Earth Processes > Cross-bedding


As sand is carried along by wind or water, it may pile up against a barrier forming a dune or ripple. As sand grains are pushed along, they move up the front surface of the pile and are deposited on the other side. As more sand is deposited, diagonal layers are formed. These layers are called crossbeds.



If you were to slice through a sand dune or the sediments at the bottom of the stream, you would get a cross-section view of crossbedding. The crossbeds in a sand dune are usually much larger. Stream crossbeds are smaller.


Over time layers of crossbeds may pile up on top of each other. Sometimes erosion levels off crossbeds. Eventually if conditions are right, a matrix glues the sand grains together, turning the sand into sandstone, and preserving the crossbeds.


When a geologist observes crossbeds in a sandstone like the Coconino Sandstone, she can infer that the environment in which the sand was deposited was either an area of sand dunes or a stream bed, depending on the characteristics of the crossbeds. Measurements of the angles and orientation of the crossbeds may also give clues to the direction the wind or water flow.

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