Mystery Geology Revealed!

As promised, I have the answer to our mystery geology.

RQ got the spot right: it’s Hohllay!

Image shows a large sandstone cave from inside. At the right, there is an arched opening looking out toward a jumbled pile of boulders and some trees. At the center and left, there is a large oval opening, with a narrow pillar down the middle. The ceiling overhead is marked with scrapes and grooves, and has many rounded hollows.
Hohllay (“hollow rock”) in Berdorf, Luxembourg. © Dietmar Rabich,, CC BY-SA 4.0, Wikimedia Commons.

Alas, twasn’t Chthulu or water that hollowed the cave and left the marks. The whole thing was made by humans:

From the Middle Ages until the 19th century, the mill stones for the numerous mills in the region were cut from these impressive caves called “Hohllay” or “Breechkaul” (Amphitheatre). This activity left its traces as bizarre patterns on the rocks.

And thus we become part of the geologic record! The sandstone itself is a marine deposit (pdf). Nearly two hundred million years ago, this part of Europe was a shallow shelf under the sea, and the sand that would become so useful for millstones was deposited in huge sandwaves up to 20 meters thick. This went on until about 190 million years ago. Calcium carbonate cemented the sand grains, leaving a huge swath of sandstone as a wavy belt across France, Luxembourg, and into Germany.

In the picture, our sandstones are brown, but if you were able to get at it with your trusty rock hammer, you’d find the fresh faces were bluish-gray. Weirdly, that color is caused by our old friend pyrite! So cool.

I do love how ancient seascapes become the raw material for our grain-grinding apparatuses. If I ever get my hands on a TARDIS, I’m totally going to go back to old Luxembourg and stand there humming “Under the Sea” under my breath as the miller grinds me a nice sack of flour on his or her Jurassic millstone. Folks who know European history better than me: what would be the most excellent era to visit? And folks who know baking better than I: what would be the appropriate foodstuff to make from our Jurassic-ground under-the-sea flour?

There’s a nice blurb about the Hohllay at Atlas Obscura. We absolutely ought to take a field trip here someday, my darlings, even if we haven’t got a TARDIS. It looks delightful!

Mystery Geology Revealed!
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7 thoughts on “Mystery Geology Revealed!

  1. rq

    Dangit. I was so hoping it would be Cthulhu after all. Ordinary human millers just… doesn’t quite have that excitement to it.
    As for when you want to visit Europe, it pretty much depends on which country you want to visit. Rather, which area, since borders have been shifty throughout history. Personally, I think now would be a good time for you to visit Latvia, since I can take you out to the country where you can see some of this old-type milling in action, and try it out yourself, too. And we can have bacon rolls, mm mmmmmm. Fresh. MM MMMMMM.

  2. 2

    our old friend pyrite

    Pyrite may be your friend but it’s not mine. It never calls, never writes, nary a birthday or Christmas card do I get from your “old friend.” Pyrite has been distinctly avoiding me, never acknowledging my very existence. Fortunately I have hematite and quartz to keep me from feeling alone.

  3. 4

    I was reading about mills in In build in the early 1800s often they used french burr stones (in particular after steam boats were in operation). One mill by contrast used a stone from western NC. BTW you can search for grist mills in In and they sell ground products. (There are a decent number in In as well as neighboring states)

  4. rq

    If this ever happens, I’m going to be there, too.
    That is, if nobody minds. Because Dana + Luxembourg + Giliell + caves? Seriously. I can make that sacrifice.

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