Hi again
I'm not sure if it can be called a tradition or more a question of ease and handiness, certainly in the present day. Some priests dictate what can and cannot be added to a grave in terms of headstone etc but I don't know of any specific tradition re grave borders. In some places particularly along the north and west coast they would have used whatever materials were available to them - much the same for the stone walls I guess, so that the built heritage around offers clues to the lives our ancestors led. Grass needs to be maintained too so pebbles are easier to maintain.
Clare Doyle
Genealogy Support