Definition of bottom rock in English:

bottom rock

noun

obsolete figurative Geology Mining
  • Rock which is at or forms the bottom of something; (as a count noun) a rock at the bottom of something; figurative = rock-bottom; Geology and Mining the stratum of rock on which a later series of deposits lies; (formerly) specifically (in plural) †those rock strata which lie below the oldest known fossiliferous rocks (obsolete).

Origin

Early 19th century; earliest use found in James Wilson.