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 coccoliths.
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 unconformities in geological jargon.
Cycles[change | edit source]
In many formations of strata there are repeating patterns. For example, clay layers alternate with sandstone layers; or carbonate layers alternate with shales. This is usually caused by repeating cycles of climate. In warmer times the sea rises, and carbonate rocks are deposited in shallow subtropical seas. Later, ice builds up at the poles and the sea level drops. Then the same area is close to shore, and gets sand and mud washed down by rivers.