The Correlation Table
Two outcrops a hundred kilometres apart share one thin ash bed and one short-lived fossil — that is enough to line up their whole histories.
Phase 0 · The phenomenon
Two cliffs, one history
Two stratigraphic columns side by side. The West column, 55 m tall, has shale, sandstone, a limestone with fossil B, a thin white ash bed at 32 m, shale and sandstone. The East column, 36 m tall, has sandstone, shale with fossil B, a thin white ash bed at 18 m, limestone, and sandstone with a second thin ash bed at 34 m. Dashed lines join the two ash beds and the two fossil B zones.
Two road cuts lie a hundred kilometres apart. Their layers do not look alike: one is mostly shale and limestone, the other starts with thick sandstone. Their thicknesses do not match either.
But each contains a pale layer of volcanic ash only a few centimetres thick, and just below it, the remains of the same small, short-lived sea creature.
Those two clues are enough to line the outcrops up layer by layer and put real dates on them. This lab teaches you how — using a constructed record so every answer can be checked.
How can two outcrops a hundred kilometres apart, made of different rocks, be matched layer by layer and given real ages?
What this lab works toward
HS-ESS1-6Use evidence from ancient rocks, meteorites and other planets' surfaces to reconstruct Earth's early history.HS-ESS2-7Argue from evidence that Earth's systems and life have changed each other over time.
Plain-English summaries. The course maps these performance expectations to this lab; the official wording is published at nextgenscience.org.
The stations
- Index FossilsWhy does a short-lived, widespread fossil date a layer so precisely?
- Ash ClockHow does a volcanic ash bed get an exact age?
- Correlation LinesWhich clue gives the right correlation, and why does matching rock types fail?
- Rate GaugeHow do two dated ash beds date every layer between them?
Each station runs the same loop: predict before you see any data, investigate the model and record runs, reveal what the measurements show, then explain the gap between your prediction and the result. A claim-evidence-reasoning checkpoint and an exit ticket close the lab.
- The outcrops, their beds and Taxa A–E are a constructed teaching record so that every answer can be checked. The ages are realistic for the Silurian Period, but the taxa are stand-ins, not real species.
- A fossil's range is drawn as a sharp first and last appearance. Real ranges are known from many sites and have uncertainty at both ends.
- The U–Pb age assumes the zircon crystals grew at the moment of the eruption, contained no lead at first, and have neither gained nor lost uranium or lead since.
- Ages between dated beds are interpolated assuming steady deposition. Real sedimentation speeds up, slows down and sometimes stops, leaving gaps.