Public graves are single interments. The family pay for a single burial rather than buying the whole plot. It was a very common practice and did not mean that the families were all short of money. I think it was more acceptable in the old days not to have a gravestone. They chose to spend their money on other things instead. If they thought it was important enough I'm sure they could have gathered the money together. I have a lot of people in public graves that came from big families it mustn't have been an issue to them.
Blue
