moraine


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moraine

(mərān`), a formation composed of unsorted and unbedded rock and soil debris called till, which was deposited by a glacierglacier,
moving mass of ice that survives year to year, formed by the compacting of snow into névé and then into granular ice and set in motion outward and downward by the force of gravity and the stress of its accumulated mass.
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. 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 epochPleistocene epoch
, 6th epoch of the Cenozoic era of geologic time (see Geologic Timescale, table). According to a classification that considered its deposits to have been formed by the biblical great flood, the epoch was originally called the Quaternary.
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 are marked by terminal moraines stretching across North America and Europe. See driftdrift,
deposit of mixed clay, gravel, sand, and boulders transported and laid down by glaciers. Stratified, or glaciofluvial, drift is carried by waters flowing from the melting ice of a glacier.
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.

Moraine

 

an accumulation of unsorted fragmental material transported or deposited by glaciers. Thus, there are moving, or mobile, moraines and deposited moraines.

Moving moraines are classified as surface, internal, and sub-glacial (ground) moraines. Surface moraines form from fragmental material that falls onto the surface of the glacier from the rocky walls of a valley or is thawed from the ice layer itself. Such moraines usually form two ridges of lateral moraines that extend along the sides of the glacier “tongue.” When glaciers merge, these lateral moraines are combined in a single ridge that extends down the middle of the glacier tongue in the form of a medial moraine. There may be several medial moraines and they all continue on, repeating the bends of the glacier and not merging. An internal moraine is located inside the ice layer and forms from debris that falls with snow avalanches into neve basins and is frozen into the ice as the neve accumulates; internal moraines are also built up to some extent at the expense of surface and ground moraines. Surface and internal moraines are not characteristic of ice sheets because elevations not covered by ice usually do not rise above the surface of the sheets. Ground moraines are characteristic of both mountain glaciers and ice sheets; they are fragmental material broken from the floor and embedded in the bottom layers of ice.

Deposited moraines consist of accumulations of fragmental material left in the wake of a receding glacier. Formed from all types of moving moraines, they are particularly developed in regions that were covered by continental glaciers during the Anthropogene. These are called ground moraines and consist primarily of material from subglacial moraines; sometimes above the material there is a thinner layer of ablation moraine or meltwater moraine that formed from the internal and upper layers of ground moraines. Local moraines are sometimes distinguished. They are crumbled and mixed material from the local bedrock of the glacier floor which has been moved only a short distance. In mountainous regions deposited moraines are composed of coarse, rubbly material mixed with varying amounts of silt. In areas that have been covered by continental glaciers, the deposited moraines consist of detrital sandy loams, loams, and clays.

REFERENCE

Rukhina, E. V. Litohgiia morennykh otlozhenii. Leningrad, 1960.

E. V. SHANTSER

moraine

[mə′rān]
(geology)
An accumulation of glacial drift deposited chiefly by direct glacial action and possessing initial constructional form independent of the floor beneath it.

moraine

a mass of debris, carried by glaciers and forming ridges and mounds when deposited
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