Editing Sedimentary rock (section)

===Primary sedimentary structures===
[[File:Crossbeddingbressay.jpg|thumb|upright=1.2|[[Cross-bedding]] in a [[fluviatile]] sandstone, [[Old Red Sandstone|Middle Old Red Sandstone]] ([[Devonian]]) on [[Bressay]], [[Shetland Islands]]]]
[[File:Flute cast - Triassic - Riba de Santiuste - Spain.JPG|thumb|[[Flute cast]]s, a type of [[sole marking]] on the base of a vertical layer of Triassic [[sandstone]] in Spain]]
[[File:Rippelmarken Hassberge ReiKi.jpg|thumb|[[Ripple marks]] formed by a current in a sandstone that was later tilted ([[Haßberge (district)|Haßberge]], [[Bavaria]])]]

Structures in sedimentary rocks can be divided into ''primary'' structures (formed during deposition) and ''secondary'' structures (formed after deposition). Unlike textures, structures are always large-scale features that can easily be studied in the field. [[Sedimentary structures]] can indicate something about the sedimentary environment or can serve to tell [[way up structure|which side originally faced up]] where tectonics have tilted or overturned sedimentary layers.

Sedimentary rocks are laid down in layers called [[Bed (geology)|beds]] or [[Stratum|strata]]. A bed is defined as a layer of rock that has a uniform [[lithology]] and texture. Beds form by the deposition of layers of sediment on top of each other. The sequence of beds that characterizes sedimentary rocks is called [[bed (geology)|bedding]].{{sfn|Tarbuck|Lutgens|1999|pp=160-161}}{{sfn|Press|Siever|Grotzinger|Jordan|2003|p=171}} Single beds can be a couple of centimetres to several meters thick. Finer, less pronounced layers are called laminae, and the structure a lamina forms in a rock is called [[lamination (geology)|lamination]]. Laminae are usually less than a few centimetres thick.{{sfn|Boggs|1987|p=138}} Though bedding and lamination are often originally horizontal in nature, this is not always the case. In some environments, beds are deposited at a (usually small) angle. Sometimes multiple sets of layers with different orientations exist in the same rock, a structure called [[cross-bedding]].<ref>For descriptions of cross-bedding, see {{harvnb|Blatt|Middleton|Murray|1980|pp=128, 135–136}}; {{harvnb|Press|Siever|Grotzinger|Jordan|2003|pp=171–172}}.</ref> Cross-bedding is characteristic of deposition by a flowing medium (wind or water).

The opposite of cross-bedding is parallel lamination, where all sedimentary layering is parallel.{{sfn|Blatt|Middleton|Murray|1980|pp=133–135}} Differences in laminations are generally caused by cyclic changes in the sediment supply, caused, for example, by seasonal changes in rainfall, temperature or biochemical activity. Laminae that represent seasonal changes (similar to [[tree ring]]s) are called [[varve]]s. Any sedimentary rock composed of millimeter or finer scale layers can be named with the general term ''laminite''. When sedimentary rocks have no lamination at all, their structural character is called massive bedding.

[[Graded bedding]] is a structure where beds with a smaller grain size occur on top of beds with larger grains. This structure forms when fast flowing water stops flowing. Larger, heavier clasts in suspension settle first, then smaller clasts. Although graded bedding can form in many different environments, it is a characteristic of [[turbidity current]]s.<ref>For an explanation about graded bedding, see {{harvnb|Boggs|1987|pp=143–144}}; {{harvnb|Tarbuck|Lutgens|1999|p=161}}; {{harvnb|Press|Siever|Grotzinger|Jordan|2003|p=172}}.</ref>

The surface of a particular bed, called the [[bedform]], can also be indicative of a particular sedimentary environment. Examples of bed forms include [[dune]]s and [[ripple marks]]. Sole markings, such as tool marks and flute casts, are grooves eroded on a surface that are preserved by renewed sedimentation. These are often elongated structures and can be used to establish the direction of the flow during deposition.{{sfn|Collinson|Mountney|Thompson|2006|pp=46-52}}{{sfn|Blatt|Middleton|Murray|1980|pp=155–157}}

Ripple marks also form in flowing water. There can be symmetric or asymmetric. Asymmetric ripples form in environments where the current is in one direction, such as rivers. The longer flank of such ripples is on the upstream side of the current.{{sfn|Tarbuck|Lutgens|1999|p=162}}{{sfn|Levin|1987|p=62}}{{sfn|Blatt|Middleton|Murray|1980|pp=136–154}} Symmetric wave ripples occur in environments where currents reverse directions, such as tidal flats.

[[Mudcrack]]s are a bed form caused by the dehydration of sediment that occasionally comes above the water surface. Such structures are commonly found at tidal flats or [[point bar]]s along rivers.