Geologic age dating sequence
The combination of these two types of geologic ages makes a complete record of earth's geologic history in terms of the order of events and in terms of how many years ago each event occurred.
Relative geologic age refers to the order in which geologic events occurred.
That means that tens of millions of years of geologic time lapsed between those two rock layers forming, and there is no sedimentary rock, no rock record, to record the details of what happened during that geologic time.
The contact between the Cambrain rock (over 488 million years old) and the Devonian rock (less than 416 million years old) is a type of unconformity, which you will read more about below.
Some rocks and minerals can have their absolute age directly measured by analyzing the ratios of certain radioactive and non-radioactive isotopes they contain.In the early 1800s, soon after James Hutton died, William Smith in England made the scientific case for what came to be called the principle of faunal succession.The key to this principle is that during a specific geologic time, only certain types of organisms existed, so if fossils of those organisms are found in a layer of rock, the rock is of that geologic age, the age when those organisms were species that lived on earth.The units commonly used for geologic age are mega-annum (Ma) for millions of years, giga-annum (Ga) for billions of years, and kiloannum (ka) ka for thousands of years.Because these units are used according to the rules of the metric system, the M in Ma and the G in Ga must be capitalized, and the k in ka must not be capitalized.