Trail of earth over a fallen tree trunk

I see these all over at elevation in Colorado and I can't figure out what causes them.
They look like the tracks Bugs Bunny used to make when he was traveling underground.
As far as I can tell they are not hollow though, and the fact that this one formed over a fallen tree makes it even more mysterious to me.

Updates after questions: The soil mounds are very rounded, and have a diameter up to about the size of a soda can.

They are also made up of what seems like ordinary unmodified dirt. eg. they are not hardened mud. They crumble like the surrounding soil, perhaps a tad easier because they are not compacted.

This photo was taken here: 40°46'50.8"N 106°40'16.6"W at approximately 9,934 ft.

  • 2
    Looks to me like it might be a mat of debris swept up by rain run-off, I've seen similar structures on my kids' playing fields after thunderstorms.
    – bob1
    Oct 20 '21 at 23:30
  • 1
    I've seen things like that but these are totally rounded mounds. I would expect earth carried by water to be deposited in a flat pattern. If it wasn't for the lack of a void underneath (and the fact that it goes over this tree) I'd say burrowing animal
    – aaronP
    Oct 21 '21 at 3:21
  • Ants or termites?
    – bob1
    Oct 21 '21 at 3:37
  • I didn't see any around (or in the mounds when disturbed) but this could have been something they left behind..
    – aaronP
    Oct 21 '21 at 3:57
  • 1
    My guess would be an extremely large termite colony, subterranean termites build similar structures, generally on a much smaller scale, but I could see it possibly getting to this size if left unchecked and with no one messing with them.
    – Nate W
    Oct 21 '21 at 22:36

Northern Pocket Gopher Eskers

I think this may actually be a pocket gopher burrow, but the key detail I was missing was that they are made under the snow.

The pocket gopher eskers (sometimes called earthcores) in this doc look very similar to my observations:

https://extension.colostate.edu/docs/pubs/natres/06515.pdf pocket gopher esker images from colorado state extension

The Northern Pocket Gopher's range includes the observation location: Northern Pocket Gopher Range Map

So my working theory is that the pocket gopher prefers to burrow where it's easiest, by pushing the topsoil up into the subnivean zone where there's an air space and a place for soil to go. That subnivean zone continued up and over this tree, because the tree is warmed by the surrounding soil and would melt an air space above it as well.

The weakest part of this explanation is that that pocket gopher dragged dirt up and over that tree as he encountered a hard wood obstacle that continued to form that dirt arch. I imagine the snow cover would work to contain his cast off dirt into that arch shape as he worked. The arch may be what's left of the dirt that lined a snow tunnel that he used all winter. As the snow melted It collapsed into a more or less solid tube of dirt.


These are mole tunnels. See also this image. Although admittedly, though hiking in the Colorado mountains for 25 years, I've never seen a tunnel cross over a tree as your picture!

  • 2
    I dont think this is the case as moles would not move dirt over a fallen tree, they would go beneath it. They also don't intentionally craft dirt tunnels like that, meaning they would not move dirt to on top of a tree to make a tunnel.
    – Nate W
    Oct 22 '21 at 21:27
  • Yeah, burrowing animal was always my prime suspect until I spotted this tree. Do moles live at 10,000 feet? (I added location data to the post)
    – aaronP
    Oct 22 '21 at 22:30
  • 2
    I think I found a doc that explains it very well (see my answer), along with a species found high in colorado. (moles are only out on the eastern plains) Upvoted jsf for pushing me in the right direction.
    – aaronP
    Oct 22 '21 at 23:21
  • Picas and marmots live in the Cascades.
    – Jon Custer
    Oct 22 '21 at 23:50

I don't know, or have evidence that this is 100% the work of termites, but I wanted to share why I think it is and provide some evidence that was much too long for an additional comment.

There are at least 45 species of termites in the US alone, and in other countries it is common to see mounds 4 to 5 feet tall or more. They have also been known to build large nests that look like a basketball sized or larger dirt ball in trees.

Some of the largest mud constructs (mud tubes) found in the US have been up to 24" wide, so the size of a soda can is not out of question by any means.

Giant Mud Tube Under House Video

Termites do not do well, and die quickly when exposed to too much sunlight so they build several types of mud tubes to get around, some are small paths back to the ground, some are large termite highways that may transport them by the thousands.

What Are Mud Tubes Used For? Termites typically use dirt combined with the saliva and their own excrement to make tubes. This gives the mud a consistency that is easier for them to build with and shape. They build these tubes around their nest and there are several different kinds.

  • Exploratory tubes lead to a food source.
  • Working Tubes: These tubes enable termites to reach food sources. Built to last, they may cover long distances and accommodate thousands of termites.
  • Swarm tubes are like a termite airport terminal. Also known as swarm castles, these allow termites to fly in and out of the nest.
  • Drop tubes connect one surface to another, often resembling the stalactites in caves. For example, you might find a tube running from a tree branch down to the ground.

Source 1 Source 2

Further more due to the shape, length and location of this particular oddity, it would make sense for it to be termites rather than deposits from running water as there are no other signs of highwater such as small sticks and other debris that would have been carried with the water and the other debris around the dirt tunnel seem untouched especially being near a fallen tree that would work as a dam.

It's either that.. or a sasquatch that had Taco Bell..

  • I really like the termite theory best so far, except for the fact that they are just solid crumbly dirt. If they were for termite travel I would expect hollow mud structures. It seriously does look like the sasquatch theory, or some mega worm left a trail of castings as it passed by!!
    – aaronP
    Oct 22 '21 at 22:15

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