One on many mile markers that the first Postmaster General on the United States Of America placed to make the mail system more efficient.
http://www.knowwarren.org/pages/milemrkr.htm and/or http://www.samnet.net/esso/Fmm.htm list other Ben Franklin Mile Markers, in general more westerly of this location.
only guessing it's 26 miles, based on GPS and another marker 1 mile away that can be read. This one is completely unreadable, perhaps gravestone rubbing might work on it. Will try that next time I'm in the area.