How can layers of rock tell you what came first?

Rock layers tell you order because sediment piles up from the bottom. This is the law of superposition: in an undeformed sequence of layers, the oldest strata are at the base. The Catholic priest Nicholas Steno set it out in 1669, together with the principle of original horizontality, which says layers are first deposited horizontally under gravity.

Fossils add a second clue. The fossils in a stack of layers succeed each other in a specific, reliable order, and that order can be recognised across wide distances. That is the principle of faunal succession. A Neanderthal bone will never be found in the same layer as a Megalosaurus, because the two lived in periods separated by millions of years.

The man who turned this into a map was William Smith, nicknamed 'Strata' Smith. From the 1790s he used strata and fossil markers to correlate rocks, and he made the first geological map of England. It works even when rock types differ: sediments of the same age can look completely different, but if two sections hold similar fossil species they were probably laid down around the same time.

In the 19th century this gave geologists the geologic time scale, built from fossils and layer order. It was a relative scale, a list of what came before what, until radiometric dating later gave it absolute numbers. It also has holes: a gap in the record of an area is called a stratigraphic hiatus.