Answer:
The idea that the oldest rock will always be the bottom layer of a series of rock layers is called the principle of superposition.
Step-by-step explanation:
According to the law or principle of superposition, the sequence of layers of sedimentary rock go from the oldest, at the bottom, to the youngest, at the top. Sedimentary rock is formed when pieces of rock and even of living organisms - once they die, of course - are deposited on a surface. As time passes, new pieces are deposited on top of the older pieces. That process happens again and again until we have different layers of sedimentary rock, the first one being the oldest, that is, the one that was first deposited there.