this is a closeup of a rock i found long long ago in a Neighbor's driveway. which made me happy as heck. i love finding things like this.
Anyway the rock is sandstone and it is simply riddled with snail impression fossils. impressions are basically like molds left of the creature. in this case the sandstone compressed around the snails and when it was firm enough to keep the shapes the snails themselves more than likely finished decomposition. since sandstone is permeable and bacteria can exist in it's depths they could still break it down and remove the material. (this is also what makes chunks of looser formed sandstone in the aquarium such a good thing. it gives a huge place for beneficial bacteria of both aerobic and anaerobic types to form and process the wastes and nitrogen in the tank. which is the principal behind the bioblocks and other such materials which are things like glass foam or open porous ceramics for the same purpose.)

anyway you can easily see the many spirals in the rock where snails used to be. the towering spiral points were the backside of the shell and the dome impressions were the front side or the side one is used to seeing in photos of snails.