What is Strata?
[[File:Dorset Coast Man-O-War Bay.jpg|thumb|right|270px|Man o' War Bay on the [[Dorset]] coast ([[Jurassic Coast]]). The strata here are nearly vertical, as a result of ancient [[orogeny]].]] [[File:Streisand Estate.jpg|thumb|right|270px|This aerial photo shows the sloping strata in the cliff quite well]] '''Strata''' are layers of [[rock (geology)|rock]], or sometimes [[soil]]. In nature, strata come in many layers. It is a term in [[Sedimentary rock|sedimentary]] and [[historical geology]]; the singular is '''''stratum'''''. The study of strata is called [[stratigraphy]]. These layers are laid down as [[sediment]], often in the sea, and are slowly changed by [[pressure]], heat and [[chemical]] action into rocks. The strata are often typical of a particular time and place, and allow geologists to relate rocks in different places. For instance, [[chalk]] was laid down in the [[Upper Cretaceous]] period, and consists mainly of the remains of microscopic [[algae]] called [[coccolith]]s. In normal strata, the later strata are laid down on earlier strata in horizontal layers. In the long passage of time, sedimentary rocks may get ''deformed'' by huge forces in the Earth: [[volcanism]], [[orogeny]] (mountain building) or other causes. Then it requires research to work out what has happened to the strata. When strata rise above [[sea level]] they get worn down by [[erosion]], such as [[weather]]. This causes gaps in the [[sequence]] of strata, which may have risen and sunk many times in Earth history. These gaps are called [[Unconformity|unconformities]] in geological [[jargon]]. == Cycles == [[File:Varve1.gif|thumb|right|230px|These are annual varves in limestone. [[Pleistocene]] strata, Scarboro Cliffs, [[Toronto]], [[Ontario]], [[Canada]]]] In many formations of strata there are repeating patterns. For example, [[clay]] layers alternate with [[sandstone]] layers; or [[carbonate]] layers alternate with [[shale]]s. This is usually caused by repeating cycles of climate. In warmer times the sea rises, and carbo