Dating relative rock
Gaps in the geologic record, called unconformities, are common where deposition stopped and erosion removed the previously deposited material.
Fortunately, distinctive features such as index fossils can aid in matching, or correlating, rocks and formations from several incomplete areas to create a more complete geologic record for relative dating.
Most ancient sedimentary rocks cannot be dated radiometrically, but the laws of superposition and crosscutting relationships can be used to place absolute time limits on layers of sedimentary rocks crosscut or bounded by radiometrically dated igneous rocks.
The geologic timescale is a chronology (calendar) of events on Earth based on obtaining ages of past events.For example, shells, wood, and other material found in the shoreline deposits of Utah’s prehistoric Lake Bonneville have yielded absolute dates using this method.These distinct shorelines also make excellent relative dating tools.Geologists generally know the age of a rock by determining the age of the group of rocks, or formation, that it is found in.The age of formations is marked on a geologic calendar known as the geologic time scale.