As it is a Nature Area, the pathways of Effie Yeaw are carefully delineated and visitors are not to stray from them. I have not seen that metal detectors are specifically prohibited, but any digging certainly would be. Effie Yeaw is just a parcel of Ancil Hoffman Park, which has a substantial amount of riparian area with acres of tailings from previous mining which is under swift water during high water flows, which I would think would be a great place for someone with a metal detector (though more for coins and jewelry than for gold).
Try at the river and but not in the forest.
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