These images are from the November 1968 edition of the now sadly defunct Desert Magazine, a special on Death Valley. The “Riddle of the Racetrack” refers to the enduring mystery of the “sailing stones” of Racetrack Playa, the remote dry lake in the northern part of the valley, and begins:
OFF the beaten path in the northwest corner of Death Valley National Monument lies a hidden valley—and a mystery. The valley contains a dry lake approximately one-and-a-quarter miles wide and three miles long. The Racetrack Playa at first glance appears like any other of hundreds of such dry lakes in the southwest.
It has one different and mystifying feature; rocks and other objects on its surface have been known to shift, move and skate about! No one has actually seen any of these objects move but the tracks left from such movement are obvious.
There are many theories explaining the phenomena. Some say it has to do with the earth's magnetism, while others claim it is related to the sunspots. Still others suspect the gravitational pull of the moon producing an effect similar to the ocean's tides. Under scientific examination, however, most of these theories can be dismissed.
Sunspots and magnetism are only a couple of the conjectures for the motivation of the sailing stones - the supernatural, aliens playing chequers (sorry, ‘alien tractor beams’) and teenage pranksters have also featured. But, while the debate has continued ever since the mystery was first observed a century ago, the preferred explanations are entirely natural. However, that debate is now – largely – settled, thanks to the work of a team led by Scripps Institution of Oceanography/UC San Diego palaeobiologist Richard Norris in collaboration with Ralph Lorenz, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory.
In a video accompanying the description of the work, Norris relates how they conducted what one of his colleagues described as “probably the most boring experiment ever.” Now, this is surely open to discussion. Experiments set up in the 1930s and 40s to record the fall of a drop of pitch (average wait eight years, and winner of the Ig Nobel Prize for physics in 2005) must vie for the honour. But it’s certainly true that the Racetrack Playa project required patience and modern technology. The Desert Magazine article commented that
It appears that the mystery will remain unsolved until some hardy soul camps at the playa's edge all winter, and waits with a movie camera for the action to begin. Any volunteers?
The Scripps team had the advantage of sophisticated weather stations, telemetry, time-lapse cameras, and GPS, so they could get things set up and leave, returning periodically to see what might – or might not – have happened. It took six years.
The winter season was clearly critical: decades of occasional observations recorded rocks being in different places from where they had been before the winter, all apparently having ploughed up the distinctive furrows in the mud of the lake bed, and some being truly huge.
The mysterious winter activity had long stimulated theories of the moving force being ice. This may be the same location of searing summer desert heat, but the playa lies at an elevation of over 1100 metres, and the winter nights are cold: any water that accumulates will freeze and, perhaps, the ice will shove the rocks around. The potential role of ice had been treated seriously over decades of research, including an important study published in the Bulletin of the Geological Society of America in 1955 (I have a PDF if any reader is interested). The author, George M. Stanley, meticulously measured the tracks of multiple stones (including an 11 kg example that had travelled 265 metres), together with the movement of ice. Although he did not directly witness any activity, his conclusions were as follows:
(1) Certain groups of stone trails on Racetrack Playa have identical signatures over areas nearly 500 feet across. They must have been formed by blown ice floes which held numerous small stones or scribers in generally fixed positions, and which rotated during travel.
(2) In several cases a single, moving ice floe released a small stone and picked it up again after a few feet of travel.
(3) Many tracks favor origin by ice floes blown across the playa by wind rather than by wind blowing the lone objects.
(4) Mathematical treatment applied to stone trails within a signature group demonstrated that the scribers moved in accordance with trigonometric relationships between separated loci in a rotating, planar body.
(5) There are ice ramparts and other signs of ice-floe action on Racetrack Playa, aside from the stone trails.
In other words, he saw ice as the motivating force, but most likely picking up the stones, blowing across the playa, and dumping them on melting There are tracks with no stone at the end, and this would seem a reasonable explanation, but Stanley noted that even lightweight objects such as the droppings of wild jackasses also created tracks and that such biodegradable materials might leave no trace. Stanley suggested that the high altitude of Racetrack Playa, and therefore the common occurrence of winter ice, explained why this phenomenon was so much more common there than on other, lower, desert playas. But he did note other, isolated examples, and a further fascinating piece of evidence in support of the power of ice on desert lakes:
On December 3, 1952, the central transcontinental telephone line east of Reno, Nevada, was suddenly disconnected for unknown causes… and a repair crew was sent out to find the difficulty. After following the line eastward from Reno the crew came to Toulon Lake, a part of Carson Sink covered with a year-round body of shallow water…
At the lake, partly ice-covered, the repair crew discovered that 3000 feet of line was missing; this included 20 poles loaded with 4 arms and 40 wires on 150-foot spans. Some poles had been set in caissons of corrugated metal 10 feet in diameter and 4 feet high, filled with boulders to protect the poles against waves on the lake.
Ice had formed for 3000 feet from the west shore and 2 or 3 miles along it, with open water to the east; the ice was 4 inches or more thick and floated on about 2 feet of water. Emergency repairs were made by dragging an insulated cable across the ice with a propeller-driven ice boat. The uprooted poles and remnants of wrecked caissons were found 300 feet south of the original position of the line. After two days the wind reversed direction, and the ice started to move north, tearing out the emergency repairs; it continued to move north under the influence of a 30-mile-an-hour wind a total distance of 700 feet and left the wrecked poles and caissons 400 feet north of the original line. Ice was pushed up several feet on the east shore.
This is really quite entertaining: another instance in which the severing of telephone cables revealed geological phenomena. It was the sequential cutting of the transatlantic submarine cables on the Grand Banks off Newfoundland in 1929 that led to our understanding of the processes and power of turbidity currents…
Investigations at Racetrack Playa continued, and, in the 1970s, Robert Sharp, then a geologist at the California Institute of Technology, and Dwight Carey, a geology student at the University of California, Los Angeles, labelled large numbers of rocks with, for some reason or another, women’s names and tracked their positions over the years. As the National Geographic reported:
Hortense (R) moved 820 feet (250 meters) in one winter. Karen (J), a 700-pound (320 kilograms) rock at the end of a 570-foot-long (174 meters) track, didn't move at all during their seven-year study and disappeared years later. Karen showed up again in 1996, when Paula Messina, a geologist at San José State University who had been mapping the paths of all the sliders on the Racetrack, found her far north of where Sharp had last seen her. "When I told him I had positively identified several of his original rocks, his reaction was a little like one would expect from a man who was just told I found his children."
But still, while plenty of witnesses had reported the before and after positions of many of the stones, nobody had actually caught them in the act – until now. Early this year, the Norris team’s patience was finally rewarded. As Lorenz reports in a New Scientist article this week:
On a Sunday night in January, a park ranger forwarded an email reporting that a tourist had seen the rocks moving. I dropped everything and went to Death Valley. There I met the Norris team, who had also seen movement. We drove up to the playa, which was partly flooded and frozen over, to retrieve our instruments…
We were standing on the cliffs, watching the morning sun melt the thin floating ice sheet, when it happened. A gust of wind, no more than 4 metres per second or so, picked up. And then we heard the crack, and saw the ice sheet slowly glide, bulldozing some rocks along and leaving others.
The action – sedate and sporadic, lasting a total of around 18 seconds – was captured in a series of time-lapse images (blue arrows, stationary rocks for reference, red arrow, the moving rock; darker areas are ice):
The team had developed small GPS devices which they embedded in a selection of rocks at the beginning of the experiment, and so the long-term data could be collected – “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.”
But one of the remarkable things about all this is that the rocks are moved by thin ‘windowpane’ ice, unlike the thick stuff that caused the destructive events on Lake Toulon. Nevertheless, some rocks achieved velocities of ten centimetres per second and travelled 60 metres. The results have been compiled into this helpful graphic in the New Scientist article:
The work is reported in Plos One, is open-access and freely available online – it makes for a fascinating read. Herewith, a captivating image from that paper, together with the description:
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.
[Thanks to Hans Begrich and Walter Vogelsberg for giving me the ‘heads-up’ on this work. It has been widely reported elsewhere (linked in the above), including on other geo-blogs, but I thought that a little of the historical background would be of interest. Wikipedia also has a good overview of the ‘sailing stones’ mystery, and, for fascinating videos etc. of this new project, the Scripps site is well-worth visiting. Image of boulder and track, James Gordon, Attribution-NonCommercial 2.0 Generic (CC BY-NC 2.0), https://www.flickr.com/photos/james_gordon_losangeles/8440177460/in/photostream/]
