Jesus liked to use construction metaphors. For example:
Therefore everyone who hears these words of mine and puts them into practice is like a wise man who built his house on the rock. The rain came down, the streams rose, and the winds blew and beat against that house; yet it did not fall, because it had its foundation on the rock. But everyone who hears these words of mine and does not put them into practice is like a foolish man who built his house on sand. The rain came down, the streams rose, and the winds blew and beat against that house, and it fell with a great crash." When Jesus had finished saying these things, the crowds were amazed at his teaching, because he taught as one who had authority, and not as their teachers of the law.[Matt 7:24-29]
Building on rock was a good construction practice in Our Lord’s day, and this practice continues today. The foundations of many modern structures either rest directly on bedrock (the topsoil is excavated to expose bedrock), or upon pilings (steel columns) driven down to bedrock. In some places, bedrock extends above the topsoil level. These formations are called “mountains.” And building on them is considered structurally sound.
St. Paul extended the metaphor of a building’s solid foundation:
The Church is built on the foundation of the apostles and prophets, with Christ Jesus himself as the chief cornerstone. In him the whole building is joined together and rises to become a holy temple in the Lord. [Eph 2:19-20]
Jesus is the cornerstone (the first stone laid of the foundation, from which all other measurements are made), and the Apostles and (Old-Testament) prophets form the rest of the foundation. By calling Simon by the name Peter or Cephas (meaning “rock”), Jesus indicated that he has a place of precedence in the “rock foundation” of the Church. Obviously the “rock” of Peter does not supersede the cornerstone, but the rest of the foundation is a subset of the “rock” of Peter (and, along with Peter, forms the complete foundation, “joined together” in Christ).