91av

I DIDN’T imagine it would be like this. A breath of wind, a crack and then it happened. After seven years of sleuthing, the mystery I’d been pursuing resolved itself instantly before my eyes. Exalted, I rushed down to the edge of the lake bed to watch more closely – slowly but surely, the rocks were moving and now I knew why (see graphic).

Death Valley National Park is an awesome, dramatic place and Racetrack Playa is no exception. The dry lake bed sits 1100 metres above sea level at the end of a 40-kilometre rocky road that climbs up among the Joshua trees. The road can take a heavy toll on vehicles. In a dozen trips, I’ve had five flat tyres and two busted shock absorbers.

The playa itself is staggeringly flat – its elevation changes by no more than a few centimetres across the whole of its 4 kilometre length. So flat, in fact, that a puddle of water can be pushed from one side to the other by the wind. Flat lake beds aren’t that unusual in the desert, although Racetrack is a little higher up than most and bounded on one side by dolomite cliffs, the source of the boulders that come tumbling down to edge of the playa.

What makes Racetrack special is that some of these boulders, even ones weighing tens or hundreds of kilograms, don’t stay where they land. They are found at the end of trails, sometimes hundreds of metres long, carved in the crazed, cracked mud.

Rocks of ages (Image: Jack Dykinga/NaturePL)

Despite Death Valley’s reputation as one of the hottest and driest places on the planet, it does rain and the playa does flood from time to time. Evidently, the boulders must have moved when the playa was wet and soft. But how? No one in this remote, harsh place has recorded it happen. This is a puzzle that has confronted scientists and tourists alike for almost a century.

Some of the sillier ideas have included earthquakes, clay that swells when wet and even alien tractor beams. More serious discussions centre on wind but there has been a long, and occasionally bitter, debate as to whether wind alone can push these rocks along, or whether ice is needed.

Ice might help in a couple of ways – a large sheet of floating ice blown by the wind could shove against a rock and bulldoze it along. Or ice frozen around a boulder could buoy it up like a life raft. As always in science, there are various embellishments to these basic ideas – that the very flat playa leads to particularly strong winds near the surface, or that algae in the mud makes it especially slippery, or that spray or water pushed by the wind gives the rocks an extra nudge, and so on.

Such geological debates are ultimately unsatisfying, coming down to a calculation of how fast the winds need to be for a given rock in a given scenario, and those windspeeds being declared more or less improbable. What was needed was hard, quantitative data.

Smart rocks and kites

Which is where I come in. After visiting the playa on a geological field trip in 2006, I was curious. I was working on a , desert whirlwinds that sometimes cause accidents on Earth and are an important driver of the climate on Mars.

The project was to develop weather sensors and use time-lapse cameras in the desert in summer. But my instruments wouldn’t be used in winter so I could move them to Racetrack instead. The playa is in a protected wilderness area, which precludes intrusive activities like taking rocks or installing bulky equipment. I also needed permission from the . Together with Brian Jackson, then at the University of Arizona, I began measuring the conditions at the playa in 2007, learning just how often it flooded or froze (see “Racetrack facts“).

We made the and showed that strong winds alone occur too rarely to explain the sliding stones.

Dutifully retrieving the apparatus each spring, we found the playa stayed wet for several weeks, and that conditions would drop below freezing as often as 50 times each winter. The playa’s high altitude, shadowing by the cliffs and a phenomenon called “cold-air pooling” all help to chill the playa floor. So my money was on the culprit being ice – by floating the rocks up, . In fact in the coastal Arctic, an environment almost a world away from the deserts of the south-west US, trails formed by ice and ice-rafted rocks are well-known.

After six years, we started to wonder why we hadn’t seen much evidence of rock movement, much less caught any in action. In 1970, a study found movements in three winters out of five. Was our experience just bad luck?

Looking at the numbers last year, it seemed unlikely. Our weather record showed that the number of freezing nights has systematically declined in the last few decades because of climate change. If ice were involved, this would explain why few .

In the face of these weary odds we set up again last November. Another research team led by Norris of the Scripps Institute of Oceanography in La Jolla and his cousin Jim who works at the electronics firm Interwoof in Santa Barbara, California, had an even better weather station set up. They had also brought in several rocks from a nearby canyon and fitted them with GPS trackers – an experiment that I thought was probably going to be the most boring in the history of science.

I couldn’t have been more wrong. 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. Interestingly, the time-lapse footage showed what the weather station didn’t, that the flooding was mostly the result of an early snowfall in November. The at a stately 5 centimetres per second or so (see graph).FIG-mg29844601.jpg

“We’ve seen the rocks moving. More importantly, we have pictures and data”

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.

We couldn’t walk on the playa as the mud would retain our ugly footprints for years, but with the wind up I was able to fly a kite, complete with cameras, to capture aerial pictures of the brand new trails. We had seen it happen with our own eyes, but more importantly we now had , measurements of the movement speed and good local weather data.

Just like that, the mystery was solved. Thin sheets of floating ice driven by the wind are the answer. Some may have enjoyed the mystique. But understanding the exceptional circumstances that prompt this remarkable natural phenomenon to happen for about 1 minute in a million only helps us appreciate what a truly amazing thing it is.

This article appeared in print under the headline “Rocks of ages”