Stratigraphy

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Chalk in Cyprus, showing sedimentary layering. The clear layers are probably caused by changes in climate, as with rhythmites.
Crumpled mudstone strata, Ceibwr Bay, Cardigan Bay, Wales.
A rare fold in the rock strata, a change from the mainly large, flat slate layers on the headland here. Boscastle, Cornwall

Stratigraphy is a branch of geology which studies rock formations (layers). It is primarily used in the study of sedimentary and layered vocanic rocks.

Historical development

Engraving from William Smith's monograph on identifying strata based on fossils

The subject was established by Nicolaus Steno whose book De solido contained the law of superposition, the principle of original horizontality and the principle of lateral continuity in layers of sediment.

The first practical large scale application of stratigraphy was by William Smith in the 1790s and early 19th century. Smith, known as the Father of English Geology, created the first geological map of England. He first recognized the significance of strata or rock layering, and the importance of fossil markers for correlating strata.

Lithostratigraphy

Lithostratigraphy deals with the physical rock type and how it changes from place to place. Changes in the rock (facies change) reflect changing environments of deposition. One of stratigraphy's basic concepts is codified in the law of superposition, which simply states that, in an undeformed stratigraphic sequence, the oldest strata occur at the base of the sequence.

Biostratigraphy

Biostratigraphy is based on fossil evidence in the rock layers. Strata from widespread locations containing the same fossil fauna and flora are laid down at the same time.

Biostratigraphy was based on William Smith's principle of faunal succession, which was one of the first and most powerful lines of evidence for evolution. It gives evidence of the formation (speciation) and extinction of species.

The geological timescale was developed during the 19th century based on the evidence of biostratigraphy and faunal succession. This timescale remained a relative scale until the development of radiometric dating, which gave it an absolute time framework.

Iron-red rocks from the Archaean era: Weano Gorge in Karijini National Park, Pilbara, Western Australia
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