The surface over which a fracture and subsequent avalanche release occurs. Can be either the ground or a snow surface.

What makes a bed surface?
You don’t need a bed surface to make an avalanche but it helps. Sometimes avalanches fracture within a thick layer of weak snow and the avalanche creates its own bed surface. Common bed surfaces include:
• Rain Crusts
• Sun Crusts
• Hard, old snow surface
• The Ground

Bed Surface – Snow:

Bed Surface – Ground: