Continental glacier deposition

Modern landscapes formerly covered by Quaternary ice sheets display a wide variety of depositional landforms, which have been extensively studied and described by glacial geomorphologists. The depositional characteristics of these features provide information about glacial processes in the past few tens of thousands of years of Earth history and provide a basis for understanding the origins of the landscapes around us. However, most continental glacial deposits are unlikely to be preserved in the stratigraphic record in the long term. This is because they mainly occur in areas that are only regions of deposition as a consequence of the glacial processes: many of the modern glacial landscapes are undergoing erosion and over time the continental glacial deposits will be reworked and removed. Glacial deposits recognized in pre-Quaternary strata are mostly marine in origin.

Moraines

Accumulations of till formed directly at the margins of a glacier are known as moraine. Several different types of moraine can be recognised. Terminal or end moraines mark the limit of glacial advance and are typically ridges that lie across the valley. Push moraines are formed where a glacier front acts as a bulldozer scraping sediment from the valley floor and piling it up at the glacier front. Dump moraines form at the snout of the glacier where the melting of the ice keeps pace with glacial advance. If a glacier retreats the melting releases the detritus that has accumulated at the sides of the glacier where it is deposited as a lateral moraine. Lateral moraines form ridges along the sides of glaciated valleys, parallel to the valley walls. Where two glaciers in tributary valleys converge detritus from the sides of each is trapped in the centre of the amalgamated glacier and the resulting deposit upon ice retreat is a medial moraine along the centre of a glaciated valley. When a cold glacier retreats, the snout of the glacier is often left with a carapace of detritus left behind as the glacier front has been melting. A few metres thickness of rock debris forms effective insulation and prevents the ice below it from melting. These ice-cored moraines (ablation moraines) give the impression of being much larger volumes of detritus than they really are because most of their bulk is made up of ice.

Other glacial landforms

Lodgement tills deposited beneath a glacier may form sheets that can be tens of metres thick, or show irregular ridges known as ribbed moraines. These tills also form smoothed mounds known as drumlins, which are oval-shaped hills tens of metres high and hundreds of metres to kilometres long, with the elongation in the direction of ice flow. In temperate glaciers partial melting of the ice results in rivers flowing in tunnels within or beneath the ice, carrying with them any detritus held by the ice that melted. The deposits of these rivers form sinuous ridges of material known as eskers and they are typically a few metres to tens of metres high, tens to hundreds of metres wide and stretch for kilometres across the area formerly covered by an ice sheet. The deposits are bars of gravel and sand that form cross-bedded and horizontally stratified lenses within the esker body. They may be distinguished from river deposits by the absence of associated overbank sediments and by internal deformation (slump folds and faults) that forms when the sand and gravel layers collapse as the ice around the tunnel melts. Kames and kame terraces are mounds or ridges of sediment formed by the collapse of crevasse fills, sediment formed in lakes lying on the top of the glacier or the products of the collapse of the edge of a glacier.

Outwash plains

As the front of a glacier or ice sheet melts it releases large volumes of water along with any detritus being carried by the ice. Rivers flow away from the ice front over the broad area of the outwash plain, also known by their Icelandic name sandur (plural sandar). The rivers transport and deposit in the same manner as a braided river. The large volumes of water and detritus associated with the melting of a glacier mean that the outwash plain is a very active region with river channels depositing sediment rapidly to form a thick, extensive braid plain deposit. Outwash plain deposits can be distinguished from other braided river deposits by their association with other glacial features such as moraines. The most spectacular events associated with glacial sedimentation are sudden glacial outburst events known by their Icelandic name as jo¨kulhlaups. The outburst can be either the result of the failure of a natural dam holding back a lake at the front of the glacier or a consequence of melting associated with a subglacial volcanic eruption. Very large volumes of meltwater create a dramatic surge of water and sediment, which may include some very large blocks onto the outwash plain. Deposits of glacial outbursts are thick beds of sand and gravel that are massive and poorly sorted or cross-bedded and stratified. Reworking of this material by ‘normal’ fluvial processes on the outwash plain may occur. The absence of widespread vegetation under the cold climatic conditions means that fine-grained sediment on the outwash plain remains exposed and is subject to aeolian reworking. Sand may be blown into accumulations within and marginal to the outwash plain, forming deposits with the characteristics of wind-blown sediment. Silt- and clay-sized grains may be transported long distances and be widely distributed: accumulations of wind-blown silt of Quaternary age (loess – 8.6.2) are thought to be sourced from periglacial environments. Clasts exposed on the outwash plain may be abraded by wind-blown sediment to form ventifacts.

Periglacial areas

In polar regions the areas that lie adjacent to ice masses are referred to as the periglacial zone. In this area the temperature is below zero for much of the year and the ground is largely frozen to create a region of permafrost. Only the soil and sediment near the surface thaws during the summer, and to a depth of only a few tens of centimetres, below which the ground remains perennially frozen. The thin layer of thawed material is often waterlogged because the water cannot drain away into the frozen subsurface. This upper mobile layer can be unstable on slopes and will slump or flow downslope. Other features of regions of permafrost are patterned ground, which is composed of polygons of gravelly deposits formed by repeated freezing and thawing of the upper mobile layer, and ice wedges, which are cracks in the ground formed by ice that subsequently become filled with sediment.
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