What geologic principle states that in horizontal layers of sedimentary rock, each layer is older than the layer above it and younger than the layer below it?

The principle that states in horizontal layers of sedimentary rock, each layer is older than the layer above it and younger than the layer below it is known as the Law of Superposition. This fundamental concept in geology helps establish the relative ages of sedimentary rock layers.

When sediments are deposited in horizontal layers, the bottom layer is formed first, followed by subsequent layers stacked on top. Therefore, the oldest layers are at the bottom, and the youngest are at the top. This principle is cornerstone in the study of stratigraphy, as it provides a clear, straightforward method for interpreting the geological history of an area.

Understanding the Law of Superposition is essential for reconstructing the timeline of Earth's history and for various applications such as dating fossils or understanding sedimentary environments.