Paul Drez had some interesting notes on fossils that don't belong where
they were found. Here are some more paleontological sources of
contamination:
 
(1) Quarrying. Sandy shell material from the Sarasota pits is widely used
as fill in southern Florida. Well-preserved Pliocene and Pleistocene shells
can turn up on driveways, improved dirt roads, harbors, and building lots.
 
(2) Dredging. Modern and fossil shells often turn up in dredged material
from rivers and offshore deposits, as collectors are well aware. People
flock to gather shells (and shark teeth) whenever offshore sand is pumped
onto beaches in South Carolina.
 
(3) Natural erosion occurring today. Rivers and tidal channels can be deep
enough in places to erode Miocene and younger shells and mix them with
modern shells. This is very common in Florida, Georgia, and South Carolina
(USA), for instance. I have also collected Pleistocene shells that were
washed up on the beach of Dauphin Island, Alabama, where erosion is taking
place on the seafloor.
 
In practice, paleontologists often find rocks and fossils out of place
(called "float"), and trace them upstream or upslope back to the source.
The famous Burgess Shale was discovered from black shale "float" that had
fallen down the slope of a mountain in British Columbia.
 
(4) Natural erosion occurring in ancient times. Storms often mixed ancient
and contemporary shells together on the seafloor, so, for instance, shells
from the base of the Tertiary often contain an admixture of latest
Cretaceous shells. Paleontologists therefore use the youngest fossils from
the mix to determine the true age of the deposit, not the oldest ones.
 
Glaciers moved vast quantities of material for hundreds of miles across
northern North America, Europe, and elsewhere. Boulders with Cambrian
fossils have been moved from Scandinavia to Germany and Poland, for
instance.
 
(5) Mixing of shells in collections. A few months ago, Conch-L hosted a
discussion on shells that jump from tray to tray when a drawer is jerked or
a cabinet is tilted. The owner may not always be aware of this.
 
It's no wonder that many paleontologists prefer to collect their own
material when they can.
 
Andrew K. Rindsberg
Geological Survey of Alabama
Tuscaloosa, Alabama, USA