Mr. Celeste,

I saw George Sangiouloglou's posting on the Conch listserver so I did a
little net surfing.  I found two resources that might answer your question.
First is a geologic map of Guam.
http://water.usgs.gov/pubs/wri/wri034126/pdf/wrir03-4126.pdf.

Find the location on the map where you found the fossil, then go to the
legend to determine which the rock layer the color represents. After you've
identified the formation, go to the next site and use the search tool (click
on the binoculars, enter the name, then click "find". There will be several
references so click on the binoculars with the curved arrow - Find Again) to
find the occurences to that name.  Eventuslly, you'll find what you're
looking for.

http://www.nps.gov/wapa/indepth/PDFs/guamseashore-1979_text.pdf

Limestone, being much less dense than basalt, "floats" on the molten basalt
as it makes its way to the surface to erupt. As the basalt moves upward, the
overlaying rock layers crack and tilt upward. That is one way your fossil
could have wound up so far above sea level.  Another possibility is that the
sea level was much higher when the limestone formed. If the climate was too
warm to permit the formation of the ice caps, sea level would have been much
higher - 200-300 feet, I believe.

Let me know how this worked for you. Thanks.