Cross-bedding

In geology, cross-bedding refers to inclined sedimentary structures in a horizontal unit of rock; such tilted structures indicate the type of depositional environment, not post-depositional deformation.  

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Cross-bedding structures form in bedforms such as ripples and dunes. Sand grains bounce up the stoss side of a ripple, and then tumble down the lee side. Separation of heavy minerals, and cyclical changes in flow cycles cause the different bedding planes.

Cross-bedding can form in any environment where water or wind flows and sand or gravel exists on the bed of the system. It is most common in stream deposits, tidal areas, as well as in sand dunes. Careful study of cross-beds can often determine ancient flow or wind directions as the dip direction of the cross-bedded sediments provides insight into flow direction. Sand dune cross-beds can be large, such as in the Jurassic-age erg deposits of the Navajo Sandstone.

Cross-bedded sediments that form along the outer portion of a river delta are termed foreset beds.

References

• Monroe, James S. and Wicander, Reed (1994) The Changing Earth: Exploring Geology and Evolution, 2nd ed., St. Paul, Minn. : West, ISBN 0-314-02833-1, pp. 113–114.

• Rubin, David M. and Carter, Carissa L. (2006) Bedforms and cross-bedding in animation, Society for Sedimentary Geology (SEPM), Atlas Series 2, DVD #56002, ISBN 1-56576-125-1