The half-life of uranium-238, for example, is 4.5 billion years.
By measuring the concentration of uranium-238 in comparison with the products of its decay (especially lead-206), a scientist can estimate the age of the rock in which the uranium was found.
Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault.
The principle of inclusions and components explains that, with sedimentary rocks, if inclusions (or clasts) are found in a formation, then the inclusions must be older than the formation that contains them.
The regular order of the occurrence of fossils in rock layers was discovered around 1800 by William Smith.