20,000 Kilometers Over Death Valley: The GPS View of the
Sliding Rocks
Paula Messina
Geology Department
San José State University
0 200 Km.
California, Nevada
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CA NV
Death Valley National Park
0 400 Km.
N
Racetrack Playa: Physical Setting• +1131 meters
• ~4 km. (north-south); ~2 km. (east-west)
• Surface: sand, silt, clay; desiccation polygons
February, 2001: Seven Inches of Snow
Grandstand
Racetrack Road
Ditch
Ponding WaterSeptember, 1997
Sliding Rocks
Why Do Rocks Slide?
• Ice rafting?
• Wind alone?
What is the Complete Network?
• Does non-parallelism among trails imply wind alone?
• How can all the trails be mapped?
Does a Rock’s Character Contribute to its Activity?
• Do rounder rocks inscribe more-sinuous trails?
• Do larger (more massive) rocks produce shorter trails?
Global Positioning System to the Rescue
• 24 satellites• 20,000 km. (12,000
mile) orbits• 24 hour global
coverage• Sub-meter accuracy
• Full operational capability: April 1995
How Does GPS Work?
Mapping Mission
• July 16 - 23, 1996
• 162 rocks/trails
• Two field mappers
• Human “digitizers” walked over 100 km.
Cartographic Entities
Points Lines
Areas
• In-field Data Download
• Post-processed differential correction
• Export to ArcView GIS Shape File format
• Export to MS Excel for Quantitative Analyses
Final Map
• Predominant motion: SW-NE (slightly uphill)
• Trail length f (rock size)• Trail straightness f (rock shape)• Parallelism is
the exception; not
the rule.
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Inferred Wind Rose
Plots Showed No Statistically-Significant Correlations
• Trail length appeared spatially controlled– (see following slides)
What Factors Control A Rock’s Ultimate Distance Traveled?
Image and trail map for Diane,the rock that inscribed the longest trail
What Factors Affect Trail Straightness?
• Straightness=Total Trail Length
StartEnd Distance
• Trail straightness appeared spatially controlled
What Factors Control A Rock’s Ultimate Path?
Crooked trails Straight trails
What Factors Control A Rock’s Ultimate Path?
• GIS-Generated Aspect Map
Intervisibility: Ray Tracing• Cross-section:
• Map View:
Intervisibility Maps for Representative Rocks
Trail Character Correlates to Surrounding Terrain
The Racetrack: A Mosaic of Microclimates
• Wind tunnels
• Turbulence
The Final Word: How Can Rocks Converge?
More Information on the Sliding Rocks:
Paula Messina’s Racetrack Web Site
Smithsonian magazine: July, 1999
California Geology: January, 2001
“Savage Planet” Episode IV: “Extremes”
Discovery Channel Online’s Dancing Rocks Web Site
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