How does the law of superposition scientists determine the ages of rocks and fossils?

The law of superposition asserts or states that the rocks that a deeper or lower in the Earth are older than the rocks that are above them.
 
For example, let's say you have 3 different layer of rocks (Layer A, layer B, and layer C). Let's say layer A is on top of layer B and layer B is on top of layer C. So, how do you know which is older? Based on the law of superposition, Layer C would be the oldest of them all and layer A would be the youngest or relatively new layer. 

For definite age of these rocks, you would need to use some radiometric dating. 

Hope that helps!  

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