when a mountain glacier's terminus doesn't extend as far downvalley as it previously did; occurs when ablation surpasses accumulation.
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 USGS photo courtesy of B. Molnia; 1941 photo courtesy of W. Field. Archived at the Long-Term Change Photograph Pairs Special Collection in the Glacier Photograph Database.)