Cross-bedding is formed by the downstream migration of bedforms such as ripples or dunes in a flowing fluid. The fluid flow causes sand grains to
saltate up the stoss (upstream) side of the bedform and collect at the peak until the angle of repose is reached. At this point, the crest of granular material has grown too large and will be overcome by the force of the moving water, falling down the lee (downstream) side of the dune. Repeated avalanches will eventually form the sedimentary structure known as cross-bedding, with the structure dipping in the direction of the paleocurrent. The sediment that goes on to form cross-stratification is generally sorted before and during deposition on the "lee" side of the dune, allowing cross-strata to be recognized in rocks and sediment deposits. The angle and direction of cross-beds are generally fairly consistent. Individual cross-beds can range in thickness from just a few tens of centimeters, up to hundreds of feet or more depending upon the depositional environment and the size of the bedform. Cross-bedding can form in any environment in which a fluid flows over a bed with mobile material. It is most common in
stream deposits (consisting of sand and gravel), tidal areas, and in
aeolian dunes.
Internal sorting patterns Cross-bedded sediments are recognized in the field by the many layers of "
foresets", which are the series of layers that form on the downstream or lee side of the bedform (ripple or dune). These foresets are individually differentiable because of small-scale separation between layers of material of different sizes and densities. Cross-bedding can also be recognized by truncations in sets of ripple foresets, where previously-existing stream deposits are eroded by a later flood, and new bedforms are deposited in the scoured area.
Geometries Cross-bedding can be subdivided according to the geometry of the sets and cross-strata into subcategories. The most commonly described types are tabular cross-bedding and trough cross-bedding. Tabular cross-bedding, or planar bedding consists of cross-bedded units that are extensive horizontally relative to the set thickness and that have essentially planar bounding surfaces. Where the set height is less than 6 centimeters and the cross-stratification layers are only a few millimeters thick, the term cross-lamination is used, rather than cross-bedding. Cross-bed sets occur typically in granular sediments, especially
sandstone, and indicate that sediments were deposited as ripples or dunes, which advanced due to a water or air current.
Trough cross-beds Cross-beds are layers of sediment that are inclined relative to the base and top of the bed they are associated with. Cross-beds can tell modern geologists many things about ancient environments such as- depositional environment, the direction of sediment transport (paleocurrent) and even environmental conditions at the time of deposition. Typically, units in the rock record are referred to as beds, while the constituent layers that make up the bed are referred to as laminae, when they are less than 1 cm thick and strata when they are greater than 1 cm in thickness. Trough cross-beds have lower surfaces which are curved or scoop shaped and truncate the underlying beds. The foreset beds are also curved and merge tangentially with the lower surface. They are associated with sand dune migration.
Sediment The shape of the grains and the sorting and composition of sediment can provide additional information on the history of cross-beds. Roundness of the grains, limited variation in grain size, and high quartz contents are generally attributed to longer histories of weathering and sediment transport. For example: well-rounded, and well-sorted sand that is mostly composed of quartz grains is commonly found in beach environments, far from the source of the sediment. Poorly sorted and angular sediment that is composed of a diversity of minerals is more commonly found in rivers, near the source of the sediment. However, older sedimentary deposits are frequently eroded and re-mobilized. Thus, a river may well erode an older formation of well-rounded, well-sorted beach sands of nearly pure quartz. ==Environments==