At or near Earth's surface, sedimentary rocks form in two ways: by the accumulation of rock grains or by the formation of a solid from minerals dissolved in water. The fragments that go into making sedimentary rocks can be as big as boulders or as small as clay particles. Over long periods of time, the upper layers of debris compress the lower layers, squeezing out excess water or air trapped between the rock fragments. Under the pressure, individual fragments eventually dissolve and stick together, or the remaining fluid within the sediment brings in other substances that act as a cement, until the sediment has turned into rock. Scientists classify many sedimentary rocks based on the size of the particles that built the rock; mudstone and sandstone, for example, originally came from fine-grained mud and sand deposits that hardened over long time periods.

Wind, water, and ice pick up rock and mineral grains, the building blocks of sedimentary rocks, and eventually deposit them in horizontal or nearly horizontal layers. Geologists call these layers strata (a single layer is a stratum, from the Latin word for cover). Humans can see multiple layers of sedimentary rock in places where natural erosion or human digging has exposed them to view. By studying the order in which the strata were deposited, a discipline of geology called stratigraphy, geologists can estimate the relative ages of the layers. As a general principle, the oldest strata are at the bottom of a stack of layers, and the youngest strata are at the top. Sometimes, though, the forces of earthquakes or plate tectonics tilt or buckle strata in various places, or even overturn the strata and bring the oldest layers to the top. In such cases, scientists must use other clues, such as ripple marks from past wind and water motion or fossils, to figure out the original orientation of the strata.