Arête
An Arete is a thin, almost knife-like, ridge of . Aretes are typically formed when two erode parallel U-shaped valleys. An arete is the thin ridge of rock that is left separating the two valleys. Aretes can also form when two glacial erode towards one another, although frequently this results in a saddle-shaped pass, called a .
Truncated spur
It is a pre-glacial spur that projected into a valley through which a glacier passed.
Hanging valley
It is a tributary valley to a larger or over-deepened glacial trough. Where the tributary valley meets the main glacial valley, it may ‘hang’ several hundred metres above the main valley floor. Hanging valleys are common features in former glaciated uplands.
Erratic
A Glacial erratic is a piece of rock carried by glacial ice some distance from the rock outcrop from which it came. Erratics can range in size from pebbles to massive pieces such as the Okotoks (16,500 tons) and Airdrie erratics found in Alberta, Canada. Geologists identify erratics by studying the rocks surrounding the position of the erratic and the rock of the erratic itself.
The Foothills Erratics Train is a deposit of rocks of many sizes. These deposits stretch in a narrow belt for about 600 km (400 miles) from the Athabasca River Valley in Alberta to south western Alberta.
Roche mountonnée
This is a rock outcrop that has been abraded and quarried by glacial action. Roche mountonnées have two contrasting slope forms. The slope that faced up-glacier has been smoothed and lowered by glacial abrasion. In contrast, the down-glacier slope has been quarried and steepened.
Crag and tail
A crag (sometimes spelled cragg, or in Scotland craig) is a rocky hill or mountain, generally isolated from other high ground. Crags are formed when a glacier or ice-sheet passes over an area that contains a particularly resilient chunk of rock (often a granite plug or other volcanic structure). The force of the glacier erodes the surrounding softer material, leaving the rocky block standing proud from the surrounding terrain. Frequently the crag serves as a partial shelter to softer material in the wake of the glacier, which remains as a gradual fan or ridge forming a tapered ramp (called the tail) up the leeward side of the crag.
In older examples, or those which are latterly surrounded by the sea, the tail is often missing, having been removed by post-glacial erosion.
Crag Tail
Fjord
Fjords are found in locations where current or past glaciations extended below current sea level. A fjord is formed when a glacier retreats, after carving its typical U-shaped valley, and the sea fills the resulting valley floor. This forms a narrow, steep sided inlet (sometimes as deep as 1300m) connected to the sea. The terminal moraine pushed down the valley by the glacier is left underwater at the fjord's entrance, causing the water at the neck of the fjord to be shallower than the main body of the fjord behind it.
This shallow threshold and the protection afforded by the valley's sides generally means that fjords are excellent natural harbours. Consequently fjords often provide a home-port to fishing fleets, and in industrialised locations have come to be used for fish farming and shipbuilding.
As late as 2000, some of the world's largest coral reefs were discovered along the bottoms of the Norwegian fjords. These reefs were found in fjords all the way from the north of Norway to the south. The marine life on the reefs is believed to be one of the most important reasons why the Norwegian coastline is such a generous fishing ground. Since this discovery is fairly new, little research has yet been done. So far, only the deep sea diver who discovered the first reef at 60 meters has visited it, and even he has only been down three times. The reefs are host to thousands of life forms such as plankton, coral, anemonies, fish, several species of sharks, and many more one would expect to find on a reef. However most are specially adapted to life under the greater pressure of the water column above it, and the total darkness of the deep sea.
New Zealand's fiords are also host to deep sea corals, but a surface layer of dark fresh water allows these corals to grow in much shallower water than usual. An underwater observatory in Milford Sound allows tourists to view them without diving.
Moraines
Moraine is the general term for debris of all sorts originally transported by or ice sheets that have since melted away. The following are commonly recognized types of moraine:
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Lateral moraine: The and other material from the sides of a glacial accumulated on the glacier and carried along with it. The mass of debris distributed along the lateral edges of the glacier are thus called lateral moraine. In the case of valley glaciers which have disappeared, their former existence may often be proved by the traces of lateral moraines left along the sides of the valley.
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Medial moraine: If one or more tributary glaciers coalesce with the main glacier the lateral moraines unite to form trains of debris on the surface of the glacier at or near its center, called medial moraines.
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Terminal moraine: When balance is maintained between the melting of a glacier and its forward advance, the debris carried on (superglacial), within (englacial), and dragged along the bottom (subglacial) is dumped at that point and builds up a heterogeneous mass of the transported material called the terminal moraine. If a glacier is slowly retreating and makes successive halts farther and farther up the valley, a series of terminal moraines are formed which are spoken of as recessional moraines.
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Interlobate moraine: If large glaciers and continental ice sheets advance irregularly so that their margins are lobate, when the margins retreat by melting the resulting terminal moraines of boulders, , and simulate the original interlobate shape of the glacier or glaciers, and therefore such moraines are called interlobate.
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Ground moraine: When a valley glacier melts completely away the debris carried on or within it are dropped on the valley floor, forming a deposit called ground moraine. The ground moraine from the melting of the great ice sheets is usually spoken of as .
Lateral moraine on a glacier joining the Gorner Glacier, Zermatt, Switzerland. The moraine is the high bank of debris in the top left hand quarter of the picture. For more explanation, click on the picture.
Glacier table
A rock or boulder that has been transported by a glacier and the surrounding ice has melted to leave the rock or boulder supported on a small piece of ice.
Outwash plain
Glacial outwash is the debris produced by glacial erosion and weathering, transported and deposited by meltwater streams. It forms stratified deposits, and glacial-fluvial landforms such as eskars, kames and sandar.
Striations/polished surfaces/chatter marks
Glacial striations or glacial grooves are gouges or grooves cut into the bedrock by glacial ice and meltwater as it slowly ground its way along during one of the Earth's Ice Ages or by mountain glaciers. Striations usually occur as multiple, straight parallel lines representing the movement of the sediment loaded base of the glacier. Large amounts of gravel and boulders were carried along and provided the scouring power to cut these grooves into the bedrock. Fine sediments in the base of the moving glacier scoured and polished the bedrock.
One very good example can be found at the National Natural Landmark at Glacial Grooves, Kelleys Island, Ohio which is 400 feet long, 15 feet deep and 30 feet wide. A much longer nearby groove on the island, the Great Grooves, was quarried out in the early 20th century. Those grooves were also about 15 feet deep and 30 feet wide but were 2000 feet long.
Rock flour
Rock flour consists of clay sized particles of rock generated by glacial erosional actions. Some of these actions include:
- glacial migration, where the glacier grinds against the land surface and erodes it away ;
- freeze thaw, where the act of water freezing and expanding in cracks helps break up rock formations;
It should be noted that these particles are not clay, but typically ground up quartz and feldspar. Rock flour is carried out from the system via meltwater streams, where the particles travel in suspension. Rock flour particles can travel great distances suspended in fluid. Dried rock flour can be carried by the wind great distances and deposited in thick layers, where it is called loess.