Richard D. Norris; James M. Norris; Ralph D. Lorenz; Jib Ray; Brian Jackson

“ Ice panels also stack up on the upstream side of large rocks, increasing the effective surface area of rocks exposed to upstream ice as well as water flowing under the ice (Fig. 8a) . The splintering of ice sheets can instantaneously decouple the movement of rocks, and may explain the movement of one rock and not another in Sharp and Carey's ‘corral’ experiment in which one rock moved out of a circle of stakes driven into the playa surface while one rock remained behind [3] . ”

“ 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. ”

“ Error analysis shows that for each rock, the velocity uncertainty is generally <0.3 m/minute (Fig. 7) . Rock velocities are consistent with time-lapse images and observations on December 21 and January 9 (Fig. 1) . In situ rock movements were detected by observing the position of moving rocks relative to stationary stones. However, the low velocities involved make it difficult to detect movement events by casual observation. Rock trails are formed under the ice, and become visible only when the muddy water is blown away by light winds. ”

“ These contrasts in rock behavior are consistent with observations that fractures in the ice can decouple the movement histories of rocks only tens of centimeters apart, while congruent movements spaced many 10 s of meters apart can sometimes occur. The floating ice sheets at breakup are initially very large, so congruent rock trails may reflect motion early in the move event before large ice panels become fragmented, as proposed by Reid et al. ”

“ Indeed, there was a stake just upstream of the unmoving stone in the Sharp and Carey experiment that may have shattered a moving ice sheet before it encountered the stationary rock. Floating ice may sometimes be ineffective at moving rocks since stones with low profile may be over-ridden by floating ice; rocks at the edges of ice panels may fail to sufficiently engage with floating ice to be moved, and rocks may be too massive for the available force (Fig. 8d) . ”

“ Time lapse cameras were installed under NPS studies DEVA-00169 and DEVA-00341.
Map of Racetrack Playa.Map shows the locations of the weather station, GPS-carrying rocks, and approximate locations of the northern shoreline of the pool and Figures 1 and 2. The weather station was obtained from the Sweeney Granite Mountains Desert Research Center (part of the University of California Natural Reserve System) and continuously recorded wind speed at 1 second intervals, along with temperature, insolation, rainfall, and GPS location. ”

“ The weather station and GPS-instrumented rocks were installed under Wilderness research permits DEVA-65173, DEVA-2012-SCI-0021, DEVA-2011-SCI-0047 and DEVA-2010-SCI-0023 from Death Valley National Park. ”

“ Racetrack Playa phenomena.Parts show: (a) thin windowpane ice over-riding a recently moved rock (January 9, 2014) , (b) water creeping onto the low gradient northern shore of the pond during the December 20, 2013 rock movement (∼11:15 am) , (c) ice windrows on the eastern shore of Racetrack Playa near the weather station (∼1 pm, December 20, 2013) ; ice panels are ∼2–3 mm thick and mixed with mud and rocks scoured from the pond bottom, (d) rock carving a wake through ice that is moving left to right ”

“ This location is along the trajectory of the longest rock trails mapped by Messina and Stoffer [1] . The weather station was equipped with Campbell Scientific CR-800 Data Logger, CS LI200X-LC Pyranometer, CS HMP35C-LC Temp and RH probe, CS GPS16X-HVS-PW, and a R.M. Young 05103-5 Wind Monitor. Rainfall collection was made with a Campbell Scientific TE525-LC Tipping Bucket Rain Gauge; note that this device was not equipped to measure snowfall or snow-water equivalents. Wind speeds were measured at 1 second intervals explicitly to determine the peak velocity of gusts. ”