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⛰️ Rocks and Rock Layers

Sedimentary rock builds up in layers like pages in a book. Learn to read those layers and the story of time they hold.

lessons

~20 min

to learn

🔬 Science

subject

Ages 6–12

level

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What you’ll learn

Three Kinds of RockName the three kinds of rock and identify sedimentary rock as the layered kind.Rocks come in three families: igneous from cooled lava, metamorphic changed by heat and pressure, and sedimentary formed in layers. Sedimentary rocks, like sandstone, are made of pressed-together sediment and often show visible layers.
Layers of TimeDescribe how sedimentary layers form and state the rule of superposition.Sedimentary rock forms as sediment settles, piles up, and hardens into flat layers called strata. New layers land on top of old ones, so by the rule of superposition the oldest layer is at the bottom and the youngest is at the top.
Reading the LayersUse superposition to order layers and fossils by age.Using superposition, scientists order layers from oldest at the bottom to youngest at the top. Fossils trapped in the layers follow the same rule: deeper fossils are older. Rock layers are like the pages of Earth's diary.

Questions this course answers

Which kind of rock forms in layers from pressed-together sediment?

Sedimentary rock forms when layers of sediment, like sand and mud, are pressed and cemented together.

Sandstone is made mostly of what?

Sandstone is a sedimentary rock made of countless sand grains that were pressed and cemented into solid rock.

What is the first step in forming sedimentary rock?

First, weathering and erosion break rock into tiny pieces of sediment. Only then can it settle, pile up, and harden.

What are the stacked flat sheets of sedimentary rock called?

The flat, stacked layers of sedimentary rock are called strata, and each one records a moment in time.

In undisturbed rock layers, where is the oldest layer?

By the rule of superposition, the oldest layer is at the bottom because it was laid down first.

A fossil found deep in a cliff is likely...

Deeper layers are older, so a fossil found low in the cliff is older than one found near the top.

Grounded in trusted sources

USGS
National Park Service
National Geographic Kids
Britannica Kids

Every Wunder lesson is built from real, reputable sources — never invented.

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