Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: Rich beds of marine fossils have been found in Florida. By dating these fossils, scientists can best determine
the period during which Florida was covered in water.
the age of the sedimentary rock layers in which they are found.