Earth Science Regents Practice Exam 2025 – Complete Study Resource

The law of superposition is a fundamental principle in geology that states that in undisturbed sedimentary rock layers, the oldest layers are found at the bottom, while the younger layers are deposited on top of them. This means that as new material is deposited, it gradually builds up over time, forming horizontal layers where the lowest layer represents the earliest period of deposition.

Thus, in an undisturbed sequence of strata, the bottom layer contains the oldest material. This principle is widely used by geologists to determine the relative ages of rock formations and the fossils they contain, establishing a chronological sequence of geological events. The top layer, being the most recently deposited, would naturally be the youngest, while the middle layer represents a stage in the middle of the sequence. The presence of fossils can provide additional information about the age of the layers, but it does not determine the overall order of stratification according to the law of superposition.