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Also if you find something is it ok to keep?
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Jmeister, cfmct gives a great link. Namely : Yes you CAN detect NFS. But sure, in-so-far as you're within ARPA cultural heritage
Ie.: You're only looking for and only finding new coins, jewelry, etc.... Presto: You can detect.
As far as the ARPA stuff, the reality of that is : Just avoid obvious historic sensitive monuments. No one is going to rush up to you, armed with a calculator, to do the math on the ages of coins you might find.
As far as the next question of yours : "Can you keep what you find ?", well ......... it depends on what you mean, and what you find. All 50 states have Lost & found laws. Born out of wandering cattle laws of the 1800s. Typically stating that if you find something worth $100 or more, that you are to take it to the police station for proper lost & found procedures. So you're welcome to "rush to the police station" if you find a ring, if you want
Or another rule that your question could bump into, is verbiage that forbids harvest & remove. These are so that no numbskull thinks he can take home the park benches, or start harvesting lumber, etc.... Could it apply to individual coins and pulltabs ? SURE ! But does anyone ever interpret it that way ? No.
So let's just make it simple : Send me everything you find, and I will make sure it gets to the proper authorities. Ok ? Problem solved