One outcrop shows only part of Earth's history. To build a longer record, geologists match layers from place to place, called correlation. Lateral continuity says a layer once ran continuously, so matching its pieces across a valley or a continent ties their histories together.
- Rock type: matching sandstone to sandstone works over short distances, where a layer can be traced.
- Key beds: a distinctive layer laid down in a geologic instant over a wide area, such as volcanic ash, is a time marker.
- Index fossils: species that lived a short time, spread widely and were common mark a narrow span of time wherever they are found.
Why it works
The same kind of rock can form many times: sand piles up on every beach in every age. So rock type alone can match layers of different ages. A key bed or an index fossil, on the other hand, marks a moment or a short span of time. When a layer holds two fossils, it formed while both species were alive, inside the overlap of their ranges, which narrows the age further.
Key idea
Correlate layers by rock type over short distances, and by key beds (such as ash) and index fossils over long ones. A layer with several fossils formed where their time ranges overlap.
Worked examples
Worked example: Overlapping ranges
Fossil X lived from 480 to 440 million years ago; fossil Y from 460 to 420. A shale holds both. When did it form?
- Both lived only during the overlap.
- The overlap begins when Y appeared, 460, and ends when X died out, 440.
- So the shale formed between 460 and 440 million years ago.
Answer: Between 460 and 440 million years ago.
Worked example: Good index fossil?
A species of clam lived for 200 million years in seas worldwide. Is it a good index fossil?
- It was widespread and common, but its range is huge.
- Finding it only says "sometime in a 200-million-year span": a poor index fossil.
Answer: No; it lived too long.
Common mistake
Matching layers by color or thickness alone is a common trap. Two red sandstones at two sites can be tens of millions of years apart. Check with time markers: a shared ash bed or index fossil is far stronger evidence than look-alike rock.