Sometimes sedimentary rocks are disturbed by events, such as fault movements, that cut across layers after the rocks were deposited.
There are three general approaches that allow scientists to date geological materials and answer the question: "How old is this fossil?
Layers of rock are deposited horizontally at the bottom of a lake (principle of original horizontality).
Younger layers are deposited on top of older layers (principle of superposition).
Despite seeming like a relatively stable place, the Earth's surface has changed dramatically over the past 4.6 billion years.
Mountains have been built and eroded, continents and oceans have moved great distances, and the Earth has fluctuated from being extremely cold and almost completely covered with ice to being very warm and ice-free.