SMPD, are you sure they didn't walk to your pond? If they came in a vechicle you could get the plate number and the warden could go talk to them. Sometimes that is enough to make them stop. They usually walk in so that people does not tear up their ride. Around here some people(not me) will destroy a vehicle that is trepassing. Call the state board and a game warden will get back to you. You are learning the hard way about property ownership. Don't let the SOB' discourage you. Good luck!! BTW,how old are those fossils? Mid thirties perhaps?
Two ponds, 13 and 15 acres on the Mattaponi River.
