A 17th-century priest worked out the basic rules for reading rock layers
In 1669 Nicholas Steno, a Catholic priest, set down a principle that sounds obvious once said aloud: in undisturbed layers of rock, the oldest lie at the bottom. That law of superposition, along with a few companions, turned cliffs and quarries into readable calendars of Earth's past.
Steno's work on fossils trapped in sediment introduced three ideas at once: superposition, the rule that layers are originally laid down flat, and the rule that they extend sideways until something interrupts them. In 1759 the Italian geologist Giovanni Arduino divided the crust into four successive orders, Primary, Secondary, Tertiary and Quaternary, a scheme often seen as the start of modern stratigraphy. The first large practical use came from William Smith, the father of English geology, who from the 1790s used fossils to match layers across distances and drew the first geological map of England. Georges Cuvier and Alexandre Brongniart applied similar methods around Paris.
Smith's observation that fossil groups follow each other in a fixed order became one of the earliest and strongest lines of evidence for evolution, documenting both the appearance and the extinction of species. Nineteenth-century geologists built the geological time scale on it, but that scale only gave relative ages until radiometric dating supplied absolute ones. Chronostratigraphy aims ultimately to date every rock in every region and so reconstruct Earth's complete history.
Missing layers matter as much as present ones. A gap in the sequence, called a hiatus, may mean sediment simply stopped arriving for a while, or that erosion stripped layers away; a fault can mimic one too. Specialised branches read other signals. Chemostratigraphy tracks shifting carbon and oxygen isotopes to trace ancient environments, and cyclostratigraphy follows rhythmic changes in layer thickness or fossil variety linked to climate cycles.
Magnetostratigraphy uses the planet's flipping magnetic field. Fine magnetic grains smaller than 17 micrometres settle through water like tiny compasses and lock in the field's direction when buried, so a column of samples records normal and reversed polarity that can be matched against a global timescale. It is especially useful where rocks lack fossils. The same toolkit also helps petroleum geologists find reservoir rocks and the seals that trap oil and gas.
Source: Stratigraphy
