How do they tell the age of fossils?
There are two types of dating – relative dating and absolute dating. Relative dating is simply telling what is older than what. Geologists use rules formulated hundreds of years ago (Steno’s Laws) to determine this. Steno’s Laws are: Horizontality (all layers originally deposited horizontally), superposition (the layers on top are younger than the layers on bottom), and cross-cutting relations (if something, like a fault or an igneous body, cuts cross another body of rock, it came at a later time). These rules helped early geologists to unravel the layers of rock and helped them to determine what was older than what. As this sequence was worked out, geologists also noticed that certain types of fossils occur in certain sequences or only occur in layers of a certain age (remember only relative ages, not in years…yet). Once they figured this out, everytime they saw a specific type of fossil (called an index fossil), they knew that a rock was a certain age. For instance, when rocks of C
