The sliding rocks of Racetrack Playa, Death Valley National Park, California: Physical and spatial influences on surface processes

摘要

The Racetrack is a dry lakebed nestled within the Panamint Range in Death Valley National Park, California. Though almost perfectly flat, it shows evidence of dynamic traction events of boulder-sized and smaller rock fragments that tumble onto it from two abutting cliffs and surrounding alluvial fans. Scars of sliding rock activity in the form of recessed furrows have been noted since the beginning of the twentieth century, yet to date no one has witnessed the actual surface process causing the rocks to slide.;The location of every rock and its associated trail was recorded using Differential Global Positioning System (DGPS) and Geographic Information System (GIS) methods in July, 1996. The resulting map shows a total of 162 rocks and trails to a horizontal accuracy of about 30 centimeters.;Examination of trail patterns shows an inferred general trend in rock movement toward the north-northeast. This is consistent with the direction of prevailing winds. However, there is a high degree of variation in trail character. Surprisingly, trail lengths and headings are not well correlated with rock shape, volume or surface-contact area.;Analyses conducted on the digital data set show that large rocks tend to produce shorter, straighter trails. However, a rock's total distance traveled and the degree to which it follows a straight-line path is more significantly influenced by its location on the playa at the onset of motion.;The Racetrack's southeastern sector, about 5 centimeters lower in elevation, is more frequently saturated by collecting rain water. In addition, the presence of three natural springs may contribute to lower friction coefficients in this region. The longest and straightest trails are preferentially distributed in the southeast, as rocks are propelled by the amplified force of horizontal winds when air is channeled through one of two topographic corridors to the south. The central part of the playa is a focal point for two such natural wind tunnels. In this part of the playa, trails are most convoluted, suggesting entrainment of rocks in wind vortices. This research quantifies the influence of the surrounding topography on inferred airflow, ultimately governing the nature and magnitude of sliding rock episodes.