Fundamentals of geology I. (lithosphere) 1 1. The formation of the Earth 1
Fig. 2.5. General model od cross stratification
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- Fig. 2.6. Model of 2D cross stratification Trough (3D) cross-beds
Fig. 2.5. General model od cross stratification Tabular (2D) cross-beds. Tabular (planar) cross-beds consist of cross-bedded units that have large horizontal extent relative to set thickness and that have essentially planar bounding surfaces. The foreset laminae of tabular cross-beds have curved laminae that have a tangential relationship to the basal surface. Tabular cross-bedding is formed mainly by migration of large scale, straight crested ripples and dunes. It forms during lower flow regime conditions and its individual beds range in thickness from a few tens of centimeters to a meter or more, but bed thickness down to 10 centimeters has been observed. 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. For larger features, the term cross-bedding is used. They occur typically in granular sediments, especially sandstone, and are indication of sediments deposited in deltas, sand dunes and glaciers (Fig. 2.6.). 
Fig. 2.6. Model of 2D cross stratification Trough (3D) 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. Cross beds are angled relative to either the base or the top of the surrounding beds. As opposed to angled beds, cross beds are deposited at an angle rather than deposited horizontally and deformed later on. 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 (Fig. 2.7.). 
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