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moraine

moraine

[muh-reyn]
moraine, a formation composed of unsorted and unbedded rock and soil debris called till, which was deposited by a glacier. The till that falls on the sides of a valley glacier from the bounding cliffs makes up lateral moraines, running parallel to the valley sides. When two or more valley glaciers unite, their lateral moraines form a medial moraine, running down the center of the glacier. When two or more lobes of a continental ice sheet unite, the debris carried by each lobe intermingles, forming an interlobate moraine. When the climate of a region becomes warmer, glaciers will start receding. The debris deposited by a melting glacier is called a ground moraine. The debris left at the edge of the glacier's extreme forward movement is a terminal moraine. Similar moraines deposited during a temporary halt in the retreat of glacial ice are called recessional moraines. After the retreat of a glacier the moraines remain as prominent features of the topography. The margins of the great ice sheets of the Pleistocene epoch are marked by terminal moraines stretching across North America and Europe. See drift.

Medial moraine of Gornergletscher (Gorner Glacier) in the Pennine Alps near Zermatt, Switz.

Accumulation of rock debris (till) carried or deposited by a glacier. The material may range in size from blocks or boulders to sand and clay, is unstratified when dropped by the glacier, and shows no sorting or bedding. Several kinds of moraines are recognized, depending on how they are deposited by the glacier; these include lateral moraines along the margins of the glacier and terminal moraines at its leading edge.

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Moraine refers to any glacially formed accumulation of unconsolidated glacial debris (soil and rock) which can occur in currently glaciated and formerly glaciated regions, such as those areas acted upon by a past ice age. This debris may have been plucked off the valley floor as a glacier advanced or fallen off the valley walls as a result of frost wedging. Moraines may be composed of silt like glacial flour to large boulders. The debris is typically sub-angular to rounded. Moraines may be on the glacier’s surface or deposited as piles or sheets of debris where the glacier has melted. Moraines may also occur when glacier or iceberg transported rocks fall into the sea as the ice melts.

Types of moraines

Lateral moraines

Lateral moraines are parallel ridges of debris deposited along the sides of a glacier. The unconsolidated debris is deposited on top of the glacier by frost shattering of the valley walls and from tributary streams flowing into the valley. The till is carried along the glacial margin until the glacier melts. Because lateral moraines are deposited on top of the glacier, they do not experience the postglacial erosion of the valley floor and therefore, as the glacier melts, lateral moraines are usually preserved as high ridges.

Lateral moraines stand high because they protect the ice under them from the elements, which causes it to melt or sublime less than the uncovered parts of the glacier. Multiple lateral moraines may develop as the glacier advances and retreats.

Ground moraines

Ground moraines are till covered areas with irregular topography and no ridges often forming gently rolling hills or plains. It is accumulated under the ice by lodgement, but may also be deposited as the glacier retreats. The ground moraine is located between the two lateral moraines.

End or terminal moraines

End moraines or terminal moraines are ridges of unconsolidated debris deposited at the snout or end of the glacier. They usually reflect the shape of the glacier's terminus. Glaciers act much like a conveyor belt carrying debris from the top of the glacier to the bottom where it deposits it in end moraines. End moraine size and shape is determined by whether the glacier is advancing, receding or at equilibrium. The longer the terminus of the glacier stays in one place the more accumulation there will be. There are two types of end moraines, terminal and recession moraines. Terminal moraines mark the maximum advance of the glacier. Recessional moraines are small ridges left as a glacier pauses during its retreat. After a glacier retreats the end moraine may be destroyed by postglacial erosion.

Recessional moraine

A recessional moraine is in the form of a series of ridges running across a valley behind terminal moraine. They form during standstills in a glaciers retreat.

Medial moraine

A medial moraine is a ridge of moraine that runs down the centre of a valley floor. It is formed when two glaciers meet and the debris on the edges of the adjacent valley sides join and are carried on top of the enlarged glacier. As the glacier melts or retreats, the debris is deposited a ridge down the middle of the valley floor. The Kaskawash glacier in the USA has a ridge of medial moraine 1km wide.

See also

References

  • Easterbrook, D. J. (1999) Surface processes and landforms. (Second Ed). Prentice Hall, Upper Saddle River, New Jersey.

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