Question: How do you determine the age of a sedimentary rock or a fossil contained within it?

Christian Maerz answered on 19 Jun 2013:

There are a number of methods you can use to date sedimentary rocks. Absolute dating is what Simon is describing below, and it is based on the properties of certain elements and their isotopes contained in most rocks in variable amounts. There are different isotope systems – carbon isotopes are really good for dating sediments as old as 50,ooo years. For older stuff, you can use beryllium isotopes, argon isotopes, uranium isotopes etc.
Once you know the absolute age of a sediment, you can have a look at the specific fossils contained in it. Certain fossils are distributed globally, and always occur in sediment of the same age. In this way, you can now look for specific fossils that mark specific ages. This is called biostratigraphy.
In addition, there is magnetostratigraphy. As you probably know, Earth has a magnetic field with a north and south pole. Every now and then, the Earth’s magnetic field changes polarity, so the north pole becomes the south pole, and vice versa. Once you know at what time certain magnetic “reversals” happened, you can analyse magnetic properties in sediment records, and identify in which magnetic zone you are.