- Cap Rock
(1) A cap rock is a rock placed in the top or uppermost layer of a constructed rock structure, such as a rock retaining wall.
(2) A natural cap rock is a hard rock layer capping softer rock (such as basalt capping sandstone).
(1) Ed put the cap rock on top of the wall to prevent erosion.
(2) You see a cap rock on top of a lot of hoodoos.