What made these shapes in the dirt?
April 13, 2019 1:40 PM   Subscribe

I spotted these, 1,2,3, in the field where I walk my dog and I'm curious what may have made them.

They weren't there in January and in February there was a big snowfall and they were there in March after the melt. They're solid and I haven't found any rodent tunnels underneath. Anyone have any suggestions?
posted by Tenuki to Science & Nature (7 answers total) 2 users marked this as a favorite
I'm thinking tire treads or similar. You start with mud or sludge, and there's a bobcat-type machine or other snow removal equipment with big tires. The tires go squishing thru the mud, filling the tire treads with mud that forms into these worm-casting things. They drop out into the snow, and then maybe the snow is plowed up into piles, which mixes up their shapes somewhat. Then the snow melts and these are left.

Are they localized just in one area of the field? Was there a snow pile where they are? Anything else near where they were?
posted by LobsterMitten at 2:11 PM on April 13 [1 favorite]

Are they on a slope? Maybe water melting beneath the snow formed some channels which mud flowed through.
posted by ShooBoo at 2:22 PM on April 13

Tire treads was my first thought when I spotted them but then I noticed them in places no equipment could go (gaps between fences.) The formations a scattered all around the field no real concentrations. The field is kind of isolated in the middle of other orchards and a cow pasture, it's about 5 acres and used to be an apples but they bulldozed the trees out about a decade ago. There were some 4-6 foot drifts there in February.
posted by Tenuki at 2:45 PM on April 13

Mole cricket tunnels perhaps? They look too random to be tire tracks.
posted by Aleyn at 2:48 PM on April 13

Those are eskers! They're made by burrowing and pushing soil into the snow, and can only be made when there's snow, by rodents like pocket gophers. Read more about them here.
posted by barchan at 2:49 PM on April 13 [14 favorites]

Your shots display the phenomenon much more clearly than the photos at barchan's very interesting link, Tenuki — but if they're not right on top of tunnels, it's not clear to me why, or even how the rodents would do this; unless they dug a tunnel in the snow first, and then filled it with dirt from the subsequent excavation of a tunnel in the soil.

Which seems like a lot of extra work, so there would have to be a good reason for it.
posted by jamjam at 3:54 PM on April 13

unless they dug a tunnel in the snow first, and then filled it with dirt from the subsequent excavation of a tunnel in the soil.

That's exactly what happened! Here's a book link that goes into a bit more explanation. I've read that a single gopher can move up to a ton of dirt a year - they can build tunnels hundreds of feet long. They are built to burrow and love to do so. Such "bioturbation" is really good for the soil and for the local ecology. Yay for gophers!
posted by barchan at 4:24 PM on April 13 [6 favorites]

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