Geological principles state that if a fragment of rock is included, that is entirely surrounded by rock, it must be older than the surrounding rock. This is because it had to be present for the rock to form around it. This principle is one of several principles geologists use to guide them in guessing possible relative dates for rock formations they find during digs and other research.
The geological principle of inclusion is only one of several principles geologists use to estimate the relative age of rocks. One of the simplest of these other principles is the principle of superposition. The principle of superposition states that in layered rocks, each layer is younger than the one below it. The principle of horizontal layering states that sediments are deposited in flat, nearly horizontal layers, so if layers of rock are inclined or folded, this occurred after they formed. The principal of original continuity states that, over any flat area, sediments are deposited relatively evenly. Thus, if a canyon cuts through a flat area of sedimentary rock, the same layers of rock should be evident on both sides of the canyon. These principles and others are all used to simplify the task of analyzing geological structures.