Glacial streams are usually braided because of their heavy sediment load. According to one theory, ice ages occur when continents are clustered near the poles. Glaciers are effective agents of erosion, but do not transport sediment effectively. In the accumulation area of a glacier, the annual rate of snow accumulation is less than the rate of melting. The unsorted drift deposited directly by a glacier is till.
Most glacial terrain is characterized by large mounds of till known as stratified drift.
The Milankovitch theroy states that variations of the Moon's orbit around the Earth caused the cycles of the Pleistocene advances and retreats. The transformation of firn to glacial ice is fastest in a very cold environment with limited snowfall. Erosional features produced by ice sheets are generally more spectacular than those produced by valley glaciers. The capacity of a glacier is greater than that of a stream, but the competence of a stream is greater than that of a glacier. The competence and capacity of a glacier are greater than the competence and capacity of a stream.