This is a close-up - but not macro - of a chipped side of the bunker door. I was curious about it, because I had read that the germans (or French work-prisonners, rather) mixed steal shreads in with their concrete to make it stronger, but we can also see that they used rubbel from the beach, as there are many rocks of that ocre color down there. At least, that is what struck me as a probability whilst looking at the site.