The bed surface usually lies immediately below the failure plane. In response to a significantly strong temperature gradient within the snowpack, grains grow flat faces by a process known as kinetic growth or simply faceting. Facets common form near the snow surface or where the snowpack is shallow during periods of cold weather. The activity in which snowmobilers drive up a steep slope, each trying to reach a higher point than the previous rider. When the sled slows at the top of the run, the rider turns down the slope.
Crusts sometimes form the bed surface for avalanches.
Propagation - The spreading of a fracture or crack. The shear fractures that spread along weak layers and release slab avalanches tend to propagate further under thicker, harder slabs than under thinner, softer slabs. Rime can accumulate on the windward side of rocks, trees or structures, or on falling crystals of snow. When snow crystals cannot be recognized because of rime, the grains are called graupel. Under sufficiently low temperature gradients , branched and angular crystals decompose into more rounded shapes called rounds.