Relative dating principles include
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.
William Smith's collecting and cataloging fossil shells from rocks led to the discovery that certain layers contained fossils unlike those in other layers (see: fossil sorting).After the uplift of the land, the forces of erosion attacked the highlands and the eroded rock debris was transported and redeposited in the lowlands.During the same interval of time in another part of the world, the land surface subsided and was covered by the seas.Using these key or index fossils as markers, geologists began to identify a particular layer of rock wherever it was exposed.
Because fossils are believed to record the slow but progressive development of life, geologist use them to identify the relative age of rocks throughout the world.
Wide rings are produced during wet years and narrow rings during dry seasons.