie Claire Playa: Jour. playa crusts: Physical properties and min- Sed. Petrology, v. 22, p. 182-186. eral content: Jour. Sed. Petrology, v. 36, MANUSCRIPT RECEIVED BY THE SOCIETY AUGUST 1958, Geological story of Death Valley: p. 377-396. 28, 1975 Palm Desert, Calif., Desert Magazine Press, McAllister, J. F., 1956, Geology of the Ubehebe REVISED MANUSCRIPT RECEIVED APRIL 26, 57 p. Peak quadrangle: U.S. Geol. Survey Map 1976 Creutz, E. C., 1962, The racing rocks: Pacific GQ 95. MANUSCRIPT ACCEPTED MAY 19, 1976 Discovery, v. 15, no. 6, p. 24-26. McAllister, J. F., and Agnew, A. F., 1948, Playa CONTRIBUTION No. 2651, DIVISION OF Di Cesare, F., and Pratelli, W., 1967, "Moving scrapers and furrows on Racetrack Playa, GEOLOGICAL AND PLANETARY SCIENCES, stones" of the Tunisian Sahara (Bir Pistor), Inyo County, California [abs.]: Geol. Soc. CALIFORNIA INSTITUTE OF TECHNOLOGY, in Guidebook to the geology and history of America Bull., v. 59, p. 1377. PASADENA

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