retreat
when a mountain glacier's terminus doesn't extend as far downvalley as it previously did; occurs when ablation surpasses accumulation.
Image
Muir Glacier, Glacier Bay National Park and Reserve's White Thunder Ridge as seen on August 13, 1941 (left) and August 31, 2004 (right).
2004: B. Molnia, USGS; 1941 W. Field., archived in the Long-Term Change Photograph Pairs Special Collection in the Glacier Photograph Database
Topic(s):
glaciers