This page was last edited on 18 December 2020, at 14:07. Mudcracks are a bed form caused by the dehydration of sediment that occasionally comes above the water surface. Any sedimentary rock composed of millimeter or finer scale layers can be named with the general term laminite. Most authors presently use the term "mudrock" to refer to all rocks composed dominantly of mud. Short astronomic cycles can be the difference between the tides or the spring tide every two weeks. Carbonate rocks predominantly consist of carbonate minerals such as calcite, aragonite or dolomite. Where the lithosphere moves upward (tectonic uplift), land eventually rises above sea level and the area becomes a source for new sediment as erosion removes material. That new rock layers are above older rock layers is stated in the principle of superposition. Compaction takes place as the sediments come under increasing overburden (lithostatic) pressure from overlying sediments. Sedimentary rocks are a result of accumulating sediments over a period of time. Sediment transported by wind is called aeolian and is almost always very well sorted, while sediment transported by a glacier is called glacial till and is characterized by very poor sorting. Normally, such material eventually decays by oxidation or bacterial activity. Relatively small changes in the orientation of the Earth's axis or length of the seasons can be a major influence on the Earth's climate. Sedimentary rocks can contain fossils because, unlike most igneous and metamorphic rocks, they form at temperatures and pressures that do not destroy fossil remains. Biochemical sedimentary rocks are created when organisms use materials dissolved in air or water to build their tissue. Concretions are roughly concentric bodies with a different composition from the host rock. Such erosional material of a growing mountain chain is called molasse and has either a shallow marine or a continental facies. Sedimentation is the collective name for processes that cause these particles to settle in place. The classification of clastic sedimentary rocks parallels this scheme; conglomerates and breccias are made mostly of gravel, sandstones are made mostly of sand, and mudrocks are made mostly of mud. Asymmetric ripples form in environments where the current is in one direction, such as rivers. Conglomerates are dominantly composed of rounded gravel, while breccias are composed of dominantly angular gravel. The size and form of clasts can be used to determine the velocity and direction of current in the sedimentary environment that moved the clasts from their origin; fine, calcareous mud only settles in quiet water while gravel and larger clasts are moved only by rapidly moving water. When these organisms die, their skeletons sink to the bottom, forming a thick layer of calcareous mud that may lithify into limestone. Define sedimentary rock. 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