Sorry, I should have been more clear. The house is my grandparent's old house, in the country village where I grew up. I know the land well and the oldies nearby have known me since I was a baby!
Instead of saying "no idea" I should have said "we've had lots of ideas but none of them seem to make sense other than that we were told by the neighbours he was given the material for free and just liked it". It's black rock that is not local, it's something you would buy from a landscaping or road building place. They are in rectangular blocks with railway sleepers for edges that mean to walk through the yard you effectively have to go through a maze.
It's on very flat land on a large village block in a very dry climate. It's hot for much more of the year than it is cold.
I'm an ecologist so I know it provides habitat, I will be leaving the ones that aren't in the way of using the yard, and aren't right next to the back door.