Sediment settles in flat layers, one on top of another, like pages added to the top of a stack. So in sedimentary rock the layers record time from the bottom up. This idea, written down by Nicolaus Steno in 1669, is the law of superposition.
Key idea
In layers that have not been overturned, the oldest layer is at the bottom and the youngest is at the top. Sediment is also laid down in flat (horizontal) layers, so tilted or folded layers were moved later.
Why it works
A layer can only settle on something that is already there. The bottom layer had to exist before anything could pile on top of it, so it is older. This gives relative age: which is older or younger, not how many years old. Finding the age in years takes a radioactive clock.
Worked example: Reading a cliff
A cliff shows, from the top down: sandstone, shale, limestone, conglomerate. The layers are flat. Which is oldest and which is youngest?
- Flat layers have not been overturned, so superposition applies.
- The bottom layer, conglomerate, is oldest; the top layer, sandstone, is youngest.
Answer: Conglomerate is oldest; sandstone is youngest
Worked example: Tilted layers
Layers in a cliff tilt at 30°. Were they laid down that way?
- Sediment settles in flat layers.
- So the layers were tilted after they formed, by Earth movements. The tilting is younger than all of the layers it tilted.
Answer: No: they were tilted later
Common mistake
Applying "oldest at the bottom" to layers that were later folded so far that they turned upside down. In an overturned fold, the order is reversed. Geologists check which way was up using clues such as mud cracks or ripple marks, which form on the top surface of a layer.