It looks to me that if all you are after is coins, you are OK. Any type of period artifacts could be trouble.
Here are the legal citations:
Code of Federal Regulations, 36 CFR 261.9: "The following are
prohibited: (g) digging in, excavating, disturbing, injuring, destroying,
or in any way damaging any prehistoric, historic, or archaeological
resources, structure, site, artifact, or property. (h) Removing any
prehistoric, historic, or archaeological resources, structure, site,
artifact, property."
USDA Forest Service Manual Direction (draft): "Metal Detector Use. Metal
detectors may be used on public lands in areas that do not contain or
would not reasonably be expected to contain archaeological or historical
resources. They must be used, however, for lawful purposes. Any act with
a metal detector that violates the proscriptions of the Archaeological
Resources Protection Act (ARPA) or any other law is prosecutable.
Normally, developed campgrounds, swimming beaches, and other developed
recreation sites are open to metal detecting unless there are heritage
resources present. In such cases, Forest Supervisors are authorized to
close these sites by posting notices in such sites."
ARPA, 16 U.S.C. 470cc: "No person may excavate, remove, damage, or
otherwise alter or deface or attempt to excavate, remove, damage or
otherwise alter or deface any archaeological resources located on public lands or Indian
lands unless such activity is pursuant to a permit. . ."
Daryl