The Law of Superposition states that unless the layers of soil or rock have been disturbed, the layer are in order from the youngest at the top to the oldest at the bottom. This is used in geology and archaeology for "reading the rocks" to see what happened in the past.

So, how could the rocks be disturbed so that the oldest was on the top? Earthquakes, folding of the rocks, and landslides are some of the possibilities. Luckily, they all leave evidence of that disturbance, letting us figure out what happened.