| Sedimentary rocks are difficult to work with because they are made up of several rocks (or organic material) cemented together, so the parts are different ages.
fossils can be useful to date a rock, Example of an index fossil is an Ammonite (index fossils are fossils of creatures that were not around for a long period of time so when you find one you automatically know the age of the rock.)
Fossils of shells can tell you the temperate of the water thousands of years ago because the composition of the shell will be different depending on how warm/cool the water was when it formed. (Shell of Globigerinoides ruber varies in ratio of calcium and magnesium)
Fossils can also tell how a creature has evolved, documenting each stage | 
Linear Mode
