the strange markings . I am completely unsure really how they truly got their I would personally guess shells in a ice age or summit where mussels Where feeding in the sea on the rock then frozen their or they collected in that place ? Could be how the shells break down over time and in this area where in large numbers this is going back truly thousands of years as I sure you no !
I think that's a pretty sound guess to be fair, im not sure how shells decay underground, I think they go grey top side but could very well stay white underground. In the absence of any other theory I think its pretty good