tillite

Till is an unsorted glacial sediment. Glacial drift is a general term for the coarsely graded and extremely heterogeneous sediments of glacial origin. Glacial till is that part of glacial drift which was deposited directly by the glacier. It may vary from clays to mixtures of clay, sand, gravel and boulders. A particularly sticky form of clay till is called gumbo. Clay in till may form in spherical shapes called till balls. If a till ball rolls around in a stream, it may pick up rocks from the streambed and become covered by rocks; thence it is known as an armored till ball.
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