Till - Definition and Overview

This article is about glacial sediments, for other uses see till (disambiguation).

Glacial till with tufts of grass

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 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. Prospectors use trace minerals in tills as clues to follow the glacier upstream to find kimberlite diamond deposits and other types of ore deposits.

In cases where the till has been cemented together into solid rock, it is known as tillite. Ancient tillites provided early evidence for continental drift. The same tillites also provide the key evidence for the current Snowball Earth theory.