Stratigraphic Superposition - Because of Earth's gravity, deposition of sediment will occur depositing older layers first followed by successively younger layers.Thus, in a sequence of layers that have not been overturned by a later deformational event, the oldest layers will be on the bottom.Calculations gave estimate of 100 million years for the age of the Earth.
By carefully digging, we have found that each trash pit shows a sequence of layers.
Although the types of trash in each pit is quite variable, each layer has a distinctive kind of trash that distinguishes it from other layers in the pits. Stratigraphy = the study of strata (layers) in the Earth's crust.
Laws of Stratigraphy Original Horizontality - sedimentary strata are deposited in layers that are horizontal or nearly horizontal, parallel to or nearly parallel to the Earth's surface.
Thus rocks that we now see inclined or folded have been disturbed since their original deposition.
Thus in the geologic column shown, the absolute ages in the far right-hand column were not known until recently.
How is the geologic column used in relative dating
Although geologists can easily establish relative ages of rocks based on the principles of stratigraphy, knowing how much time a geologic Eon, Era, Period, or Epoch represents is a more difficult problem without having knowledge of absolute ages of rocks.
Nonconformities occur where rocks that formed deep in the Earth, such as intrusive igneous rocks or metamorphic rocks, are overlain by sedimentary rocks formed at the Earth's surface.
The nonconformity can only occur if all of the rocks overlying the metamorphic or intrusive igneous rocks have been removed by erosion.
Determination of equivalence is based on: fossils present - Fossils are key indicators of relative age (life has evolved through time) and environments of deposition.
Over the past 150 years detailed studies of rocks throughout the world based on stratigraphic, paleontologic, and correlation studies have allowed geologists to correlate rock units throughout the world and break them into time stratigraphic units.