A line separating tracts of unglacierized terrain of strikingly different appearance, the appearance of one of the tracts being interpretable as due to recent deglaciation. The trimline usually separates terrain with more mature vegetation, deglaciated in the more distant past or never glaciated at all, from terrain exposed during the retreat of glaciers from their Little Ice Age maximum extents. The separation may also be marked in part by terminal moraines. The trimline can be used to reconstruct former glacier extent and volume. Reliably dated trimlines have been used in this way to estimate long-term average mass balance.