Raven spreads two pages on the Sky Deck: the canyon photo and a table of ages from the textbook.
"The flat layers on top are 540 to 250 million years old," she reads. "The tilted ones below are 825 to 1,250 million."
Rocket does the subtraction on his fingers. "So the gap between them is almost 300 million years. Where did it go?"
Nova hovers over the tilt. "Tilted, then worn flat, then buried," she says. "The missing time left no pages."
"What do you notice?" Raven asks. "The rules still work. Bottom older, top younger, tilt before the flat."
Rocket sits back. "A gap is still a clue." Nova hums. "Read the whole wall before you decide."
The open textbook Physical Geology describes a wall in the Grand Canyon in Arizona. The tilted rocks at the bottom belong to the Grand Canyon Group.
Flat layers sit on top of them. The boundary between the two is called an unconformity, an interruption in the laying down of sediment.
The lower rocks were tilted and then eroded to a flat surface before the younger rocks were deposited. Any record of that time is now gone.
Read the table, then use the rules.
| Part of the wall | Position | Age range from the textbook | What happened |
|---|---|---|---|
| Flat-lying layers | top | 540 to 250 million years ago | deposited flat on the eroded surface |
| The boundary | between | a gap of nearly 300 million years | tilting and erosion; no layers were kept from this time |
| Tilted layers of the Grand Canyon Group | bottom | 825 to 1,250 million years ago | deposited flat, then tilted, then worn down |
The United States Geological Survey says no precise rate of deposition can be determined for most rock layers. So the actual length of time a layer represents is usually unknown.
The Next Generation Science Standards put it this way. Analyses of rock strata and the fossil record provide only relative dates, not an absolute scale.
The ages in the table came from other methods you will not study in this course. The rules from Tuesday give the order on their own.
| Statement | True or false? |
|---|---|
| Reading layers alone tells you the exact number of years each layer took. | ? |
| Rock strata give relative dates: older and younger. | ? |
| The tilted layers were tilted before the flat layers were deposited. | ? |
| An unconformity means the layers were laid down without any break. | ? |
Excellent detective work. Tomorrow fossils join the evidence, and you find layers in everyday life.