Raven pins a photo of a canyon wall to the Sky Deck board. Stripes run across it like a stack of blankets.
"Oldest at the bottom," Rocket says at once. "Done."
"What do you notice near the bottom?" Raven asks. "Those layers lean. The ones above are flat."
Rocket squints. "So the bottom ones got tilted, then something flat landed on top. There is a gap in the story."
Nova hovers over the photo. "Geologists have a few simple rules," she says. "Superposition is only the first."
"Rules for detectives," Raven says. Nova hums. "Gather the evidence. Then say which came first."
The open textbook Physical Geology says the simplest way to date features is to look at the relationships between them.
Superposition: sedimentary layers are deposited in sequence. Unless the stack has been turned over or broken by faults, the bottom layers are older than the top ones.
Inclusions: any rock fragment included inside a rock must be older than the rock that holds it.
Cross-cutting: any feature that cuts across or disrupts another feature must be younger than the feature it disrupts.
The survey adds Steno's Law of Original Horizontality: most sediments were laid down flat. Tilted layers were moved later, often during mountain building.
| Rule | What it says | What you conclude |
|---|---|---|
| Superposition | layers pile up one on another | bottom bed is older than any bed above it |
| Original horizontality | sediments are laid down nearly flat | tilted beds were moved after they formed |
| Inclusions | a fragment inside a rock was already there | the fragment is older than the rock around it |
| Cross-cutting | a crack or vein cuts through older rock | the thing that cuts is younger than what it cuts |
| Statement | True or false? |
|---|---|
| A fault that breaks a layer is younger than that layer. | ? |
| Tilted layers were tilted before they were deposited. | ? |
| The three rules give relative order, not years. | ? |
| Superposition works even if the whole stack has been flipped over. | ? |
Sharp reasoning. Tomorrow is Sky Deck Lab: the sediment bottle, shaken, timed and measured.