Problems with understanding fossils dating

[heartless]

We're using, Concepts in Modern Biology. On page 410, it says that layers of rock determine the age of a fossil, and few lines later that using a fossil we may determine the age of the layer. Does it work in this way that, they determine the age of the fossil using radioactive dating and then identify the age of rock, or they don't use the ractive dating for that? And guess?
Thanks,

 

[matthyaouw]

It does work both ways, but only if you have a good understanding of the sequence of rocks and the fossils they contain. If you find a new exposure with fossils you know appear only in mid jurassic rocks, you can place a fairly safe bet that these rocks are mid jurassic too. There are some very good indicator fossils that changed form quickly and lived short durations that can pin a rock age down to within a few million years. likewise you can use stratigraphy to determine the relative ages of a fossils- one higher up in a sequence will just about always be younger. For an absolute date (the fossil is x million years old) you will need some radioactive dating at some point, but sometimes relative dating (x is older than y, which is older than z) is enough. Pretty much all of the sedimantary rock in the UK had been given a relative age and assigned to a period on the geological column before radioactive dating was invented, so it goes to show that stratigraphy and fossil evidence work pretty well for dating rocks relative to each other.