Dear WW: I have an idea that a nut embedded in the hickory cross section was one a bird hid in crevice of bark, close enough to the cambium (?the rapidly growing layer) to become surrounded. I think the nuthatches may do this, but I don't have my bird CD installed on new computer. I'll try searching about this.

Hey, WW! Have I got a URL for you! Shows nuthatch doing it!

http://www.fcps.k12.va.us/StratfordLandingES/Ecology/mpages/white-breasted_nuthatch.htm