Most fossils lie in sedimentary rock, which can't be dated directly (its grains are older than the rock). So how do we know the dinosaurs died out 66 million years ago? By combining relative order with dated igneous rocks: ash beds, lava flows and dikes.
Why it works
Each dated igneous rock gives a limit. A layer resting on an ash bed is younger than the ash. A layer covered by an ash bed is older than that ash. A layer cut by a dike is older than the dike. So a sedimentary layer between two dated beds is bracketed: it formed sometime between their ages. Sediment does not pile up at a steady, known rate, so the honest answer is a range, not a single number. More dated layers make the brackets tighter; today, ash beds near the end of the dinosaurs are dated to within tens of thousands of years.
Key idea
A sedimentary layer is younger than any dated rock it rests on, and older than any dated rock that covers it or cuts it. Report its age as the range between those limits.
Worked examples
Worked example: Between two ash beds
A shale with ammonites lies on an ash bed dated 95 million years and under an ash bed dated 92 million years. How old is the shale?
- On the 95-million-year ash: younger than 95.
- Under the 92-million-year ash: older than 92.
- So it formed between 95 and 92 million years ago, and so did its ammonites.
Answer: Between 95 and 92 million years old.
Worked example: Using a dike
A limestone rests on an ash bed dated 120 million years and is cut by a dike dated 101 million years. How old is the limestone?
- Younger than the ash below it: less than 120.
- The dike cuts it, so it existed before 101.
- Between 120 and 101 million years old.
Answer: Between 120 and 101 million years.
Common mistake
It is tempting to give one age, often the average, when the evidence only brackets a range. A shale between ash beds of 95 and 92 million years could be anywhere in between, and its lower part is older than its upper part. Check what each dated rock actually tells you: one limit each.