I once learned about something similar.For what it's worth, I'll share it with no guarantee that this is the whole story.
I once toured a cave whose ceiling was covered with that kind of formation. Our guide said it was called box work.
My imperfect memory of the explanation: ages ago, fissures developed in the rock. The fissures got filled with (molten?) rock of a different type.
The river that flowed through the cave caused erosion. But the filled-in rock was the harder of the two, so it remains while the softer rock around it has eroded away.
@ScreamingFox I wondered fossil too but I'm pretty sure it's a natural formation. I have some vague memory of seeing it before, maybe in school.
I considered taking it home but scrambling up a steep ravine and walking 20 minutes back to the car with a rock the size of a skateboard under my arm didn't seem fun. But I put it somewhere obvious so I can find it again if I change my mind.
To my pretty ignorant eye, it kinda looks like shale that got fractured in sedimentary rock and then stuck there. But without actually feeling it, i really wouldn't know for sure It looks very cool.
@OogieBoogie now that you say it, the part with the relief does have that reddish- shale look to it, different from the other half of the rock. I'm thinking you nailed it.
