Mostly rocks in foundation with concrete holding it together. Found scrap lumber for forms, dumped in rocks (I have a never ending supply here on the Canadian shield) and added concrete.
The house I lived in before was built in the early 1900's. Exact same foundation method. Smooth concrete on the outside, chunks of redrock of all sizes and concrete on the inside. I pity the concrete boring company that comes across one of those without knowing it beforehand. I don't think their bits are meant to go through solid rock.