What Stories Do Rocks Tell?
ES2903  The Principle of Superposition

Using relative dating principles and the position of layers within rock, it is possible to reconstruct the sequence of geologic events that have occurred at a site. In the image below, cliffs along the Snake River show signs of volcanic activity and deposition. The thick, dark, gray layer at the bottom is made of basalt. Red-colored layers are sandstone.

  !   Examine the cliffs in the photograph below. Try to visualize the processes responsible for forming them. Click the photograph to see an animation showing how the layers may have formed.

Larry Ridenhour, BLM/Jennifer Loomis, TERC

3. Which layers were deposited first and are therefore the oldest?

4. Which layers were deposited last and are therefore the youngest?

You have just used the principle of superposition to interpret the relative ages of the layers. This principle states that in a sequence of undisturbed sedimentary layers or lava flows, the oldest layers are at the bottom.

 Step:   1   2   3   4   5   6   7   8   9   10   11