It was a HAha style ditch which was hidden by the ivy covered wall in front. I think the stones were to prevent them going down into the ditch and so out of view. So no grazing for these zebras either!
This area used to be several long narrow parallel paddocks extending out from the original old Antelope/Zebra house(not this one) One held Ostrich, two more had the Grevy Zebras(seperated pair) which they bred a couple of times. The old Stallion's loud call would ring around the zoo about every half an hour or so.
