The principle of superposition says that in a sequence of rock strata that has not been overturned or heavily disturbed, the oldest layers are at the bottom and the layers become progressively younger toward the top. This is a basic rule geologists use to work out the relative ages of sedimentary rocks and lava flows. In practice, this means any given layer is older than the one above it and younger than the one below it, as long as the sequence is still in its original order. Geologists rely on this principle, together with other stratigraphic rules, to reconstruct the order of events in Earth’s history by reading rock layers like pages in a book.