The principle of original horizontality refers to an assumption by geologists that all layers of sedimentary rock were horizontal when they were formed. This is one of the basic principles of stratigraphic succession, which refers to the way that geological processes build up and change layers of rock over time.
These principles are used to determine the age of rocks through relative dating. Scientists also employ numerical dating, which looks at radioactive decay to figure out the actual age of rocks. A second assumption that geologists make regarding relative dating is that the oldest rock layers are closest to the bottom, and the newest layers are closest to the top. This principle is known as the law of superposition.