Abstract The engraved trails of rocks on the nearly flat, dry mud surface of Racetrack Playa, Death Valley National Park, have excited speculation about the movement mechanism since the 1940s. Rock movement has been variously attributed to high winds, liquid water, ice, or ice flotation, but has not been previously observed in action. We recorded the first direct scientific observation of rock movements using GPS-instrumented rocks and photography, in conjunction with a weather station and time-lapse cameras. The largest observed rock movement involved >60 rocks on December 20, 2013 and some instrumented rocks moved up to 224 m between December 2013 and January 2014 in multiple move events. In contrast with previous hypotheses of powerful winds or thick ice floating rocks off the playa surface, the process of rock movement that we have observed occurs when the thin, 3 to 6 mm, “windowpane” ice sheet covering the playa pool begins to melt in late morning sun and breaks up under light winds of ∼4–5 m/s. Floating ice panels 10 s of meters in size push multiple rocks at low speeds of 2–5 m/min. along trajectories determined by the direction and velocity of the wind as well as that of the water flowing under the ice.

Citation: Norris RD, Norris JM, Lorenz RD, Ray J, Jackson B (2014) Sliding Rocks on Racetrack Playa, Death Valley National Park: First Observation of Rocks in Motion. PLoS ONE 9(8): e105948. https://doi.org/10.1371/journal.pone.0105948 Editor: Vanesa Magar, Centro de Investigacion Cientifica y Educacion Superior de Ensenada, Mexico Received: March 20, 2014; Accepted: July 29, 2014; Published: August 27, 2014 Copyright: © 2014 Norris et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: The authors confirm that all data underlying the findings are fully available without restriction. All relevant data are within the paper and its Supporting Information files. Funding: This work was supported by National Aeronautics and Space Administration NNX07AL32G and NNX12AI04G (to RDL, BJ); Contributions from Interwoof (JMN, JR) and Scripps Institution of Oceanography (RDN) were self-funded. NASA provided support in the form of salaries for authors (RDL and BJ), but did not have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. The specific roles of these authors are articulated in the ‘author contributions’ section. The Commercial Firm Interwoof likewise provided salary support to JMN and JR for the study, but did not otherwise play a role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: Funding from Interwoof does not alter the authors' adherence to PLOS ONE policies on sharing data and materials.

Introduction Racetrack Playa in Death Valley National Park, is well known for the phenomenon of tracks left by hundreds of rocks plowing across the nearly flat playa surface (Fig. 1). Rock movement by pebble to boulder-size pieces of dolomite and granite leaves tracks in the playa surface showing the direction of motion via groves cut in the playa mud. Remarkably, multiple rocks commonly show parallel tracks (Fig. 2), including apparently synchronous high angle turns and sometimes reversals in travel direction [1], [2], [3], [4]. The phenomenon of rock motion has excited considerable interest, and there is a scientific and popular literature extending back to the first report in 1948 [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. Since then, theodolite mapping surveys, repeat photography and, most recently, the use of high resolution submeter GPS to map the rocks and their trackways have shown that the rocks move very episodically, often with no motion for several years to a decade or more [1], [2], [3], [4]. Various mechanisms for rock motion have been proposed, but owing to the harsh nature of the playa surroundings, and the difficulty of access, there has been no unambiguous determination of the mechanisms for rock motion. PPT PowerPoint slide

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larger image TIFF original image Download: Figure 1. Time lapse images of a moving rock. Image acquired with a handheld digital camera on January 9, 2014. Image on the left shows the wide-angle view; interior black frame indicates the view in other frames. In close-up frames, blue arrows show stationary rocks and red arrow—a rock in motion (moving from left to right). Total movement lasted ∼18 seconds. Dark, flat areas on the pond are panels of ∼3 mm thick ice surrounded by rippled water several centimeters deep. Ice thickness estimated from inshore ice panels. Broken ice panels accumulated on the upstream side of the moving rock in the last two images. Images have been cropped but not otherwise edited. https://doi.org/10.1371/journal.pone.0105948.g001 PPT PowerPoint slide

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larger image TIFF original image Download: Figure 2. View from the ‘source hill’ on the south shore of Racetrack Playa. View is looking north on December 20, 2013 at 3:15 pm. Steady, light wind, 4–5 m/s has blown water to the northeast exposing newly formed rock trails. Lower image shows overlay of lines to emphasize the congruent shape of adjacent rock trails as well as the proximity of rock trails to rocks that did not move. Image has not been enhanced. https://doi.org/10.1371/journal.pone.0105948.g002 All authors agree that rocks are most likely to move when the playa surface is wet, creating a slick surface, and that wind must be involved. The first scientific study of the Racetrack suggested that rock motion was driven by dust devils [5]. This idea was tested using the wash of an aircraft propeller over wetted surfaces of Racetrack Playa [13]; these experiments showed that winds more than 20 m/s could move natural rocks. Shelton [13] suggested that other factors, including the presence of algal films might help to lower the frictional forces resisting rock motion under strong winds. W. Sharp [14] carried out static and dynamic friction tests using rocks towed across wet and dry mud surfaces and calculated that wind velocities of 33–45 m/s would be needed for rock movement. Additional calculations for rocks of various sizes and sail heights showed that most rocks would move across a wetted playa surface where the coefficient of static friction is about 0.15 and wind velocities were >40 m/s [7]. Still other static friction tests suggest the need for even higher wind velocities (up to 80 m/s), particularly to move rocks with relatively low profiles [2]. All these experiments suggest that very high winds are needed for rock movement. Other authors, led first by Stanley [4], argued that rocks are frozen into sheets of ice that reduce the friction with the underlying lake bed and increase wind drag [4], [8], [10], [11]. Most of these authors also note that multiple rocks can follow almost identical tracks, suggesting that they were moved while frozen onto a large layer of ice floating on liquid water. Reid et al. [2] made extensive observations of rock trails and showed parallel movements between rocks up to 830 m apart, implying very large sheets of ice. These authors also noted that parallel trails can involve rocks of different sizes that usually do not rotate or tumble during movement—both observations that suggest ice, rather than wind alone, is responsible for rock movement. It has also been noted that rocks encased in ice can actually partly float off the surface of the playa mud leaving shallower tracks than would be expected for a rock moving by wind alone across a muddy surface [8], [11]. In a test of the ice sheet hypothesis, R. Sharp and Carey [3] performed a now famous “corral” experiment, in which they drove a series of stakes into the playa surface around several rocks. The goal was to test whether the rocks would move independently as might be the case for wind-driven movement in the absence of ice. One rock moved out of the corral during the next winter while another rock remained inside the circle of stakes—a pattern Sharp and Carey [3] interpreted as evidence that ice is not the driving mechanism for rock motion. Finally, Messina and Stoffer [1] mapped the locations of the rocks and traced the visible trails using submeter differential GPS. Although there are broad similarities in the tracks of many rocks, deviations in trails suggest that the rocks were likely moving independently of one another rather than being propelled by a single ice sheet [1].

Conclusions A necessary condition for the rock motion we observed is the existence of a playa pool deep enough to submerge the southern section of the playa, yet shallow enough to leave many rocks partly exposed at the pond surface. Other repeating features of rock movement events that we observed include the presence of floating ice, temperatures and sunlight sufficient to create melt pools in the ice, and light breezes that are steady enough to drive floating ice. Although the ice breaks up around rocks, even thin moving ice sheets can generate sufficient force to drive rocks across the pool. All observed rock movement events occurred near mid-day when sufficient ice melting had occurred to allow ice break-up. Creation of rock trails is difficult to observe because trails form below the ice-covered pool surface where they are often not evident until the ice has melted, and liquid water has been removed. In addition, rock movement is slow and relatively brief—our GPS instrumented stones traveled at speeds of 2–5 m/minute for up to 16 minutes—so casual observation is likely to miss rocks in motion. Weather station data show that the freezing temperatures necessary for ice formation, and winds in excess of 3–5 m/s are common phenomena at Racetrack Playa during the coldest few weeks of winter. Therefore, the extremely episodic occurrence of rock motion (years to decades) is likely due to the infrequency of rain or snow events sufficient to form winter ponds.

Supporting Information Table S1. Weather data collected from Racetrack Playa, Death Valley National Park. Data period: Nov-20-2013 to Jan-9-2014. Records of hour-total rainfall (column 2), as well as hourly average insolation (column 3), air temperature (column 4), and wind velocity (column 5) with the time stamp given in column 1. The record of maximum wind gust strength (in column 7) is calculated to the nearest minute with a time stamp given in column 6. Station located at N36.6823, W117.5515. Instrument package specifications reported in the text and table header. https://doi.org/10.1371/journal.pone.0105948.s001 (CSV) Table S2. Movement data for GPS-instrumented rocks on Racetrack Playa, Death Valley National Park. Data obtained for three rocks (A3, A6, and A11) that recorded position and velocity. For each rock, movement data are date (column 1), time stamp (to nearest second UTC, column 2), latitude (degrees, column 3), longitude (degrees, column 4), and velocity (m/minute, column 5). Rocks A3 and A6 had total trail lengths longer than recorded by their GPS instrument packages (Table 1), showing that they moved at least one more time after their GPS batteries had been depleted. GPS instrument packages are custom designed units by Interwoof. https://doi.org/10.1371/journal.pone.0105948.s002 (CSV)

Acknowledgments RDN and JMN shared equally in the conception of the study, interpretation of the phenomenon, and drafting of the text. RDN and JMN observed rock movement in-situ on Dec 20/21; JMN and RDL observed movements on Jan 9. JMN and JR designed and built the GPS loggers and performed data reduction on GPS logger and weather station data. The weather station was provided by the UC Natural Reserve System, Sweeney Granite Mountain Desert Research Center; we thank reserve directors, Jim Andre and Tasha La Doux. RDL and BJ contributed long-term time lapse time series to interpret the history of snowfall and the history of the resulting pond, funded under NASA grants NNX07AL32G and NNX12AI04G (both to RDL). RDL and BJ contributed to interpretation of the phenomenon and writing of this manuscript. We particularly thank NPS Ranger C. Callahan for his assistance in setting up the experiment. Thanks also to R. X. Crane and M. Hartmann for assistance during equipment service and deployment visits. Interwoof provided support in the form of authors' salaries and research materials. We acknowledge the large number of members of the “Slithering Stones Research Initiative” who contributed effort and enthusiasm to the study. Membership of the “Slithering Stones Research Initiative” includes the authors, and, in alphabetical order: Jim Andre, Robert Brown, Dianne Cox, Russ X. Crane, Aaron Dodson, Betsy Dodson, Robert Dodson, Roger Eggers, Ken Ethier, Matt Forrest, Lauren Freeman, Simon Freeman, Maggie Fusari, Dennis Galloway, Denis Goodwin, Mary Goodwin (dec), Mike Hartmann, Peter Hartmann, Betty Johnson, Pete Johnson, Tom King, Arden Kysely, Tasha LeDoux, James Matheson, Susan Matheson, Jeff Mcfarland, Jon Miller, Ariel Norris, Ben Norris, Christine Norris, Don Norris, Philly Norris, Robert Norris (dec), Teresa Norris, Tom Norris, Virginia Norris, David Nye, Elvia Nye, Bill Ortendahl, Lori Rafferty, Jane Ray, Dave Romer, Mark Saunders, Caitlin Scully, Robert Sengebush, Robert Sloan, Bruce Tiffney, Curtis Wathne, Kail Wathne, Tatsu Yamaguchi.

Author Contributions Conceived and designed the experiments: RDN JMN RDL JR BJ. Performed the experiments: RDN JMN RDL JR BJ. Analyzed the data: RDN JMN RDL JR. Contributed reagents/materials/analysis tools: RDN JMN RDL JR BJ. Contributed to the writing of the manuscript: RDN JMN RDL JR BJ.