Most of them here are either chip bark or sand, but some are gravel. Also, some are chopped up tires. And speaking of gravel - I have had lots of luck detecting gravel parking areas. Both drive ways and parking lots. Many coins to be dug there. It's amazing how the coins take on the coloring of the gravel. I have found coins, gold rings, old silver coins - just about everything in them.
One park I did last year had a parking area along it for about a block long, all with gravel. I took 300 coins and a silver ring out of there. Think outside the box. steve in so az