"Strata" means layers of sedimentary rock or volcanic rock. Rocks are formed in layers that tell a story of the Earth. This is called the "rock record". By studying the arrangement of a rock formation (stratigraphy) we can tell the order of the rock layers, which layer came first and how old the rocks are.
- Layers deposited by water are usually horizontal (though layers deposited otherwise are usually tilted.)
- When layers have not been disturbed, a layer is older than the one above it and younger than the one below it.
- If something cuts across a layer, whatever is cutting across is younger than the layer it cuts across.
- Fossils in lower layers are older than fossils found in top layers.