Yeah, why clay balls?... Pottery? waterproofing? camouflage? mini oven? (my dad told me as a kid that hobo's used to cook hedgehogs in clay, as, when done and opened, the spines staye in the clay making it easier to eat). Guess if wet clay was present near a fire, the irreversable hardening process must have been a little weird. If you've ever made 'seed bombs' with clay, it's a great way to get things to grow at a later date, raw clay, whilst thick and tough, is a great medium for growing stuff, pretty nutrient rich.

I can understand chalk balls to some degree (flint, associations I guess? and in certain parts of the country, it would be your subsoil).