Thanks for that Sweety, now I know. I never payed them any heed for thinking they were manmade, you learn something new everyday, well maybe every other day.
Perhaps the sinkhole opened up during their time and it was felt necessary
to construct the row right there and then, some kind of protection from it happening again, or ( and it cloys to suggest but...) as part of a healing process for the earth, acupuncture like.