tillArticle Free Pass till, in geology, unsorted material
deposited directly by glacial ice and showing no stratification.
Till is sometimes called boulder clay because it is composed of
clay, boulders of intermediate sizes, or a mixture of these. The
rock fragments are usually angular and sharp rather than
rounded, because they are deposited from the ice and have
undergone little water transport. The pebbles and boulders may
be faceted and striated from grinding while lodged in the
glacier. Some till deposits show limited organization of the
fragments: large numbers of stones may lie with their long axes
parallel to the flow direction of the glacier. This could
provide more accurate information about flow direction than
other glacial indicators. Although difficult to distinguish by
appearance, there are two types of till, basal and ablation.
Basal till was carried in the base of the glacier and commonly
laid down under it. Ablation till was carried on or near the
surface of the glacier and was let down as th