Mr. Nicolas Steno, b. 1638, in Copenhagen, was a fellow of many abilities. He was a paleontoogist, geologist, a medical doctor, and ultimately, a Catholic bishop. A busy man, all over Europe. He is remembered for his Theory of Stratigraphy; he published his ideas in 1669.
Stratigraphy is the idea that each layer of rock forms on the rock already there, and builds up with time. The new layer is fairly even, but eventually the earth buckles or shifts, disrupting the layers- not in order, but in relative location. By studying these layers, We can see what was going on when the layer formed, what plants and animals are fossilized in each layer. The top layer is the newest. It takes more time than any Rhino could imagine.
Like many great discoveries, it all seems obvious, once somebody points is out. It makes Mr. Steno a Founder of Modern Geology. (Back in Denmark, Mr. Steno was called Neils Steensen, FYI.)