wunder beta🪨 Reading Rock Layers
Rock layers are the pages of Earth's history book. Learn the simple rule that lets scientists read them, oldest to newest.
What you’ll learn
- Layers of TimeExplain that rock layers (strata) form as sediment piles up and hardens, newest on top.The coloured stripes in cliffs and canyons are rock layers, or strata. They form as sand and mud settle and pile up, usually underwater, then harden into sedimentary rock. New layers always settle on top of older ones, building a stack that records time.
- Law of SuperpositionState the law of superposition and use it to order undisturbed rock layers by age.The law of superposition says that in undisturbed rock layers, the oldest layer is at the bottom and each layer above is younger. It works because new layers settle on top of old ones, like clothes in a laundry basket. This lets scientists put events in order from oldest to newest.
- Fossils in the LayersExplain that fossils follow the law of superposition and how index fossils match distant layers.Fossils inside rock layers follow the same rule: a fossil in a lower layer is older than one above it. Special index fossils, from creatures that lived worldwide for only a short time, let scientists match layers in faraway places and know they formed at the same time.
Questions this course answers
What are the stripes of rock in a canyon wall called?
The stripes are rock layers, also called strata — each one like a page of Earth's history.
How do new rock layers form on top of old ones?
Bits like sand and mud settle and pile up, usually underwater, then harden into sedimentary rock.
In undisturbed rock layers, where is the OLDEST layer?
By the law of superposition, the oldest layer is at the bottom and younger layers sit on top.
The laundry basket example helps explain that...
In a laundry basket, the first item dropped in is at the bottom — just like the oldest rock layer.
If one fossil is in a lower layer than another, what do we know?
A fossil in a lower layer is older than a fossil in a layer above it, following the law of superposition.
How do index fossils help scientists?
Index fossils lived worldwide for a short time, so matching ones show that distant layers are the same age.
Grounded in trusted sources
- United States Geological Survey (USGS)
- Natural History Museum
- Smithsonian's National Museum of Natural History
- Britannica Kids
Every Wunder lesson is built from real, reputable sources — never invented.
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