The top fracture surface of a slab avalanche.

Credit: Flathead Avalanche Center

In the wake of a slab avalanche, the remaining wall of snow where the slab broke away from the snowpack is called a fracture line. The fracture line is a smooth surface that is perpendicular to the bed surface. The crown or crown line is the uppermost part of the fracture line, exposed by tensile fracture along the top of the slab avalanche. The height of the crown can be used to measure the thickness of the upper part of a slab avalanche after the event. 

Credit: Crested Butte Avalanche Center

This crown, and thus the slab, are approximately a meter thick. Credit: Flathead Avalanche Center