As promised, I have the answer to our mystery geology.
RQ got the spot right: it’s Hohllay!
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!