Final answer:
The principle of original horizontality, formulated by Nicolas Steno, is true and states that sediments settle out of fluid suspension into nearly horizontal layers. Any subsequent tilting or folding of these layers indicates geological processes that have occurred after deposition.
Step-by-step explanation:
The principle of original horizontality states that beds of sediment are nearly always deposited in nearly horizontal layers. This is true. The principle, formulated by the 17th-century geologist Nicolas Steno, posits that due to gravity, sediments settle out of fluid suspension as horizontal or nearly horizontal layers. Any deviation from this horizontality, such as tilting or folding, suggests that subsequent geological processes have altered these layers after their initial deposition. This principle remains fundamental in the study of sedimentary rocks, as geologists utilize it to interpret Earth's history.
Even when considering scenarios where underlying strata are not horizontal (for example, on slopes), new layers of sediment will conform to the pre-existing angle of repose until natural forces such as gravity cause them to break away. When these sediments come to rest, the principle of original horizontality still applies. It is only through later geological events such as tectonic activity that these once horizontal layers may become tilted, folded, or otherwise disturbed, giving rise to features such as angular unconformities, which produce an observable angle between older tilted layers and younger horizontal strata.