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Daniel Smith |
Depositional
landforms |
| Back to Hyperglac home |
Daniel Smith |
Depositional
landforms |
DEPOSITIONAL PROCESSES
| Glaciers and ice sheets deposit sediment in different areas and in many different ways. Some of the controlling factors are; type of glacier, thickness of glacier, sediment supply (bedrock), and climate. Sediment can be deposited under glaciers, and along the margins. Glacial deposits are called till. Till is classified according to what conditions the sediments are deposited under. Till that is located under a glacier is either basal till, lodgment till, or sub- glacial melt out till. Basal till is deposited in areas where there is an airspace under the ice. Lodgment till is deposited where a cavity in the bedrock has overlying ice pushing down on it. Subglacial and supra glacial melt out tills can be foliated and as the name suggests they form from melting debris rich ice under, along, or on top of a glacier. |
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Drift: glacial deposits
| Lodgment till is very dependent upon glacial pressure and flow conditions. For
lodgment till to be present the correct pressure and flow conditions of the glacier are
required. If these conditions are not met the glacier will be eroding instead of
deposition under the glacier bed. Within different areas of a glacier both processes
can be taking place depending upon velocity and pressure. Figure
1 shows constant pressure with increasing velocity moving downward on the
diagram. Where the lines extend above the bedrock deposition is taking place and
where the lines intersect the bedrock erosion is taking place. Lodgment till can be
thick enough to create a bed of sediment for the glacier to ride on, this is called basal
till. Also where bedrock obstructions are present under a glacier cavities can be found on the down ice side of the obstruction (fig 2). The till on the up ice side is held in place by the pressure of the overlying ice. Till can also build up within cavities due to melt out, although cavities are rare. |
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| Fig 1 (modified after Boulton, 1982) | Fig 2 (modified after David Sugden and Brian John, Glaciers and Landscape, 1976) |
Melt Out Till
Subglacial
| Subglacial melt out till is located under the ice and reflects the amount of sediment in transport at the time of melting. The sediment deposition is coming from slow moving debris rich ice and also stagnant ice. Ice under a glacier or ice sheet can melt and when this happens sediment that is being carried is released. The images below show both fine and coarse till, fines on the left with steps for scale and coarse on the right. Since melt out till reflects the glaciers sediment load it can be anywhere from fine to coarse and any combination of grain sizes in between. |
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| Supra glacial melt out till is located at the margins of a glacier. This includes lateral and terminal moraines where a glacier is depositing till where melting is taking place along the edges of the ablation zone. Lateral and terminal moraines are composed mostly of coarse materials with some fines. The lack of abundant fines comes from melt water carrying these particles away from a glaciers terminus. The coarse materials are left behind since flowing water has an upper limit of particle sizes that can be carried. Glaciers have no upper size limit and that is why they are associated with such large size boulders that rivers are not. |
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| Figure 3 compares a glacier with a conveyer belt. This shows how material that gets put into transport is carried by a glacier to it's terminus. Debris gets deposited at a glaciers terminus by melt out of the slow moving ice. Debris can also be "rafted" on top of a glacier to it's terminus where it gets deposited. Debris rich ice can become separated from a glacier and become incorporated in a terminal moraine. |
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Fig 3 (modified after Tom Mcknight, Physical Geography, Prentice Hall 1996) |
