underclay
/ˈʌndərˌkleɪ/
noun
- A layer of clay that lies beneath a coal seam or other mineral deposit.
- Miners had to dig through a thick layer of underclay before reaching the coal.
- Geologists studied the underclay to understand how the ancient swamp formed.
- The underclay in this region is rich in minerals and often used for pottery.
Synonyms