Pictures of absolute dating
The possibilities for what is in the rocks immediately beneath the unconformity are (1) layers of sedimentary or volcanic rock (strata) that have been tilted or folded prior to development of the unconformity; (2) a stratum is parallel to the unconformity and parallel to the stratum above the unconformity; or (3) plutonic or metamorphic rocks, which originated much deep in the earth's crust rather than at its surface.An angular unconformity is an unconformity beneath which the strata were tilted or folded before deposition of the younger layers of sediment above the unconformity.They are summarized as the Principles of Relative Geologic Age Determination, sometimes referred to as the Principles of Relative Dating.By the end of the 19th century, geologists had used these principles to put together an outline of the geological history of the world, and had defined and named the eons, eras, periods, and epochs of the geologic time scale.They did not know how many thousands, millions, or billions of years ago the Cambrian period began, but they knew that it came after the Proterozoic Eon and before the Ordovician Period, and that the fossils unique to Cambrian rocks were younger than Proterozoic fossils and older than Ordovician ones.In the 20th century, radiometric methods of absolute age determination were developed.
Ask yourself how the things that are happening in the world today might end up being recorded in the sediments that are now or soon will be deposited.Because granitic and metamorphic rocks form deep in the earth's crust, a significant amount of time is required for uplift and erosion to expose them.Nonconformities mark major chapter breaks in the geologic history of an area.There are no sedimentary strata to record what happened during the intervening interval.
Instead, there is just an unconformity, a buried erosional or non-depositional surface.
If so, the granite is younger and the boundary between the granite and the conglomerate is an intrusive contact rather than a nonconformity.