(A name for) the rock stratum which is the lowest that is generally worked in the quarries of a particular district; (more widely) the lowest stratum of a rock formation.
Origin
Early 19th century.
Definition of bottom bed in US English:
bottom bed
noun
(A name for) the rock stratum which is the lowest that is generally worked in the quarries of a particular district; (more widely) the lowest stratum of a rock formation.