Glacial deposits

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Glacial deposits

Glacial deposits

The great bulk of the material eroded by a glacier remains in the lower layers of ice and is seen redeposited. However, some material works its way upward through the ice on shear planes to emerge at the glacier surface and then to be carried supra glacially. Sometimes boulders are transported to great distances, such erratics, moved far from their original bedrock, are useful of the direction of ice movement. In addition to direct ice transport, meltwaters may pack up debris and redeposit it. Hence a twofold division of glacial deposits is usually recognized: material deposited directly by ice is known as glacial tilt; that deposited by meltwater streams is referred to as fluvioglacial material. The general term glacial drift is given to all debris deposited in a glacial environment.

Till Deposition. Till is a very heterogeneous deposit composed of a seemingly chaotic assemblage of stones of various sizes set in a finer mass (matrix) of clay, silt or sand. Till is sometimes known as boulder day, but many tills have neither boulders nor clay. The individual stones in till are usually subangular - that is, they are not rounded, but neither do they posses sharp edges.

Lodgement till is till that has been plastered on to the ground by the sole of the glacier, perhaps against an obstruction. The lower debris-rich layers of ice become separated from faster-flowing and clearer ice above. The forward motion. The lower debris-rich layers of ice becomes separated their long axes are parallel to the direction of flow. When a stagnant glacier downmelts on the spot, englacial and supraglacial material will accumulate in heaps as ablation till. In this case we would not expect the stones to be aligned in any particular direction. Material brought to the ice surface on shear planes sometimes flows rather like porridge, filling up any hollows on the glacier surface and forming a flow till. Hence with these several different possibilities for till deposition, a single glacier can build up quite complicated drift sequences.

Moraines are depositional landforms, usually composed of both till and fluvioglacial material. Lodgement till normally makes up most of the groundmoraine of the glacier, creating a fairly featureless area or plain, unless drumlins are also present. At the margins of the glacier, more definable ridges or mounds are created as part of the end-moraine complex. The lateral moraine builds up at the side of the glacier, the terminal moraine at the glacier snout. End-moraines take a wide variety of form and usually contain fluvioglacial material as well as till. The largest end-moraines may reflect a long standstill of the glacier margins in one position, and may contain multiple ridges piled up by a slight readvance of the ice over previously deposited drift. Recessional moraines mark standstill stages in the retreat of a glacier.

Drumlins are streamlined egg-shaped features, largely composed of lodgement till, although some may have a rock core. Individual drumlins can be up to a kilometer in length and they usually have their long axes aligned parallel to the direction of flow; their steepest (stoss) end faces up-glacier. The expression basket-of-eggs topography arises because drumlins usually occur in clusters.

Next: Glacial erosion

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