Early geologists had no way to determine the absolute age of a geological material. What they could do was determine the ages of materials relative to each other.
If they didn’t see it form, they couldn’t know if a rock was one hundred years or 100 million years old. Using sensible principles they could say whether one rock was older than another.
The layer containing the gravel must be younger than the layer from which the clasts originate.
But in some families, multiple marriages, delayed childbearing, extended childbearing or other variations mixes up generations so that Aunt Julia may be five years younger than her nephew. How do Steno’s laws help geologists to decipher the geological history of a region? In many places, especially the national parks, the formations are fantastic. The principles discussed below are easily seen around the Southwest.Remember Nicholas Steno, who determined that fossils represented parts of once-living organisms? Superposition refers to the position of rock layers and their relative ages (Figure below).Steno also noticed that fossil seashells could be found in rocks and mountains far from any ocean. Steno proposed that if a rock contained the fossils of marine animals, the rock formed from sediments that were deposited on the seafloor. Relative age means age in comparison with other rocks, either younger or older.Steno was born as Niels Stensen, but he is better known by the Latinized forms of his name, Nicholas Stenonis or Nicholas Steno.A native of Copenhagen, Denmark, Steno left Denmark in 1660 to study medicine at the leading center for medical education of his time, the University of Leiden in the Netherlands.Steno formalized the laws of superposition, original horizontality, original continuity and inclusions in his publication entitled states that any inclusion is older than the rock that contains it.