Slashdot Mirror


Sliding Rocks Bemuse Scientists

An anonymous reader writes "Scientists can't figure out why these rocks — weighing up to several hundred pounds each — slide across a dry lake bed. The leading theory proposes that wind moves the rocks after a rain when the lake bed consists of soft and very slippery mud.

64 of 433 comments (clear)

  1. Mark Newman Poster by stoolpigeon · · Score: 5, Informative

    Mark Newman has a very nice sliding rock poster with a good shot of rock and trail in a variety of sizes.

    --
    It's hard to believe that's how Micronians are made. Why don't we see it right now by having you both kiss one another?
    1. Re:Mark Newman Poster by An+ominous+Cow+art · · Score: 4, Informative

      I was there in August this year, and it was quite windy. It's very easy to imagine that if the ground had been muddy, the wind could slide the rocks around.

    2. Re:Mark Newman Poster by stoolpigeon · · Score: 4, Insightful

      In the desert the ground is really, really hard. It is completely plausible that a thin layer on the top could be slimy mud, while hard clay lies beneath.
       
      I don't think you appreciate how remote this site is and what an effort it would be to pull off something like that. I really don't think it is someone messing around or that the wind theory is as unlikely as you think.

      --
      It's hard to believe that's how Micronians are made. Why don't we see it right now by having you both kiss one another?
    3. Re:Mark Newman Poster by radish · · Score: 3, Funny

      Yeah, I mean heaven forbid tourists actually come and spend money at their tourist-related businesses. I can imagine they're so glad when all that money goes back where it came from! Then their hotels can be nice and empty like they should be.

      --

      ---- Den ene knappen er powerknapp, den andre er Bender voice knapp "Bite My Shiny Metal Ass"

    4. Re:Mark Newman Poster by ChameleonDave · · Score: 2, Informative

      Indeed.

      When my heavy beer glass gets a tiny bit of water between it and the hard table, it starts sliding around all by itself, with no wind at all. I can imagine that these stones slide similarly.

    5. Re:Mark Newman Poster by clam666 · · Score: 3, Funny

      In soviet desert, rocks move you?

      --
      I'm a satanic clam.
    6. Re:Mark Newman Poster by Basehart · · Score: 4, Interesting

      It's not just beer. I was bathing the other day and my pint sized glass of heavily iced water all of a sudden moved across the smooth plastic surface of the toilet seat lid and fell to the floor.

      The toilet seat lid was covered in a fine layer of condensation from the bath water at the time.

      I'm betting if the stones are cooled way down to almost freezing by the wind, or maybe frozen overnight and still cold when the rains hit, and the top surface of the mud turns into a slurry of fine particles, the stone will move around all on its own just like my cold glass of water on a fine layer of condensation.

      Either that or space aliens.

    7. Re:Mark Newman Poster by vought · · Score: 4, Interesting

      Just seems to me that instead of crazy rocks sliding round on their own, some damn kids were up there fucking round with rocks. Considering how difficult it is to get out to the Racetrack, I doubt this. Otherwise, I would think someone might camp out there during a storm and find out if they really skate around on their own.

      Problem is that storms come up rarely but suddenly there (usually) and it takes almost two hours to get to the Racetrack from the nearest paved road - three hours from the Death Valley visitor's center - and if you get out there before a storm, there's no guarantee that even a very capable 4x4 will get you back afterwards.
    8. Re:Mark Newman Poster by Roger+W+Moore · · Score: 2, Insightful

      Problem is that storms come up rarely but suddenly there (usually) and it takes almost two hours to get to the Racetrack from the nearest paved road - three hours from the Death Valley visitor's center - and if you get out there before a storm, there's no guarantee that even a very capable 4x4 will get you back afterwards.

      Isn't this what remote camera's were invented for? I doubt this location is so remote that there isn't some way to link it up or at least to store the data and then periodically retrieve it. The question is have these rocks moved recently or is this a rare (i.e. once/century say) type occurance?

    9. Re:Mark Newman Poster by vought · · Score: 5, Funny

      It sounds like fun, and it would only take a few years to get results. Compared to raising children the cost is low and the results are fast. I agree. You do it.
    10. Re:Mark Newman Poster by MillionthMonkey · · Score: 2, Funny

      I love places that try to require a tip on the bill. Those places, shoudl I happen to wind up at one unknowingly, will never get a cent for a tip. I'll figure out the actual check total and pay exactly that. The can go scratch on the tip for being retarded about it.

      In Death Valley? They'll remember your face and tomorrow you'll end up driving an extra 50 miles for lunch.

    11. Re:Mark Newman Poster by eonlabs · · Score: 2, Funny

      "there's no guarantee that even a very capable 4x4 will get you back afterwards"

      So ride the rocks!

      --
      I wouldn't consider the mad hatter mad. Just reality impaired. He sure can make a mean cup of tea.
    12. Re:Mark Newman Poster by BigDogCH · · Score: 2, Funny

      Yeah, but you won't know where the rock is when you return! It will have moved!

    13. Re:Mark Newman Poster by ConceptJunkie · · Score: 2, Funny

      It's not the wind. It is the earth rotating under the rocks. Not sure of the latitude of the location but the earth is rotating at something around 1000 miles per hour at the equator.

      Well, you know of course that why the famous experiment with marbles on a smooth surface works. It's well known that any object on a relatively frictionless surface will slowly drift to the West because of the Earth's rotation. This is why that old design for "air hockey" was unfeasible. That's why marbles on balanced glass always roll Westward. It's how compasses work.

      That's also why ice rinks nearer the equator need to be tilted slightly to compensate for the effects of the Earth rotating from under the skaters. I understand that an ice rink built in Quito in 2004 had to be slanted at nearly 7 degrees.

      I understand that the government of Indonesia is building a 7 mile long maglev runway that will allow airplanes to be launched at almost 400 miles per hour (wind resistance cuts it down from the full 1000), merely by placing the airplane on the track and letting the Earth's rotation fling it into the air. At that point the engines cut in. The system is expected to cut fuel costs for air travel by almost 30%.

      This same effect is what powers that "perpetual motion" turbine that generates power for the Amundsen-Scott South Pole Research Station.

      --
      You are in a maze of twisty little passages, all alike.
    14. Re:Mark Newman Poster by Linux_Bastard · · Score: 3, Informative

      What is happening there is that the cohesive water is acting as a flexible seal around the bottom of your glass, and it is resting on a cushion of trapped air.

      When the glass is put down, if the water seal forms before the glass has fully contacted the surface, the air pressure will lift the glass as it evens out the pressure on the air cushion. This will cause it to be riding on an air bearing, and slide very easily.

      Usually it will only go until the water seal is broken, releasing the air pressure that forms the cushion. If the surface is moist enough, the cohesive water can renew the seal as it glides.

      If you try this with a very flat, nearly level surface and a glass with a concave bottom you can get good results.
      Hot liquids can actually expand the air under them and suddenly lift up and slide.

      --
      F X=0:1:9999 F D=2:1 Q:((X>2)&(X#D=0)!((D>X/2)&(X'=1))) I D>(X/2) W:$X>75 ! W X,?$X+5-$l(X) Q
  2. It's a Horta! by loftwyr · · Score: 5, Funny

    I saw this on TV once! It was this documentary about these very things! They're called Hortas and their intelligent. Apparently they can be taught to mine.

    1. Re:It's a Horta! by iggymanz · · Score: 2, Funny

      you also don't want a female Horta to sit on your face if you wear glasses, that hydrofluoric acid they secrete when they're horny eats right through spectacles.

    2. Re:It's a Horta! by sokoban · · Score: 5, Funny

      Yes, and though the flow of water surrounding these things can be directed, these Horta do not readily absorb moisture.

      Thus, you can lead a Horta water, but you can't make it drink.

      --
      09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0 is the magic number.
    3. Re:It's a Horta! by GeekZilla · · Score: 2, Insightful

      "Instead of testing individual theories by leaving stakes sitting around, how about getting a definite answer by leaving a webcam?"

      THANK YOU! Oh my god! And these are actually "scientists" studying these things? Sure, maybe hooking up to the internet is a little dicey in Death Valley, but there are other ways. They need some college kid hyped up on caffeine to wire together a solar-powered, weather proof DVR and finally solve this mystery. I mean, come on! How long have we known about these tracks? Decades?

      Sorry, I tend to get a little touchy when there is a mystery and 1) the experiment that could solve it is REALLY simple and 2) The mystery has been around for years. I think it would make a good thesis...no, I think it would make a decent term paper for some high-school senior interested in Geology.

      --
      Veritas patesco per quaestio questio. Truth is revealed through questions.
    4. Re:It's a Horta! by camperdave · · Score: 2, Insightful

      Plant a camera out there for a year, take a snapshot every minute (or use motion detection), collect weather data (humidity, dew point, temperature, evaportion rates, wind speed/dir) and corrolate that to the time-series snap shots.

      ... and hopefully you'll be able to find out who made off with your expensive camera.

      --
      When our name is on the back of your car, we're behind you all the way!
    5. Re:It's a Horta! by xSauronx · · Score: 3, Funny

      Sit on my face, and tell me tha--- Oh god my eyes... THE BURNING! it burns so bad!

      --
      By and large, language is a tool for concealing the truth. -- George Carlin
    6. Re:It's a Horta! by Carnildo · · Score: 2, Informative

      There are a number of reasons why they haven't:
      * The rocks don't move very often -- typically once every two or three years.
      * Cheap webcams have only been around for a few years, and I don't know if there have been any movement episodes during this time.
      * It's an incredibly hostile environment for electronic equipment: surface temperatures of 150+ degrees F during summer days, temperatures below zero F during winter nights, violent rainstorms, and intense direct sunlight.
      * There is no electricity. There is no internet service. There is no wireless phone service. During the rainstorms when the rocks are expected to be moving, there is no satellite service.
      * There's the ever-present risk of theft.

      --
      "They redundantly repeated themselves over and over again incessantly without end ad infinitum" -- ibid.
    7. Re:It's a Horta! by jfdawes · · Score: 4, Funny

      .... the goggles ... they do nothing ....

    8. Re:It's a Horta! by martinX · · Score: 3, Funny

      They won't move if they know they're being watched.

      --
      When they came for the communists, I said "He's next door. Take him away. Goddam commies."
    9. Re:It's a Horta! by hazem · · Score: 3, Funny

      Easy! You just put a camera to watch the camera...

      I wonder if they can just "tag" the rocks like they do with sharks, elephants, walruses, etc. I mean, I know the rocks don't have ears or collars, but there has to be a way.

    10. Re:It's a Horta! by dfn_deux · · Score: 2, Insightful

      The Horta culture is really complex like that ;)

      --
      -*The above statement is printed entirely on recycled electrons*-
    11. Re:It's a Horta! by jamstar7 · · Score: 2, Insightful

      So many great setups, so few modpoints.

      --
      Understanding the scope of the problem is the first step on the path to true panic.
    12. Re:It's a Horta! by vought · · Score: 3, Informative

      Given the hottest temperature ever recorded in the United States is 56.7 degrees celsius (134.1 degrees fahrenheit) I find it impossible to believe this small part of America regularly gets temperatures of above 65.6 degrees celsius (150F). A friendly nitpick: surface temperatures on the desert floor - even on light-colored surfaces like the Racetrack - can often rise above 200 degrees F. Note that the ambient temperature may be far cooler than the ground surface.

      From a dependable source:

      In the heat of summer, Death Valley roasts in temperatures greater than 120 degrees, cool when compared with the surface temperature of the salt pan. "The ground temperature gets to over 200 degrees [f] at some points here," says Dr. Douglas. I'd wager that the surface temperatures at the Racetrack in early afternoon during high summer range above the boiling point of water at sea level*, since the racetrack's playa is lower and darker than the salt pan at Badwater. In other words, don't fall; you'll skin and burn your knees.

      If you've never been to Death Valley in the summer, you should give it a try. If you're from a mild climate, I suggest March instead; the regular 90 degree temperatures before April has shown it's face will give you a little idea of the radical heat that this region experiences.

      *The Racetrack and Badwater are both below sea level, so you'd need to get up to at least 240f to boil water.
    13. Re:It's a Horta! by Walkingshark · · Score: 2, Insightful

      Why not grab a 3m x 3m x 10cm sample of the surface, take it to a lab, hose it down, drop a rock on it, and turn on a fan?

      --
      The world you experience is only a close approximation of reality.
  3. Obvious Answer!! by explosivejared · · Score: 3, Funny

    Anyone who has seen an M Night Shamylan movie or been involved in a Usenet discussion about UFO's can readily see that there is one glaringly obvious answer...

    IT'S ALIENS GUYS!! RUN FOR YOUR LIVES!!

    --
    I got a catholic block.
  4. So this is... by Anonymous Coward · · Score: 5, Funny

    ...nature's version of desert curling?

  5. Answer on page 42 ... by foobsr · · Score: 5, Interesting

    ... literally:
    Quote: "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." (Messina, 1997, p. 42)"
    http://sophia.smith.edu/~lfletche/deathvalley.html

    But it seems they have no real conclusion too.

    What about 'The Force"?

    CC.

    --
    TaijiQuan (Huang, 5 loosenings)
    1. Re:Answer on page 42 ... by orclevegam · · Score: 5, Funny

      GAH! MY EYES!!! Put a warning on that link, geez.

      --
      Curiosity was framed, Ignorance killed the cat.
  6. Re:Any word on magnetic influence? by stoolpigeon · · Score: 2, Funny

    has anyone checked the mitichlorian levels in the area?

    --
    It's hard to believe that's how Micronians are made. Why don't we see it right now by having you both kiss one another?
  7. Begs the question by orclevegam · · Score: 3, Interesting

    This begs the question, why hasn't someone setup a webcam to record these rock movements and solve this thing once and for all? I mean, if they can setup cameras in the arctic circle, death valley shouldn't be that hard to handle.

    --
    Curiosity was framed, Ignorance killed the cat.
    1. Re:Begs the question by Maint_Pgmr_3 · · Score: 2, Insightful

      I agree, if "they" [USGS] can put spiders on MT ST Helens to see how the dome is growing and moving, you would think that someone could tape a small GPS on the rock. Duct tape shouldn't change the movement by much.

      better yet, put a small weather station, ala north pole, on a sled and leave it in the middle of the playa and see what the weather conditions are and when it moves. 12 volt battery and an automotive condenser should discourage people from disturbing the sled.

    2. Re:Begs the question by Bob(TM) · · Score: 3, Insightful

      Yeah, but ... how you gonna keep the webcam from sliding? :)

      --

      The little guy just ain't getting it, is he?
    3. Re:Begs the question by fbjon · · Score: 5, Insightful
      It still seems strange. The place is really dry, meaning there's lots of sun. Just make a small package with a GPS receiver, some simple weather instruments, a radio uplink to a nearby relay, a small camera and a solar panel with battery. If the GPS detects any movement, or the weather instruments detect any drastic changes, turn on all other stuff and start piping data to the relay, which passes it on by whatever means.

      If anybody steals the package, it'll sound an alert and record who took it, and where they're taking it.

      --
      True confidence comes not from realising you are as good as your peers, but that your peers are as bad as you are.
    4. Re:Begs the question by orclevegam · · Score: 2, Informative
      --
      Curiosity was framed, Ignorance killed the cat.
    5. Re:Begs the question by Random+Destruction · · Score: 2, Informative

      I'm siding with Paul Brians on this one.
      http://wsu.edu/~brians/errors/begs.html

      Raises the question != Begs the question.

      --
      :x
  8. Isn't it obvious yet? by CitznFish · · Score: 5, Funny

    These stones don't want to gather any moss.

    --
    'mmmmmmmmm.... forbidden donut'
  9. no buildup in front by egburr · · Score: 4, Insightful

    In all those pictures, I don't see any buildup of dust in front of the rocks, though there is plenty on the sides of the paths. Usually, when I push something through the dirt/mud/snow/whatever, I end up with a good buildup in front, too. I wonder where that has gone.

    --

    Edward Burr
    Having a smoking section in a restaurant is like having a peeing section in a swimming pool.
    1. Re:no buildup in front by Reality+Master+101 · · Score: 4, Interesting

      In all those pictures, I don't see any buildup of dust in front of the rocks, though there is plenty on the sides of the paths. Usually, when I push something through the dirt/mud/snow/whatever, I end up with a good buildup in front, too. I wonder where that has gone.

      I had to think about this for a second... I think the answer is that if a rock was digging into the mud, you wouldn't have this effect, because of having to shove the mass of the mud. If you look at the pictures, the fronts of a lot of them tend to be sticking up, implying they're "surfing" over the mud.

      --
      Sometimes it's best to just let stupid people be stupid.
  10. I am pretty sure ... by Culture · · Score: 5, Funny

    ... that the rocks slide because the lateral forces exerted on the rocks exceed the static and dynamic frictional force cause by the gravity induced weight of the rock acting across the mud-rock interface. I guess I could be wrong and there are worm-holes involved.

    --
    ----- There are two kinds of people in this world, my friend; those with loaded guns, and those who dig.
  11. god doesn't play dice by circletimessquare · · Score: 4, Funny

    but he does hold magnets under the surface of the table, moving objects on top as if by magic, just to bemuse and entertain us

    --
    intellectual property law is philosophically incoherent. it is your moral duty to ignore it or sabotage it
  12. One thing I know for sure by mcg1969 · · Score: 5, Funny

    is that posting this article in Slashdot is sure to produce a definitive solution to the mystery...
    or rather, 100 of them.

  13. Re:Any word on magnetic influence? by $RANDOMLUSER · · Score: 3, Funny

    Actually, it's caused by the continential tilt. It causes all the loose cannons and nuts and bolts to roll towards California.

    --
    No folly is more costly than the folly of intolerant idealism. - Winston Churchill
  14. Its not that hard to figure out! by Henneshoe · · Score: 2, Informative

    If there is really this much interest in figuring out how the rocks move, its pretty easy to do. Mount a solar powered camera with a motion detector. If the rock moves start shooting. If you really want to get fancy, you could do a continual time lapse to catch the movement if it is too slow for the motion detector. I think they do this kinda thing when you want to get pictures of wild animals in there natural habitat and the cameras are avaliable at your nearest outdoor outfitter.

  15. FSM by Anonymous Coward · · Score: 2, Funny

    They were obviously moved by his noodly appendages.

  16. Re:Ice Sheet by mpathetiq · · Score: 2, Insightful

    From TFA:

    Some researchers have found highly congruent trails on multiple rocks that strongly support this movement theory. However, the transport of a large ice sheet might be expected to mark the playa surface in other ways - these marks have not been found.

    Other researchers experimented with stakes that would be disturbed by ice sheets. The rocks moved without disturbing the stakes. The evidence for ice-sheet transport is not consistent.

  17. Silly scientists... by VE3MTM · · Score: 5, Funny

    Clearly the Flying Spaghetti Monster is moving them with his Noodly Appendage.

    --
    09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0 Whoops, silly middle mouse button...
  18. Re:Magnetism? by Rob+T+Firefly · · Score: 3, Funny

    Do the rocks have a large iron content? I hope not. If there's one thing I can't stand, it's ironic rock.
  19. Re:Any word on magnetic influence? by TempeNerd · · Score: 4, Funny

    Obviously, the rocks were casually lurking on Slashdot, when they read "Move along, nothing to see here..."

    {rimshot}

  20. Re:Global Warming by AlamedaStone · · Score: 5, Funny

    because teutonic plate theory was just too crazy to accept...
     
    ... although it replaced the even more silly Gaulish plate theory, quickly discarded by history.

    --
    "All these years believing you're the signified monkey, only to find out you're just a big hunk of nobody cares."
  21. Re:Magnetism? by spun · · Score: 4, Funny

    Ironic rock is worse than rai-e-ain on your wedding day, or a free-ee ride, when you already paid.

    --
    - None can love freedom heartily, but good men; the rest love not freedom, but license. -- John Milton
  22. MOD PARENT UP! by denzacar · · Score: 2

    Thank you!

    I remember this thing from back in '80s and even back then it was obvious that its water+wind+ice that makes the rocks move.

    What is next? Channels on Mars?

    --
    Mit der Dummheit kämpfen Götter selbst vergebens
  23. One possibilty by edwardpickman · · Score: 5, Interesting

    I'd be curious if the under sides of the sliding sliding stones were concave? Why I mention it is I still remember a certain chinese restaurant's tea cups had a habit of sliding across the table. The table tops were resin coated and the concave cups tended to capture moisture under them so when the tea heated the moisture under the cup the expansion provided enough lift to break the friction and allow them to slide. They would move randomly in different directions then stop for a few minutes then slide again. Since the area is hot a unique combination of heated rocks with slippery mud and wind could in combination cause the effect. I remember that some rocks slid and others didn't as well as the direction changes.

  24. Re:Amazing how no-one bothers to actually CHECK. by Ambiguous+Coward · · Score: 2, Insightful

    Come now...while I agree that the video is (mildly) interesting, it is not what you claim. But beside that, the article doesn't say "there's no possible explanation" at all. In fact, the article presents at least a couple possible explanations. In fact, and I say this with every ounce of *gasp* I can muster, the article presents an explanation for the moving rocks that is identical to the one presented in the video! Did you happen to read the article? You ought to. It's relevant to this discussion.

    -G

    --
    Their may be a grammatical error, misspeling, or evn a typo in this post.
  25. Everything must be ruled in or out, but... by ackthpt · · Score: 2, Informative

    Re:Any word on magnetic influence? I'd guess it isn't wind...are these rocks ferrous? Or...maybe the earth is tilting on its side...weird stuff like that always happens here...I think our perspective of it is just off a bit.

    I've camped a few times at Texas Spring campground in Death Valley. Nice place in the right times of the year. One year, however, the wind blew all night at about 40 knots. Nearly took me and my tent away. There are sand dunes to the north of the valley, too. I expect the winds there are more than up to the task of pushing around rocks on moist clay. Perhaps most enigmatic is the question, 'Why don't these larger rocks sink into the mud?' Though with strong enough winds, I imagine they could get a move on again.

    --

    A feeling of having made the same mistake before: Deja Foobar
  26. Re:Amazing how no-one bothers to actually CHECK. by DigitalCrackPipe · · Score: 4, Informative

    In true /. form, you either failed to RTFA or to WTFV, as the video clearly does not show the rocks moving. It shows water and miscelaneous floating scum moving, and posits the same theories as in the article (just claiming them to have been proven).

    And as to the foolishly simple explanation, H.L. Mekcken is quoted to have said, "Every complex problem has a solution that is simple, direct, plausible, and wrong".

  27. That was Strangely Topical. by Smordnys+s'regrepsA · · Score: 4, Funny

    Rick Rolled while Reading about Rolling Rock Research, by a link Represented as Relevant.

    I Require you Rectify this Rankling Repugnance.


    Regards,
    Ryan

    --
    Just -1, Troll talking to another.
  28. Problem with the ice or even water theory by SmallFurryCreature · · Score: 3, Interesting

    The cracked effect is what you get when muds dries out, the effect is NOT visible in the trails. How can this be IF the rocks moved when the mud was still there? There is a cracked effect in the trail but it is crushed, the effect you would expect if the rocks had been moved AFTER the ripple effect had already started to form, AFTER the mud started to dry or even when it was already dry.

    But if the rocks moved on ice then AFTER the ice melted there would be mud, that if dried would show the same pattern all around the newly positioned rock with just the ridges of the trail left. NOT flattened dried mud.

    As for purely the wind moving them, how fast do the winds get there anyway? Wind can be extremely powerfull even in areas with lots of obstructions, in open areas, well if it can pick up/move trucks, why not rocks? Far heavier things are lifted up by air alone, how do you think aircraft work?

    --

    MMO Quests are like orgasms:

    You may solo them, I prefer them in a group.

  29. Re:Amazing how no-one bothers to actually CHECK. by evanbd · · Score: 3, Insightful

    Did you watch the video carefully? They offer a very authoritative sounding explanation of the water / ice theory presented as one option in the article, but don't actually capture rocks in motion on film. There's water moving, and some sort of foam on top of the water, but that's about it. As the original article says, there's lots of evidence in favor of this theory, but also evidence against it being that simple -- like experiments involving undisturbed stakes and such. As far as I can tell, that seems to be a piece of the explanation, but not the whole explanation.

  30. Another theory by Herby+Sagues · · Score: 3, Interesting

    I have another theory for this.
    Perhaps, when the ground is wet, there's a mud layer slightly beneath the surface that becomes fluid. Then, tidal foces make the surface to move up and down creating waves. The waves migh be very low amplitude (a few cm could be enough) and low speed/frequency, but enough to make the rocks slowly slide on the dry (or slightly moist) surface.
    Does it sound plausible?