Original horizontality relative dating
There are two techniques geologist use to obtain the Earth's age: Radioactive dating and Relative dating. The exposed rock reveals layers of different types of rocks.
Geologists observe the geologic layers and try to reconstruct the sequence of events.
- The principle of superposition is simply the fact that the lowest layer of a rock formation is the oldest layer while the youngest layer is towards the top. Think of it as a boundary layer between igneous rocks and 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?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.
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When geologists study layers of rocks, they get a glimpse of the past.