A trim line, also written as trimline, is a clear line on the side of a valley formed by a glacier

Glacier

A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...

. The line marks the most recent highest extent of the glacier. The line may be visible due to changes in color to the rock or to changes in vegetation on either side of the line.

The term "trim line" is also used for similar looking phenomena caused by floods, volcanic activity, or tsunamis.