They're actually dolomitized sponge reefs that grew around a volcanic core (a setting similar to the Bahamas today, but then with sponges instead of corals), mostly from the Triassic and Jurassic. The entire tectonic plate they're on got squashed between Europe and Africa, causing it to be uplifted, but without deforming it.

You're basically looking at what the seafloor looked like ~150 million years ago.