They are the "initial working hypotheses" to be tested further by data.
In more complicated situations, like in a mountain belt, there are often faults, folds, and other structural complications that have deformed and "chopped up" the original stratigraphy.
An early summary of them is found in Charles Lyell's .
In no way are they meant to imply there are no exceptions.
Much of the Earth's geology consists of successional layers of different rock types, piled one on top of another.
The most common rocks observed in this form are sedimentary rocks (derived from what were formerly sediments), and extrusive igneous rocks (e.g., lavas, volcanic ash, and other formerly molten rocks extruded onto the Earth's surface).