1. Principle of Superposition: This principle states that in an undisturbed sequence of sedimentary rocks, the oldest rocks will be at the bottom and the youngest rocks will be at the top. Think of it like a stack of pancakes - the first pancake you put down is the oldest, and the last one you add is the youngest.
2. Principle of Original Horizontality: This principle states that sedimentary rocks are originally deposited in horizontal layers. While we see tilted or folded rocks in many places, they were initially laid down horizontally.
3. Principle of Lateral Continuity: This principle states that sedimentary layers extend laterally (horizontally) in all directions until they either thin out or terminate at the edge of their depositional basin. This means that similar rocks found in different locations could have been originally part of the same layer.
4. Principle of Cross-Cutting Relationships: This principle states that any geologic feature that cuts across another feature is younger than the feature it cuts. For example, if a fault cuts through a layer of rock, the fault is younger than the rock layer.
Bonus Principle: While not always included in the "Big Four," the Principle of Fossil Succession is crucial to stratigraphy. It states that fossils appear in a predictable order within the rock record. This allows geologists to correlate rock units across vast distances and determine relative ages.
These principles are the cornerstones of stratigraphy and are used to understand the history of the Earth, its rocks, and the life that has existed on it.
