Case 3 · Free · Geology
Grand Canyon rock layers: How can rock layers tell us what came first?
In the app: “Reading the Canyon Walls”
Grand Canyon, Arizona, USA · 1.8 billion to 270 million years ago
Why this question matters
Rock layers let geologists read time where nothing was written down. Nicolas Steno's principle of superposition (1669) gives the order of the layers. Radiometric dating gives ages in years. The Grand Canyon is one of the clearest places on Earth to see both ideas work together.
The case briefing
The walls of the Grand Canyon stack almost 2 billion years of Earth's story. Use a 350-year-old idea and some fossils to put them in order.
The evidence
The exhibits you examine in this case, labelled by the kind of evidence each one is.
- Written later1669
Steno's principles
Nicolas Steno, Dissertationis prodromus
Steno argued that layers of sediment settle one on top of another, so in an undisturbed stack the lowest layer is the oldest. This is called superposition.
- Scientific findingMeasured today
The canyon layers
Grand Canyon National Park geology
Dark, twisted Vishnu Schist lies at the bottom, about 1.75 billion years old. Above it are flat sandstones, shales and limestones. The Kaibab Limestone at the rim formed about 270 million years ago.
- Scientific findingCollected since the 1800s
Fossils in the layers
Grand Canyon fossil record
The Bright Angel Shale holds trilobites, sea animals from about 500 million years ago. The Kaibab Limestone holds sponges and shellfish from a later sea. No dinosaur bones occur in the canyon walls.
- Scientific findingSince the early 1900s
Radiometric dating
Uranium–lead and other methods
Radioactive elements in some crystals decay at a known rate. Measuring them gives an age in years, which superposition alone cannot do.
How the investigation works
After the briefing and exhibits, you complete three steps. Every answer is explained, and the verdict lists the sources.
- 1
Chronology
Put the events back in order, oldest first. BC and AD are handled correctly, and there is no year zero.
- 2
Time and place
Match discoveries and events to where and when they happened, without inventing precision.
- 3
Supported conclusions
Sort claims into supported, going beyond the evidence, or contradicted. Every answer explains why.
What is still unknown
When the canyon was carved is still argued about. Most of it is thought to be 5–6 million years old, but some parts may be much older.
Sources
Case content last checked 2026-10-13. Not reviewed by an independent historian. Found an error? Tell us.
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