Crossbedded

Crossbedding, or cross-stratification, refers to internal layers that are inclined relative to the main layering of a sedimentary rock. These inclined units, called foresets, form as sediment is transported by flowing air or water and deposited on the downwind or downstream face of migrating bedforms such as dunes, ripples, or bars. The result is a set of crossbeds that record the direction of transport and the dynamics of the original surface.

Crossbeds develop in both air- and water-dominated settings. In aeolian environments, sand dunes and ripples migrate under wind; in subaqueous settings, migrating dunes and bars in rivers, tidal channels, or deltaic systems produce cross-bedded units. The angle of the foresets, the morphology of the bounding surfaces, and the scale of the crossbeds reflect the transporting current or wind strength and the size of the bedforms. Common crossbed types include trough cross-bedding, with curved foresets typical of dune or ripple fields, and planar or simple cross-bedding, associated with more uniform flow or dune fields.

Geometrically, crossbeds are bounded by erosional surfaces and show truncation where the active foresets are buried by subsequent deposition. Their dips typically range from a few to a few tens of degrees, depending on environmental conditions. In rocks, crossbedding is preserved as markers of paleocurrent directions and can be measured to infer ancient wind regimes, flow directions, and sediment supply. Diagenesis can modify or erase crossbeds, but well-preserved examples remain valuable for reconstructing sedimentary environments and stratigraphic relationships.

Crossbedding is thus a key indicator of past depositional processes and paleogeography, helping geologists interpret ancient deserts, rivers, deltas, and related depositional systems.