But when does a purely geological item become an archaeological one? Surely the act of selection (eg the ammonite at the entrance to Stoney Littleton) is equally as valid as a purely act of construction; valid in so much as selecting something, in this case something natural, involves an act of considerable human discernment.

The fact that something is not manmade does not, imho, automatically place it outside the realm of the archaeological; if that were the case we would disregard the remains of the flowers recently found in the 4,000 year Bronze Age dig as merely botanical :-)