Steve, no I didn't notice or pay attention to the part of the sign that said "no digging". I was only paying attention to the "artifact or historic materials" part of the sign wording. I guess the reason I subconsciously don't see the "digging" part of rules/signs, for ANY park, is that if you start down that rabbit trail, you might as well give up now. Because think of it: If you go in to ANY park, even where md'ing has never been a problem, and say "Can I dig in the park?" What do you think their answer will be? Of course they will say "no". Because their knee-jerk image will be un-filled holes and morons with shovels, right? So to me, I don't consider myself doing "defacing" and damage and stuff, if I intend to fill in my hole, to the point where you can't even tell I was there. To think otherwise, you can give it up now, and stick to private property only. Because there is no public institution that will ever say "go ahead and dig in our park, school yard, etc..." (barring maybe a sandbox or beach?)
Next you say: "Of coarse it says NJ public lands. Its it a public park?" I don't understand your question. You have to remember that the term "public" has different levels: All public land is not the same, simply because it's "public". The different levels (fed, state, county, and city) can have different rules for land under their jurisdiction. Therefore the sign in the Buried-crap's post, applies to the jurisdiction of the entity that posted it (ie.: state owned park land, not county, not city, etc...)
I've even heard of persons getting into a legal semantics battle, trying to say they're not "digging", but only "probing" (because the use a screwdriver to gingerly pry out coins, with a minimum of soil movement, etc...) and loosing the battle. But ironically, in those same places, I bet if they'd just gone and done it, no one would have even paid attention to them. Ie.: "no one cares, UNTIL you ask" type of psychology. For example: in Treasure_Hunter's post, he cites city and county parks that he detects with no problems. Like, even Florida police coming over to say "hi" and "good luck" etc... But I bet you that if Treasure_Hunter had gone into the various city or county's offices, and asked enough desk clerks "can I dig?" I bet you he'd have gotten a "no". But notice he goes and hunts to his heart's content, and no one gives a r*tt's *ss!