The second principle is that sedimentary rocks form as broad layers that extend laterally until they "pinch out" or thin at the edge of a depositional basin, or grade into another environment. This is called the Principle of Lateral Continuity. In the cartoon below, for example, we can see that layers of sediment deposited in the shallow marine environment may extend for miles all directions until they grade into beach deposits or deposits ofthe reef environment or deep marine environment.