When one slit is covered, the pattern is what would be expected: a single line of light, aligned with whichever slit is open.Intuitively, one would expect that if both slits are open, the pattern of light will reflect two lines of light aligned with the slits.This concept known as the Principle of Lateral Continuity allows us to assume that similar layers of rock or sediment that are separated by a valley or other erosional feature were once continuos.
The Principle of Original Horizontality states that due to the influence of gravity all sediment is originally deposited horizontally.
Long before geologists tried to quantify the age of the Earth they developed techniques to determine which geologic events preceded another, what are termed "relative age” relationships.
These techniques were first articulated by Nicolas Steno, a Dane living in the Medici court of Italy in the 17th C.
Using this principle any fault or igneous intrusion must be younger than all material it or layers it crosses.
Much like the principle of cross-cutting, the Principle of Inclusions relatively dates objects based on their placement within other earth materials.