Through dating a sample of that rock, we determined the extrusion to be 250 million years old. An intrusion extends from the bottom layer into the fourth oldest layer. Through dating a sample of that rock, we determined the intrusion to be 100 million years old. The principle of superposition states that the oldest sedimentary rock units are at the bottom, and the youngest are at the top. Based on this, layer C is oldest, followed by B and A. So the full sequence of events is as follows: Layer C formed.