Pinned Pavement

We spent the afternoon with two grandchildren at Salmesbury Hall, midway between Preston and Blackburn. 

We went for the play area, which was a success again, but I noticed that the flagstones all had bolts through them.  I have not seen (noticed) this before, and am wondering why it was done and how long ago it was done.

Perhaps it is to keep the stones in place? 

I have also posted two backblips today.