Natural bed of stone and its importance

Natural bed

In a stratified (sedimentary or metamorphic) rock, the layers formed during deposition lie in parallel planes. The plane along which the layers were deposited is the natural bed (bedding plane). The stone obtained from the quarry has these layers, and the bed is easily seen as the plane of weakness.

   ______________   <- layers (bedding planes)
  |______________|
  |______________|
  |______________|

Importance

1. The stone is strongest against loads that act perpendicular to the natural bed, and weakest along it, because layers can split or slide.
2. In masonry, stones are laid with their natural bed perpendicular to the direction of the load. In arches the bed is placed radial, i.e. normal to the thrust. In cornices and projecting parts the bed is kept vertical.
3. If the bed is placed vertical in a wall, water enters between layers and frost makes the layers flake off (scaling).
4. Identification of bed is therefore necessary during dressing and placing to obtain strength and durability.

Unstratified igneous rocks like granite have no natural bed.