What are the older rocks that lie under younger rocks?

The older rocks that lie beneath younger rocks are referred to as "bedrock" or "subsurface strata." In geological terms, this relationship is described by the principle of superposition, which states that in an undisturbed sequence of sedimentary rocks, the oldest layers are at the bottom and the youngest are at the top. These older rocks may include various types of sedimentary, metamorphic, or igneous formations that have been buried and subsequently overlain by newer deposits.