1. The top or bottom of a joint, natural bed; surface of stone parallel to its stratification. 2. In granites and marbles, a layer or sheet of the rock mass that is horizontal, commonly curved and lenticular, as developed by fractures. Sometimes also applied to the surface of parting between rock sheets. 3. In stratified rocks, the unit layer formed by sedimentation; of variable thickness, and commonly tilted or distorted by subsequent deformation. It generally develops a rock cleavage, parting, or jointing along the planes of stratification.