Reading Earth's History in Rock Layers
Aligned to MS-ESS1-4 — Next Generation Science Standards.
What this lesson teaches
Sediment piles up in layers over time, so in undisturbed rock the oldest layers lie at the bottom and the youngest on top, like a stack of newspapers. Fossils trapped in each layer act as time markers, letting geologists match up rocks and build the geologic time scale that organizes Earth's 4.6-billion-year history.
Worked example
In the walls of the Grand Canyon, a hiker walking down passes through layers laid over hundreds of millions of years. Marine fossils high in the canyon reveal that a shallow sea once covered the region long after the deeper, older rocks below had already formed.
Practice questions
- Drop layers of different-colored sand or dirt into a clear jar and identify which layer is oldest and why.
- Explain how the same fossil found in two far-apart cliffs tells geologists the rocks are about the same age.
- Describe what a tilted or folded rock layer tells you must have happened after it was first deposited.
Watch the lesson
Every lesson comes with a video taught in English and Spanish — the same video the QR code in the printed workbook opens.
▶ Watch this lessonEn español
Leer la historia de la Tierra en las capas de roca
El sedimento se acumula en capas con el tiempo, así que en roca no alterada las capas más antiguas quedan abajo y las más jóvenes arriba, como una pila de periódicos. Los fósiles atrapados en cada capa funcionan como marcadores de tiempo, que permiten a los geólogos correlacionar rocas y construir la escala de tiempo geológico que organiza los 4,600 millones de años de historia de la Tierra.
Ejemplo: En las paredes del Gran Cañón, un excursionista que baja atraviesa capas depositadas durante cientos de millones de años. Los fósiles marinos en la parte alta del cañón revelan que un mar poco profundo cubrió la región mucho después de que ya se habían formado las rocas más profundas y antiguas de abajo.
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