How is cross bedding formed

Cross beds also form in sand dune environments--these however, are usually much larger scale than those created by water. Additionally, the grain size of wind blown sand is generally very fine. Cross beds in Cambrian Prospect Mtn. Quartzite, E. Light layers were deposited by storms or floods, and briefly interrupted algal growth. When cross-bedding forms, sand is transported as sand-dune like bodies sandwave , in which sediment is moved up and eroded along a gentle upcurrent slope, and redeposited avalanching on the downcurrent slope see upper half of picture at left.

After several of these bedforms have migrated over an area, and if there is more sediment deposited than eroded, there will be a buildup of cross-bedded sandstone layers. The inclination of the cross-beds indicates the transport direction and the current flow from left to right in our diagram.

The style and size of cross bedding can be used to estimate current velocity, and orientation of cross-beds allows determination direction of paleoflow. Cross-bedding in a sandstone that was originally deposited by rivers. The deposition currents were flowing from right to left. Cross-bedding can also be produced when wind blows over a sand surface and creates sand dunes. The picture on the left shows ancient sanddunes with cross-bedding. Diagram illustrating the formation of a graded bed turbidite.

Once it reaches the flat deep sea regions, it slows down due to friction, and gradually the sediment settles out of suspension. Larger grain sizes settle out first, and then successively smaller ones. Example of a graded bed. Largest grains occur at the base, and the grain size gradually decreases. Current ripples in a creek in Arlington. Ripples are asymmetrical and have a gentle slop on the right and a steep slope on the left. Photo taken near New Kensington, PA These matched sets of cross-beds tilting in opposite directions are called herringbone cross-beds.

Such beds are most commonly produced by tidals moving in and out of a shallow sea. These are carbonate sands deposited in a tidal lagoon about million years ago. Marble Mountains, near Cadiz, CA. Cross Bedding Sedimentary rocks are normally deposited as horizonal layers. Photo taken near New Kensington, PA. These matched sets of cross-beds tilting in opposite directions are called herringbone cross-beds. E-mail C. Jones with comments or corrections. Geology and Planetary Science HomePage.