Relative time scale
Unlike tree-ring dating -- in which each ring is a measure of 1 year's growth -- no precise rate of deposition can be determined for most of the rock layers. Therefore, the actual length of geologic time represented by any given layer is usually unknown or, at best, a matter of opinion.
By comparison, the history of mankind is similarly organized into relative units of time. We speak of human events as occurring either B.C. or A.D. ... Geologists have done the same thing to geologic time by dividing the Earth's history into Eras -- broad spans based on the general character of life that existed during these times -- and Periods -- shorter spans based partly on evidence of major disturbances of the Earth's crust.
Charles Darwin's "Origin of Species" was published in 1859, and soon the simple basis of stratigraphy founded in ideas derived from catastrophism began to disintegrate as palaeontologic interest became focussed on establishing evolutionary trends and on the search for ancestors and descendants. (Annual Review of Earth and Planetary Sciences, Vol. 6, page 354)
Using fossils to correlate from area to area, geologists have been able to work out a relative worldwide order of rock formations and to divide the rock record and geologic time...
Nowhere on Earth is there a complete section that shows strata deposited over the entire history of the Earth. ... In the past, some areas were above sea level and being eroded and other areas were below sea level where deposition was occurring. Thus, in order to develop a complete record, correlations must be undertaken in order to see how everything fits together.
Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained, and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration or electromagnetism.
Superposed strata in sedimentary rocks are believed to have been formed by successive layers of sediments deposited periodically with interruptions of sedimentation. This experimental study examines possible stratification of heterogeneous sand mixtures under continuous (non periodic and non-interrupted) sedimentation.
Detail research of Cambrian — Ordovician Sandstones of St.-Petersburg region shows that the sequence is result of single deposition cycle that develops from clay of Siversk Formation (underluing to Sablino Member) to the lower boundary of Ladoga Member (regressive phase); and from Ladoga to Tosno (and overlying Koporie shales) as transgressive phase according to changing of paleohydraulic conditions. Inner erosion surfaces were result of variations of intensity and competence of the flow rather than long interrupt of sedimentation and erosion of strata in subaerial conditions.
Find out why paleohydrology is a crucial factor which is sometimes overlooked.
Sedimentary rocks are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. ... The sedimentary rock cover of the continents of the Earth's crust is extensive, but the total contribution of sedimentary rocks is estimated to be only 8% of the total volume of the crust. Sedimentary rocks are only a thin veneer over a crust consisting mainly of igneous and metamorphic rocks. Sedimentary rocks are deposited in layers as strata, forming a structure called bedding.
Sedimentary rock covers about three fourths of the land area, and most of the ocean floor. Where the earth's crust is deformed or eroded, large areas of buried sedimentary rock may be exposed. In some places, such as the mouths of rivers, the sedimentary rock is 12,000 meters thick.
A polystrate fossil is a fossil of a single organism (such as a tree trunk) that extends through more than one geological stratum. This term is typically applied to "fossil forests" of upright fossil tree trunks and stumps that have been found worldwide, i.e. in the Eastern United States, Eastern Canada, England, France, Germany, and Australia, typically associated with coal-bearing strata.
This study reveals that silicified wood can form under suitable conditions in time periods as short as tens to hundreds of years, and contributes to the understanding of the mechanisms forming silicified wood.