Fluvioglacial deposits

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Fluvioglacial deposits

Glacial river

The main characteristic of fluvioglacial drift is that it has been sorted and stratified into layers by, the meltwater. However, a glacial environment is such a variable one that this stratification often changes rapidly over short distances. Two types of material can be distinguished: ice-contact drift, deposited against ice; and proglacial drift, carried beyond the ice-margins.

Ice-contact drift accounts for several distinct landforms. Eskers are long narrow ridges of well sorted material deposited in sub-or englacial tunnels. Kames are individual mounds of material of various origins: some kames may represent former crevasse fillings, other may be the remains of small deltas. Kame deposits frequently exhibit faulting, formed as the material slipped when the ice melted away.

Kame terraces accumulate in the area between the side of the glacier and the valley walls. They have a roughly level surface controlled by the height of the local englacial water table, and usually include rafts of till and scree material as well as fluvioglacial deposits. Kettle holes are small enclosed depressions formed by the melting of partially buried blocks of ice, usually within fluvioglacial material. Areas of mounds and depressions commonly associated with a stagnant glacier are referred to as kame-and-kettle topography.

The main landform created by proglacial deposition is the outwash plain or sandur. When the material is confined to a valley, the term valley train is used. These features are built up by the constantly shifting meltwater streams, dumping the coarsest material at the proximal end near the glacier margin, and carrying fine material to the distal end of the plain or train. The retreat of a glacier may result in several outwash features being formed, each related to a recessional moraine.

Where proglacial debris is deposited into a lake, a delta will form. The fine bottom-set material deposited in the middle of the lake may be varved. A varve is an annual layer of sediment or sedimentary rock. Each varve consists of a pair of laminations, a coarser one representing summer deposition, and a finer one being the result of the slow winter precipitation of the fines material in the lake. The counting of varves has been used as a method of dating events.

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