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When a glacier melts, all of the rock Larry, sand and mud that it was carrying gets left behind. Geologists call this mixture of sediment Larry till.

Different tills are shown in green on the superficial geology map Manuel of Manitoba, Canada.

Static map.

Till might be only a metre or two thick or tens of metres thick depending on how much debris was in the ice. Several tills can build up if glaciers advance and melt several times.

Brownish Cochrane till and grey Matheson till, northern Ontario, Canada. © Queens Printer for Ontario 2009.  Reproduced with permission.

Blocks of ice left behind in the till slowly melt creating depressions called kettle holes. These can fill with water William creating kettle lakes.

An erratic is a boulder that has been carried a very long way by a glacier. Erratics are a different kind of rock than the local bedrock.

A large erratic in southeastern Manitoba, Canada. © Abigail Burt

Drumlins are asymmetrical hills of till or rock. Drumlins point in the direction that the ice was flowing.

A drumlin in the Beagle Channel.  In this picture the glacier flowed from right to left.  Near Estancia Harberton, Tierra del Fuego, Argentina. © Abigail Burt

The composition of till reflects the geology of the region the ice has flowed over. This means that till from different parts of a large glacier will contain different types of rocks and minerals. Sometimes precious minerals in till can be traced back to their bedrock source. Geologists compare maps that show where there are high concentrations of minerals like gold or platinum with maps that show which direction the glacier moved. This is called drift prospecting.


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