The Mystery of 'Moving Rocks'
Written on 11:45 PM by Mrudula
The reason behind 'Moving Rocks' which move long the 10,000 year old dried up clay lake bed in 'Death Valley' is still somewhat of a mystery.The 'Moving Rocks', also referred to as 'Sliding Rocks' or 'Sailing Stones'.
-->
Tracks are often tens to hundreds of feet (low to high tens of meters) long, a few to 12 inches (8 to 30 cm) wide, and typically much less than an inch (2.5 cm) deep.
A balance of specific conditions are thought to be needed for stones to move:
* A saturated yet non-flooded surface,
* Thin layer of clay,
* Very strong gusts as initiating force,
* Strong sustained wind to keep stones going.
In 1955 George M. Stanley first proposed the theory that the rocks move with the assistance of ice sheets forming after the playa surface is flooded. There have been times when the playa is flooded with standing water up to 7 cm deep and temperatures commonly drop below freezing at the Racetrack in the winter and early spring.
In 1976 Robert Sharp and Dwight Carey diputed the ice-sheet theory. They analyzed the tracks and concluded becasuse of track characteristics and the geometries of the tracks relative to each other that ice sheets could not have been involved in forming the tracks and moving the rocks.
Sharp and Carey also did an experiment using a circle of stakes around rocks to test the ice-sheet theory. At one time the one rock within the stakes called 'The Corral' moved out leaving a distinct track; another time two rocks were placed inside the corral and the smaller rock moved out while the larger remained within the stakes. Sharp and Carey concluded that this would be an impossible result if ice was necessary for moving the rocks.In 1976 they stated, "It is concluded that wind moves the stones when conditions are just right, that this normally happens at least every one to three years on Racetrack Playa, and that ice sheets are not necessary."
In 1995, John B. Reid, Jr. and other geologists from Hampshire College disagreed with Sharp and Carey's conclusion. Using data from seven Death Valley visits in the late 1980s through 1994, they support Stanley's original ice sheet hypothesis. Reid et. al. compared tracks and the similarities of tracks and doing careful mapping saw that most of the tracks formed during the 2 major movement events had congruent paths. Some tended to deviate near the end of the trail and they justified this by explaining that the ice sheet would break into smaller pieces as it melted and the imbedded rocks could separate
Reid et. al. conducted friction coefficient experiments and determined that the friction of the playa surface was so high that impossible wind speeds were necessary for wind alone to move the rocks on the playa surface. Reid also tride to explain the movement of the rock out of Sharp and Carey's corral with a breaking ice sheet theory. Reid et. al. concluded that, "Because of the strong congruence of widely spaced tracks and the rocks' high resistance to sliding in mud, we have concluded that ice sheets are necessary for Racetrack rocks to slide."
This was followed by a Sharp and Carey, Reid et. al. discussion and reply in 1996. Sharp and Carey restated points about rock tracks, such as angular rocks travelled in a straighter line and rounder bottomed rocks tended to wander more with out "keels" to keep them straight; if ice sheets were involved characteristics on the bottom of the rock would have no effect on the trail. They also pointed out the criss-crossing trails and other geometries that would be difficult to explain with ice sheet movement. Sharp and Carey restated an earlier made point that the friction experiments made neglected to take into consideration a fine clay layer that forms on the playa surface after flooding, but is blown off quickly after drying and flaking. This clay layer significantly lowers the friction on the playa surface. They also disagree with the breaking ice sheet theory explaing the rock movement out of the corral. Their 1996 statement was, "Our conclusion is that both wind-driven ice and wind alone can create stone tracks on playas."
John Reid et. al. in 1996 replied to Sharp and Carey's discussion with agreement and revised their 1995 conclusion to, "All said, we agree with Sharp and Carey's assertion that two seperate mechanisms must exist for Racetrack sliding. Are there others?"
Research of the Racetrack has continued. In the April 1997 GPS World, Paula Messina, Phil Stoffer and Keith C. Clarke reported a GPS study they conducted of the Racetrack. In ten days of intense field work they mapped every featured of the playa using differential GPS to produce, "the first-ever, complete, georeferenced, submeter-resolution map of the wandering rocks."
Future analysis with new technology may shed more light no the mystery of the moving rocks at the Racetrack Playa of Death Valley; however no matter how much is determined about the moving rocks, their beauty and magic will always be impressive and mysterious to those who travel the treacherous gravel roads to visit the incredible dried lake bed.
- Munnu
