bedrock (noun) 

Bedrock (BED-rahk) is the solid layer of rock underlying loose rocks and soil. The word is also used to refer to the fundamental principles upon which something is based. As I used it today: “Today, let’s talk about this idea of Western Civilization. The idea that America and Western Europe share a common culture that dates back to the ancient Greeks and Romans. And that this culture is the bedrock that has supported the accomplishments of the West for the last 2600 years.”