rock bed
/ˈrɒkbɛd/noun
1A bed of rock; a rocky floor, bottom, or base; (in early use) specifically solid rock underlying loose deposits such as soil or alluvium; bedrock.
2 Figurative. The fundamental principles on which something is based.
Origin
Late 18th century; earliest use found in James Beresford (1764–1840), writer. From rock + bed.