From the beginning of this course, we have stated that the Earth is about 4.6 billion years old.How do we know this and how do we know the ages of other events in Earth history?The most obvious feature of sedimentary rock is its layering.This feature is produced by changes in deposition over time.
These techniques were first articulated by Nicolas Steno, a Dane living in the Medici court of Italy in the 17th C.
Grand Canyon National Park [see Photo 1] is one of the best places in the world to gain a sense of geologic, or “deep,” time because the canyon exposes a great swath of geologic history.
Rocks exposed in Grand Canyon are truly ancient, ranging from 1840 million years old (m.y.), or 1.84 billion years old (b.y.), to 270 m.y.
Conversely, the igneous rocks are younger than the sedimentary rocks.
Other examples of cross crutting relationships can be related to faults (fault has to be younger than the rock it is found in) and unconformities (see below).