Today, with the help of isotopic dating methods, we can determine the ages of rocks nearly as well as we map the rocks themselves.
For that we can thank radioactivity, discovered at the turn of the last century.
Bishop James Ussher, a 17th-century Irish cleric, for example, calculated that creation occurred in 4004 B. There were many other such estimates, but they invariably resulted in an Earth only a few thousand years old.
By the late 18th century, some naturalists had begun to look closely at the ancient rocks of the Earth.
Sedimentary rock on the other hand consists of sedimentary particles which were removed and deposited somewhere else by some sort of fluid (generally wind and water).
The sedimentary particles predate the rock which they form.
This technique is generally used to date igneous and metamorphic rock, which are rocks that were once melted due to extreme heat and pressure.