The surface of the ground in this area adjacent to Sherman glacier is covered with a coarse glacial till. Chugach Mountains, Chugach National Forest, Alaska. Credit: USGS.
Till, also known as boulder clay, is the unsorted material left behind on the land after the retreat of a glacier. Till is a heterogeneous mixture of different sized material deposited by moving ice (lodgement till) or by the melting in-place of stagnant ice (ablation till). After deposition, some tills are reworked by water.