After all, a dinosaur wouldn’t be caught dead next to a trilobite.
Unlike people, you can’t really guess the age of a rock from looking at it.
So carbon 14 is used to date materials that aren’t that old geologically, say in the tens of thousands of years, while potassium-argon dating can be used to determine the ages of much older materials, in the millions and billions year range.
Chart of a few different isotope half lifes: In reality, geologists tend to mix and match relative and absolute age dates to piece together a geologic history.
Absolute age dating is like saying you are 15 years old and your grandfather is 77 years old.
To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This rule is common sense, but it serves as a powerful reference point.