Glacial till
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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 balls called till balls. If a till ball rolls around in a stream and picks up rocks from the bed of the stream and becomes covered with them it may become an armored till ball.
Till is deposited at the terminal moraine and along the lateral moraines of a glacier. As a glacier melts, especially a continental glacier large amounts of till are deposited by the rivers flowing from the glacier and in any proglacial lakes which may form. Till may contain alluvial deposits of gems or other valuable ore minerals picked up by the glacier during its advance, for example the diamonds found in Wisconsin, Indiana, and Canada.
In cases where the till has been cemented together into solid rock, it is known as tillite.