Concepts of Stratigraphic Time
The field of stratigraphy is a branch of geology that deals with rock succession through time and space. Stratigraphy is largely based on the law of superposition, originally brought into vogue by Nicholas Steno in the 1600's. In geology, this law states that in a vertical succession of rocks a time sequence is formed with the oldest layers at the bottom with younger ones at the top. Geologists have to be careful in some cases when applying this law because tectonic forces are capable of tilting of strata which may obscure layer succession. For this reason, Steno introduced the principle of original horizontality, which states that all layers of sedimentary rocks were originally deposited in flat layers.
One classic example that illustrates law of superposition is the Grand Canyon where thick stacks of sedimentary rocks, known as layer cake geology, offers a look through time at multiple stages of sedimentary rock deposition since the Paleozoic Era.
Another law important to the field of stratigraphy is the principle of fossil succession. This law goes hand in hand with the law of superposition and basically states that animals change through time.
Stratigraphy is a complex and intriguing form of geology which tells the story of the Earth through time. The principles introduced here are important axioms which form the foundation on which stratigraphy is built.