Relative dating of sedimentary rocks
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.How would today's sediments appear to a geologist millions of years in the future examining outcrops of sedimentary rock that originated in our time?Nonconformities mark major chapter breaks in the geologic history of an area.In the example below, the contact between the conglomerate and the granite beneath it appears likely to be a nonconformity.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.
They are summarized as the Principles of Relative Geologic Age Determination, sometimes referred to as the Principles of Relative Dating.
To determine the nature of the contact - whether it is an intrusive contact or a nonconformity - further evidence from field investigations would be needed.
Evidence such as angular pieces of conglomerate surrounded by the granitic intrusion, and contact metamorphism of the conglomerate adjacent to the granite, would indicate that the granite is younger and intruded the older conglomerate.
A nonconformity is an unconformity with sedimentary or volcanic strata on top and, beneath it, either plutonic rock such as granite or metamorphic rock such as schist.
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.