Fossils and Geological Time

Sedimentary rock builds up one layer at a time, oldest on the bottom, youngest on top. That is the law of superposition, and it turns a plain cliff face into a timeline: walk up the exposed rock and you walk forward through time. If something cuts across the layers, like an igneous dike that punches up through the bottom few beds and stops, that feature has to be younger than every layer it cuts through but older than any layer deposited on top of it undisturbed. So you can reconstruct a whole sequence of events just from geometry: which layers formed first, when the intrusion happened, and what piled on afterward.

Fossils let you date rock by what is inside it rather than the minerals themselves, because the same organism often shows up in completely different rock types across the globe. The best fossils for this job, index fossils, lived in huge numbers over a wide area but only for a short stretch of geologic time. Rarity does not make a fossil useful for dating; a rare creature might not turn up in enough rock layers to compare anything. What matters is that the species was common and widespread while it existed, then vanished, so finding it anywhere pins that rock to the same narrow slice of time.

Radiometric dating adds a number to this story. A radioactive parent isotope decays into a daughter isotope at a fixed rate described by its half-life: the time it takes for half of the remaining parent atoms to decay. Each half-life that passes cuts the parent amount in half again, so after n half-lives only 1/2ⁿ of the original parent remains. Measuring that remaining fraction in a sample tells you how many half-lives have passed, and multiplying by the half-life length gives the age directly.