Radiometric dating of the oldest rocks on earth
Most sediment is either laid down horizontally in bodies of water like the oceans, or on land on the margins of streams and rivers.
Each time a new layer of sediment is deposited it is laid down horizontally on top of an older layer.
" First, the relative age of a fossil can be determined.
Relative dating puts geologic events in chronological order without requiring that a specific numerical age be assigned to each event.
Layers of rock are deposited horizontally at the bottom of a lake (principle of original horizontality).
Younger layers are deposited on top of older layers (principle of superposition).
The study of strata is called stratigraphy, and using a few basic principles, it is possible to work out the relative ages of rocks.
Just as when they were deposited, the strata are mostly horizontal (principle of original horizontality).
By comparing fossils of different primate species, scientists can examine how features changed and how primates evolved through time.
The age of the fossil must be determined so it can be compared to other fossil species from the same time period.
Understanding the ages of related fossil species helps scientists piece together the evolutionary history of a group of organisms.
This is the principle of original horizontality: layers of strata are deposited horizontally or nearly horizontally (Figure 2).
Thus, any deformations of strata (Figures 2 and 3) must have occurred after the rock was deposited.
In addition to being tilted horizontally, the layers have been faulted (dashed lines on figure).