A typical valley glacier will add snow at its head and lose to melt at its foot. The snow line is the line below which the annual snow cover is lost in summer. If it gains more than it loses, its terminus will advance. It loses more than it gains, it will retreat. Snowpatch Spire in the Bugaboos.
Glacial erosion is a powerful force that leaves a terrain with very high relief.
The dark ice is ablating or melting. Here Bugaboo Glacier tumbles between Pigeon and Snowpatch Spires. When a glacier encounters an increase in the slope of its bed, crevasses form where the surface is in tension and close where it is in compression. When a glacier encounters a very steep slope in its bed flow may become chaotic as in an icefall . The irregular ice blocks are called seracs and may be extremely unstable.