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Depositional Features Formed by Advancing and Retreating Glaciers

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Introduction

David Pearce Depositional Features Formed by Advancing and Retreating Glaciers The advance and retreat of a glacier can create many varying features on a landscape. Till is responsible for the creation of most of the landforms. Till is the sediment deposited in contact with a glacier, it contains material that was picked up and carried by the ice. Often the pebbles in the till are striated, and have broken edges. Till may be very hard, compacted by the weight of the overlying ice. It is an ice-contact deposit, and may be formed under the glacier from the debris layers that ride along, and at times may carry along, the glacier base. Till also can be a proglacial deposit; it is dumped at the glacier front when the glacier pauses or retreats. Till usually is a poorly sorted sediment. The particle size of the till depends to some extent on where the ice has travelled. ...read more.

Middle

When two glaciers flow together, two lateral moraines can merge to form an interior belt of debris, called a medial moraine. Till can be sculpted into drumlins. These are large mounds of glacial debris that have been shaped into features that are streamlined parallel to ice flow. Often, they have a rough tear-drop shape with the tail pointing in the direction of ice flow. Drumlins often form in groups or clusters. Drumlins come in a variety of dimensions. Lengths can range from 100 to 5000 meters and heights can sometimes exceed 200 meters. The diagram below shows a plan and profile of a typical drumlin. Erratics are another feature formed by till. Boulders are picked up and carried by ice, often for many kilometres, to be deposited in areas of completely different lithology. The large boulder in the picture below is a good example of an erratic. ...read more.

Conclusion

Glaciers can also contain sinuous flows of meltwater that occur in ice tunnels at the base of the ice. The beds of these sub-surface glacial streams are composed of layers of sand and gravel. When the ice melts from around the meltwater tunnels, the beds of sand and gravel are deposited on the Earth's surface as long twisting ridges known as eskers. The slightly curving thin ridge in the centre of this photo is an esker located near Lac du Sauvage. When glaciers retreat rapidly, numerous blocks of ice can become detached from the main body of the glacier. If glacial drift is then placed around the ice, a depression on the surface called a kettle hole can be created when the ice melts. Kettle holes are commonly found on moraine and outwash plain deposits. Large kettle holes that reach below the water table can form into lakes. The photo overleaf shows some kettle lakes in glacifluvial outwash complex located in the North-western District of Mackenzie. Some kettle holes develop into wetlands such as bogs, swamps, and marshes. ...read more.

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