Older layers of rock are usually found deeper underground., , New layers of dirt slowly build up over old layers over time., , “This fossil is millions of years old,” said the scientist., , The Law of Superposition explains how Earth’s layers form over time., , Archaeologists dig carefully to study ancient objects, bones, and fossils., , Volcanoes can cover the land with ash, smoke, and hot rock., , After a flood, rivers sometimes leave behind mud and sand., , Deeper rock layers are usually older than shallow layers: this helps scientists study history., , Earthquakes, landslides, and construction can sometimes mix up the layers., , Scientists ask important questions, such as, “Which layer is the oldest?”, .

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