What does the principle of original horizontality state?

The principle of original horizontality asserts that layers of sediment are deposited under the action of gravity in a horizontal or nearly horizontal manner. This principle is fundamental in geology because it helps in understanding how sedimentary rocks form. When sediments settle in a body of water, they tend to accumulate in layers that are laid down horizontally over time. Any deviation from this horizontal arrangement, such as tilting or folding, indicates that subsequent geological processes (like tectonic activity) have acted on the sediments after their deposition.

This principle allows geologists to infer the history of the geological formations being studied and to interpret past environments. For example, when examining sedimentary strata that appear tilted or folded, one can conclude that tectonic forces have occurred post-deposition. Therefore, understanding that sediment layers initially form horizontally provides a baseline for interpreting later geological changes and interactions.