If your talking about the book of The Lighting Thief then they got onto the Greyhound Bus I believe to go back to Manhattan
The correct answer is - The Principle of Superposition.
The Principle of Superposition suggests that the newest rocks are on the top while the oldest rocks on the bottom of the layers, or with other words, the new rocks are always deposited on top of the older layers, thus making a horizontal type of layers in proper order from the oldest to the youngest layers as we go from the bottom to the top. By this principle, it comes as the lava flows or the deposited sediments are covering the layers that are already in place and are naturally ordering themselves by age.
That it is weak? i dont know