The dirt was brought over from another patch of land on the property where they are building a parkinglot, there's a possibility it wasn't really checked out or the rocks weren't visible off hand so it was overlooked, if anyone has helped in a track build here they know this dirt has enough clay that when it clods up after being wet it clumps into boulders the size of basketball and as dense as concrete so the rocks could have hid in the boulder sized clumps.....