Examples from the Web for underclay
Historical Examples of underclay
These roots were Stigmaria, and the stuff into which they penetrated was an underclay.
It rests on the underclay b b', filled with roots of trees in situ, and it supports a growing forest (C D).A Manual of Elementary Geology
I wish Ehrenberg would undertake a microscopical hunt for infusoria in the underclay and shales; it might reveal something.More Letters of Charles Darwin Volume II