Absolute dating on rocks
As more half-lives pass, the number of parent atoms remaining approaches zero. This work is licensed under a Creative Commons Attribution-Share Alike 4.0 International License.
Although they had no way to measure the exact age of a rock, they were able to figure out by simple observation that some rocks were higher or lower on the geological column that others.
From this they were able to deduce that the lower rocks were generally older than the higher rocks.
They also observed that certain fossils were found in certain layers, and were not found in other layers.
When they did this, they estimated that the Earth is many millions of years old.
We now know that this estimate is far, far too young*.
Nobody knew whether the cretaceous era was five million years ago, or ten million years ago.