An aggregate of material - unsorted soil consisting of sand, gravel, clay, and unsorted stones, and deposited directly by a glacier or ice sheet. All grades of particle size may be found. Till is sometimes called boulder clay because it is composed of clay, boulders of intermediate sizes, or a mixture of these. Ice does not sort the material it carries and the range of particle sizes, as well as the range of rock types, depends on the geology over which the ice-sheet or glacier has flowed. There are two types of till, basal and ablation. Basal till is that which was carried in the base of the glacier and commonly set under it. Ablation till is that which was carried on or near the surface of the glacier and came down as the glacier melted.