primary stratification

Stratification refers to the layering found in most sedimentary rocks and some igneous rocks formed at Earth's surface. These layers, called strata, can range from millimeters to meters in thickness and vary in shape and extent. Stratification planes separate these layers, which can be horizontal or inclined depending on the depositional site.1

These layers form due to changes in texture or composition during deposition, or pauses in deposition that allow older sediments to change before new ones are added. Variations in the transporting ability of agents like water and wind, which sort sediments by size and weight, are common causes of stratification. The record of Earth's past movements is preserved in deformed strata, aiding in the location of mineral deposits, petroleum fields, and groundwater. Primary sedimentary structures, such as bedding, graded bedding, and cross-bedding, are formed concurrently with the rock itself.12