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Stratigraphy is the study of rock layers (strata) and their spatial and temporal relationships, governed by Steno's principles: superposition (younger beds overlie older), original horizontality (beds are deposited flat), and lateral continuity (beds extend laterally until they thin or terminate). Unconformities—surfaces of missing time where erosion removed strata or deposition ceased—record gaps in the geological record and are classified as angular unconformities, disconformities, or nonconformities. Correlation matches rock units across separated outcrops using lithological similarity, key beds (volcanic ash layers, impact ejecta), or fossil content; radiometric ages anchor correlations absolutely. Sequence stratigraphy relates stratigraphic packages to relative sea-level cycles, predicting rock architecture in sedimentary basins.
Interpreting a stratigraphic column diagram—identifying the oldest and youngest units, locating unconformities, and determining the relative sequence of events—directly applies all four of Steno's principles. Cross-correlating two widely separated columns using index fossils or a distinctive volcanic ash layer demonstrates how stratigraphy builds a regional picture from local outcrops.
From your study of sedimentary rocks, you know that sediments accumulate in layers, and from the geological time scale, you know that Earth's history spans billions of years divided into named intervals. Stratigraphy is the discipline that connects these ideas: it provides the principles for reading the order, age, and meaning of rock layers — turning exposed cliff faces and drill cores into a narrative of Earth history.
The foundational rules come from Nicolas Steno (17th century) and remain the starting point for any stratigraphic analysis. The law of superposition states that in an undisturbed sequence, each layer is younger than the one below it and older than the one above — a principle so intuitive it seems trivial, yet it provides the basic logic for relative dating. Original horizontality holds that sedimentary layers are deposited in approximately horizontal sheets; if you find tilted or folded strata, something has deformed them after deposition. Lateral continuity states that layers originally extend in all directions until they thin out at the basin edge or grade into a different sediment type. Together, these principles let you reconstruct the original geometry of a sedimentary sequence even when erosion, faulting, or folding has disrupted it.
Unconformities are the most important features in stratigraphy because they represent missing time — intervals when deposition ceased or when previously deposited rock was eroded away. An angular unconformity is dramatic: tilted or folded layers are truncated by erosion and then overlain by flat-lying younger strata, recording an entire cycle of deposition, deformation, uplift, and erosion before burial resumed. A disconformity is subtler: the layers above and below are parallel, but a time gap (recognizable from missing fossils or a weathered surface) separates them. A nonconformity separates sedimentary rocks from underlying igneous or metamorphic rocks, indicating that deep crystalline basement was once exposed at the surface. Recognizing unconformities is critical because the geological record is more gap than record — at any given location, more time is represented by missing strata than by preserved strata.
Correlation is the practice of matching rock units across separated outcrops to build a regional or global picture. Lithostratigraphic correlation matches similar rock types (a distinctive red sandstone, a thick limestone), but this is unreliable over long distances because the same environment can produce different rocks in different places. Biostratigraphic correlation uses index fossils — species that were widespread, abundant, and short-lived — to match time intervals. If two outcrops 500 km apart both contain the same ammonite species, those layers are approximately the same age regardless of rock type. Chronostratigraphic correlation anchors the relative sequence to absolute time using radiometric dates from volcanic ash layers or other datable materials. Sequence stratigraphy adds another dimension by relating packages of strata to cycles of relative sea-level change: during transgression (rising sea level), fine-grained sediments blanket the shelf; during regression (falling sea level), coarser sediments prograde seaward. These predictable patterns allow geologists to reconstruct basin architecture and even predict the location of petroleum reservoirs from stratigraphic principles alone.