Stratigraphy is the study of rock layers (strata). Three of the rules below (superposition, original horizontality and lateral continuity) were first written down in 1669 by Nicolas Steno, a Danish scientist working in Italy. They work almost everywhere and need no equipment beyond your eyes. Together they give you relative age, meaning which rock is older, even when you can't say how old in years.

Later in the module we add the missing piece: absolute age, a number in years, which comes from the steady decay of radioactive atoms.

Each rule is common sense once you picture how sediment piles up and how rocks get disturbed afterwards. None of them needs a laboratory.

Real outcrops rarely test one rule at a time. A good habit is to work from the bottom up. Superposition gives the starting order. Original horizontality tells you whether anything happened to the layers after they formed. Anything that slices across them (a fault, a dike, an erosion surface) slots in by cross-cutting. Pebbles and fragments slot in by inclusions.

To match rocks between places, you use the other two rules. Lateral continuity works over short distances, where you can trace a layer or see it on both sides of a valley. Over longer distances, rock types change because the environment changed: a beach passes into a muddy sea floor. Then faunal succession takes over, because the same fossil species lived across a whole sea at the same time. Matching layers of the same age between places is called correlation.

The result is a sequence of events, oldest to youngest, with no numbers attached yet. For a full puzzle with seven events, try the field exercise in Intrusions & unconformities.

The rules give you the order of events. To put numbers on them, geologists use radiometric dating.

Atoms of one element can come in slightly different versions, called isotopes, which differ in how many neutrons they carry. Most isotopes are stable. A few are radioactive: their nuclei are unstable and, sooner or later, change into a different element. The original atom is the parent and what it becomes is the daughter. Uranium-238, for example, ends up as lead-206.

Nobody can predict when one particular atom will decay. But in a large crowd of atoms, a fixed fraction decays in any given time. The time it takes for half of the parent atoms to decay is the half-life. After one half-life, half the parent is left. After two, a quarter. After three, an eighth. Heat, pressure and chemistry barely change half-lives, which is what makes them reliable clocks.

The clock starts when a crystal forms. Zircon, a tough mineral that grows in cooling magma, takes uranium into its structure but rejects lead. So a newly formed zircon has parent atoms and almost no daughter. From then on, daughter atoms pile up inside it. Measure the ratio of daughter to parent and you can read off how many half-lives have passed.

Here is the catch. Most fossils sit in sedimentary rocks, and you usually can't date a sedimentary layer directly. Its grains were eroded from older rocks, so a radiometric age tells you when the source rock crystallised, not when the sand was laid down.

Instead, geologists look for igneous rocks within the sequence, whose crystals did form at the moment the rock appeared. The best are volcanic ash beds: ash settles over a wide area in days to years and carries fresh zircon crystals. Lava flows and dikes work too. Then the stratigraphic rules do the rest. A layer above a dated ash bed is younger than it, and a layer below is older. A layer sandwiched between two dated beds is bracketed: its age lies between theirs.

Most numerical ages on the geological time scale, such as the roughly 66-million-year age of the extinction that ended the non-bird dinosaurs, come from exactly this: dated volcanic layers bracketing fossil-bearing ones.