Till is the general term for any sediments that were deposited solely by glacial ice. Till is distinguished from other glacial deposits formed by forces other than ice, such as glaciofluvial (or glacial melt water) deposits. A similar term is moraine, but it connotes more specific depositional mechanisms and spatial relationships to the glacier than does till.

Tills are produced by virtue of the formation, advance, and retreat of glaciers. The immense weight of an advancing glacier causes it to rip up rock and soil and incorporate them into the ice. These sediments then migrate forward as the glacier creeps downhill. When sediments reach the leading edge of the glacier where it is constantly melting, they are turned out as till.

This depositional mechanism results in tills being characterized by a physical heterogeneity; the sediments are unsorted, random in size, and may consist of a large range in particle size—from tiny clays to huge boulders. Tills are also generally unstratified, showing no sedimentary layering. The sediments in till exhibit a variable degree of rounding to the sediments, although some rounding is almost always observed. Despite their random origin, tills sometimes exhibit some degree of consistency in composition, allowing them to be described by the dominant size sediment they contain, such as gravelly or sandy tills.

Although tills may contain rocks from anywhere the glacier came in contact with, and sometimes do show evidence of sources hundreds of miles away, most tills are locally derived. They usually consist of rocks and soils picked up by the glacier within a few miles of where they were deposited. As a result, tills often provide evidence of the local bedrock and aid in determining the geology of areas that are now covered with glacial deposits.

See Also

Glacial Landforms; Glaciation