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cap rock Layer of hard, impervious rock which lies immediately above a source rock and which, because of its impervious nature, acts as a barrier, preventing the migration of hydrocarbons or water up-sequence. Such impervious rocks include clay-rich sandstones, limestones, and evaporite deposits associated with salt diapirs. Where it lies above an oil trap it prevents the upward migration of petroleum. Above a salt dome, the layer is usually composed of anhydrite and gypsum. In coal mining the term refers to a sandstone layer above shale overlying a coal seam.