Explain that sedimentary rock forms in layers over time, with older layers on the bottom and younger layers on top, and use layers to reason about relative age.
When sediment, like sand, mud, and small rock pieces, gets deposited over and over across a long time, it eventually piles into layers, and pressure over a long time can turn those layers into sedimentary rock. Because each new layer of sediment settles on top of whatever was already there, the layers end up stacked in a very predictable order.
That predictable order is called the law of superposition: unless something has disturbed the layers, the oldest layer is on the bottom and the youngest layer is on top. Scientists can look at a cliff face or canyon wall with many visible layers and read it almost like a timeline, oldest at the bottom, newest at the top.
Mastery looks like: They can order a set of described rock layers from oldest to youngest using the law of superposition and explain the reasoning behind the rule.
Common stumbles: Assuming the top layer is always the oldest; forgetting that the rule only applies cleanly to layers that have not been disturbed.