Tutorial:Glacial Deposition
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[edit] Glacial deposits and landforms
The deposits laid down by glaciers are known as till or boulder-clay. The characteristics of till are set out below. These deposits usually contain the weathered remains of a wide range of rock types and are often very fertile.
| 1 | Poorly sorted with particles up to boulder size, supported in a clay matrix. |
| 2 | Unstratified, except where modified by water. |
| 3 | Mixed rock types. |
| 4 | Contain particles with striations and abraded surfaces. |
| 5 | Preferred orientation of stones along axis of ice flow. |
| 6 | Compacted by pressures below ice, and by freeze-thaw processes. |
| 7 | Particles predominantly angular and sub-angular. |
Adapted from Summerfield (1991)
Material deposited over the valley floor with no visible structures is simply called till, or occasionally ground moraines. Till can also be pushed up beneath the ice or by melt water to form low hills known as drumlins, whose long axis is aligned parallel to the flow of the ice.
Moraines are accumulations of till formed at the snout of the glacier (terminal moraine), as the glacier retreats (series of recessional moraines), along the edges of the glacier (lateral moraines), along the centre of two merged glaciers (medial moraine), or where the glacier readvances (push moraine). Elongated hills (drumlins) can form in the till below glaciers.
[edit] Fluvio-glacial deposits
Glaciers, even when advancing, generate huge quantities of melt water. The material they transport can be deposited either in contact with, or beyond, the glacier.
Eskers and kames are the main landforms to develop in contact with the glacier. Eskers are long sinuous ridges of sand and gravel, which were originally deposited in melt water channels on the surface of the glacier, but were then dumped on the valley bottom as the ice retreats. Kames are mounds of material formed in a similar way from fluvio-glacial deposits within cavities in the ice.
Material carried beyond the snout of the glacier form outwash plains. Coarse material is deposited immediately beyond the glacier, with finer gravels, sands and silts being carried further. The unconsolidated sediment forming outwash plains is dissected by multiple shifting streams (braided stream).
Periglacial conditions adjacent to ice sheets and glaciers produce their own suite of landforms. Ice lenses can form in till, when this melts circular depressions (kettle holes) are left. Ice wedges, patterned ground, and involutions are formed by processes of freeze-thaw. Freeze-thaw also leads to rock shattering producing scree and in unconsolidated deposits causes solifluction leading to the development of solifluction lobes.
