Law of relative dating
Fossils are important for working out the relative ages of sedimentary rocks.
Throughout the history of life, different organisms have appeared, flourished and become extinct.
Brongniart was the first to use fossils to date rock strata., which states that geologic events are caused by natural processes, many of which are operating in our own time.
Put another way, the natural laws that we know about in the present have been constant over the geologic past.
The layers on top could only be laid down on top of the bottom layer so must be younger.
However the relative ages of rocks is more commonly determined by the presumed ages of the fossils found in the sedimentary layers.
Relative dating uses the principles or laws of stratigraphy to order sequences of rock strata.
Relative dating not only determines which layers are older or younger, but also gives insight into the paleoenvironments that formed the particular sequence of rock.
Geologists have studied the order in which fossils appeared and disappeared through time and rocks. Fossils can help to match rocks of the same age, even when you find those rocks a long way apart.
This law was independently discovered by William Smith (1769-1839), a British engineer, while working on excavations for canals in England (Winchester, 2002 p.
131) and by Georges Cuvier (1769-1832), a French anatomist, and Alexandre Brongniart (1770-1847), a French naturalist and geologist, during their work on the deposits of the Paris Basin.
Stratigraphy is a branch of geology that studies rock strata with an emphasis on distribution, deposition, age and evidence of past life.
Nicolas Steno, William Smith, Georges Cuvier, Alexandre Brongniart, and James Hutton developed the basic rules for the science of stratigraphy.