Final answer:
The idea that the oldest rocks are at the bottom in a sequence of sediments and younger sediments occur sequentially upward is known as the law of superposition.
Step-by-step explanation:
The idea that the oldest rocks are at the bottom in a sequence of sediments and younger sediments occur sequentially upward is known as the law of superposition. This principle states that in layers of horizontal sedimentary rocks, the oldest rock layer is at the bottom and the youngest is at the top. It is a fundamental concept in relative dating, which is used to determine the relative ages of different rock layers.