As Andrew Rindsberg points out, fossil-bearing rocks are often dislocated by
human activities, making it very difficult to figure out their original
source. In Ann Arbor, Michigan, I used to find mollusc fossils among the
rocks filling the dividing strips in parking lots. Near my current house in
Maryland, there is an artificial lake that is only about 15 years old with an
area on its shore reserved for landing row boats. That section of the shore
has been covered with pebbles among which I sometimes find gastropod fossils.
I suppose such fossils have no real value for their exact source cannot
reliably be located.
 
 
>*An interesting exception to this general rule is the rock used as ship
>ballast up to fairly recent times......
 
The practice still continues at least in the Aegean. Less than a month ago,
while strolling the harbor at the town of Foca in western Turkey, I came upon
a small ship with a pile of rocks next to it on the dock. Soon a guy appeared
on the deck & threw another rock onto the pile. After I questioned him, the
guy told me that they were used as ballast.
 
While the use of water for ballast introduces aquatic organisms to places
where they didn't exist before, it has been suggested that the use of
limestone rocks for ballast by the Aegean sailors resulted in the unnatural
distributions of some rock-dwelling land snails, for example, Albinaria.