When your garden used to be a farmyard, you dig up all sorts of surprising stuff... including the boulders of which these are just a tiny part of the very large number that were buried here - we now have a lot of rockeries and dry-stonewalls in the garden!
There are many things buried around like the boat on York Beach that only shows up after a big, big storm. Maybe someday someone will be digging up somethings of ours that scooted away in the wind or dropped into some unseeable area.