He concluded, after studying rocks at many outcrops, that each layer represented a specific interval of geologic time.
Further, he proposed that wherever un-contorted layers were exposed, the bottom layer was deposited first and was, therefore, the oldest layer exposed; each succeeding layer, up to the topmost one, was progressively younger.
With the sinking of the land surface, sediments were deposited on the ocean floor.
Such recurring events as mountain building and sea encroachment and believed to be recorded in rock layers that comprise units of geologic time.
Prior to the availability of radiocarbon dates (and when there is no material suitable for a radiocarbon date) scientists used a system of relative dating.
Relative dating establishes the sequence of physical or cultural events in time.
In order to date a sample of unknown age, the proportion of 14C relative to 12C are carefully determined, then compared to the proportion of a modern, measured sample.Geologists have divided 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.The "relative" positions of layers and fossils to assign estimated dates to strata.The archaeologists record and analyze the changes in types and styles of human-made items from different levels according to the principle explained below.Sediments are usually laid down in horizontal beds.Using these key or index fossils as markers, geologists began to identify a particular layer of rock wherever it was exposed.