Editing Glacier (section)

== Glacial geology ==

=== Erosion ===
[[File:Arranque glaciar-en.svg|thumb|Diagram of glacial plucking and [[Abrasion (geology)|abrasion]]]]
Glaciers erode terrain through two principal processes: [[Plucking (glaciation)|plucking]] and [[abrasion (geology)|abrasion]].{{sfn|Huggett|2011|loc=Glacial and Glaciofluvial Landscapes|pp=263–264}}

As glaciers flow over bedrock, they soften and lift blocks of rock into the ice. This process, called plucking, is caused by subglacial water that penetrates fractures in the bedrock and subsequently freezes and expands.{{sfn|Huggett|2011|loc=Glacial and Glaciofluvial Landscapes|p=263}} This expansion causes the ice to act as a lever that loosens the rock by lifting it. Thus, sediments of all sizes become part of the glacier's load. If a retreating glacier gains enough debris, it may become a [[rock glacier]], like the [[Timpanogos Glacier]] in Utah.

Abrasion occurs when the ice and its load of rock fragments slide over bedrock{{sfn|Huggett|2011|loc=Glacial and Glaciofluvial Landscapes|p=263}} and function as sandpaper, smoothing and polishing the bedrock below. The pulverized rock this process produces is called [[rock flour]] and is made up of rock grains between 0.002 and 0.00625&nbsp;mm in size. Abrasion leads to steeper valley walls and mountain slopes in alpine settings, which can cause avalanches and rock slides, which add even more material to the glacier. Glacial abrasion is commonly characterized by [[glacial striation]]s. Glaciers produce these when they contain large boulders that carve long scratches in the bedrock. By mapping the direction of the striations, researchers can determine the direction of the glacier's movement. Similar to striations are [[chatter mark]]s, lines of crescent-shape depressions in the rock underlying a glacier. They are formed by abrasion when boulders in the glacier are repeatedly caught and released as they are dragged along the bedrock.[[File:PluckedGraniteAlandIslands.JPG|thumb|right|Glacially plucked granitic bedrock near [[Mariehamn]], [[Åland]]]]The rate of glacier erosion varies. Six factors control erosion rate:
* Velocity of glacial movement
* Thickness of the ice
* Shape, abundance and hardness of rock fragments contained in the ice at the bottom of the glacier
* Relative ease of erosion of the surface under the glacier
* Thermal conditions at the glacier base
* Permeability and water pressure at the glacier base
When the bedrock has frequent fractures on the surface, glacial erosion rates tend to increase as plucking is the main erosive force on the surface; when the bedrock has wide gaps between sporadic fractures, however, abrasion tends to be the dominant erosive form and glacial erosion rates become slow.<ref>{{Cite journal |last1=Dühnforth |first1=Miriam |last2=Anderson |first2=Robert S. |last3=Ward |first3=Dylan |last4=Stock |first4=Greg M. |date=2010-05-01 |title=Bedrock fracture control of glacial erosion processes and rates |journal=[[Geology (journal)|Geology]] |language=en |volume=38 |issue=5 |pages=423–426 |doi=10.1130/G30576.1 |issn=0091-7613 |bibcode=2010Geo....38..423D}}</ref> Glaciers in lower latitudes tend to be much more erosive than glaciers in higher latitudes, because they have more meltwater reaching the glacial base and facilitate sediment production and transport under the same moving speed and amount of ice.<ref>{{Cite journal |last1=Koppes |first1=Michéle |last2=Hallet |first2=Bernard |last3=Rignot |first3=Eric |last4=Mouginot |first4=Jérémie |last5=Wellner |first5=Julia Smith |last6=Boldt |first6=Katherine |title=Observed latitudinal variations in erosion as a function of glacier dynamics |journal=[[Nature (journal)|Nature]] |volume=526 |issue=7571 |pages=100–103 |doi=10.1038/nature15385 |pmid=26432248 |bibcode=2015Natur.526..100K |year=2015 |s2cid=4461215}}</ref>

Material that becomes incorporated in a glacier is typically carried as far as the zone of ablation before being deposited. Glacial deposits are of two distinct types:

* ''Glacial till'': material directly deposited from glacial ice. Till includes a mixture of undifferentiated material ranging from clay size to boulders, the usual composition of a moraine.
* ''Fluvial and outwash sediments'': sediments deposited by water. These deposits are stratified by size.

Larger pieces of rock that are encrusted in till or deposited on the surface are called "[[glacial erratic]]s". They range in size from pebbles to boulders, but as they are often moved great distances, they may be drastically different from the material upon which they are found. Patterns of glacial erratics hint at past glacial motions.

=== Moraines ===
[[File:MorainesLakeLouise.JPG|thumb|Glacial moraines above [[Lake Louise (Alberta)|Lake Louise]], Alberta, Canada|left]]
Glacial [[moraine]]s are formed by the deposition of material from a glacier and are exposed after the glacier has retreated. They usually appear as linear mounds of [[till]], a non-sorted mixture of rock, gravel, and boulders within a matrix of fine powdery material. Terminal or end moraines are formed at the foot or terminal end of a glacier. Lateral moraines are formed on the sides of the glacier. Medial moraines are formed when a glacier meets its tributary glacier and merge, and the lateral moraines of each coalesce to form a moraine in the middle of the combined glacier. Less apparent are [[drift (geology)|ground moraines]], also called ''glacial drift'', which often blankets the surface underneath the glacier downslope from the equilibrium line.<ref name="NeuendorfOthers2005a">Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds., 2005. ''Glossary of Geology'' (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. {{ISBN|0-922152-76-4}}</ref><ref name=dkp/> The term ''moraine'' is of French origin. It was coined by peasants to describe alluvial embankments and rims found near the margins of glaciers in the French [[Alps]]. In modern geology, the term is used more broadly and is applied to a series of formations, all of which are composed of till. Moraines can also create moraine-dammed lakes.

=== Drumlins ===
[[File:Drumlins_around_Horicon_Marsh_in_Wisconsin.jpg|thumb|Drumlins around [[Horicon Marsh]], Wisconsin, in an area with one of the highest concentration of drumlins in the world. The curved path of the [[Laurentide Ice Sheet]] is evident in the orientation of the various mounds.]]
[[Drumlin]]s are asymmetrical, canoe-shaped hills made mainly of glacial sediments.<ref name=dkp/> Their heights vary from 15 to 50&nbsp;meters, and they can reach a kilometer in length. The steepest side of the hill faces the direction from which the ice advanced (''stoss''), while a longer slope is left in the ice's direction of movement (''lee''). Drumlins are found in groups called ''[[drumlin field]]s'' or ''drumlin camps''. One of these fields is found east of [[Rochester, New York]]; it is estimated to contain about 10,000 drumlins. Although the process that forms drumlins is not fully understood, their shape implies that they are products of the plastic deformation zone of ancient glaciers. It is believed that many drumlins were formed when glaciers advanced over and altered the deposits of earlier glaciers.

=== {{anchor|Glacial valleys}}Glacial valleys, cirques, arêtes, and pyramidal peaks ===
[[File:Glacial landscape.svg|thumb|Features of a glacial landscape|left]]

Before glaciation, mountain valleys have a characteristic [[V-shaped valley|"V" shape]], produced by eroding water. During glaciation, these valleys are often widened, deepened and smoothed to form a [[U-shaped valley|U-shaped]] glacial valley or glacial trough, as it is sometimes called.<ref>{{cite web |url=https://nsidc.org/cryosphere/glaciers/gallery/troughs.html |title=Glacial Landforms: Trough |publisher=[[National Snow and Ice Data Center]] |website=nsidc.org}}</ref> The erosion that creates glacial valleys truncates any spurs of rock or earth that may have earlier extended across the valley, creating broadly triangular-shaped cliffs called [[truncated spurs]]. Within glacial valleys, depressions created by plucking and abrasion can be filled by lakes, called [[paternoster lake]]s. If a glacial valley runs into a large body of water, it forms a [[fjord]].

Typically glaciers deepen their valleys more than their smaller [[tributary|tributaries]]. Therefore, when glaciers recede, the valleys of the tributary glaciers remain above the main glacier's depression and are called [[hanging valley]]s.<ref name=dkp/>

At the start of a classic valley glacier is a bowl-shaped cirque, which have escarped walls on three sides but is open on the side that descends into the valley called the "lip". Cirques are where ice begins to accumulate in a glacier. Two glacial cirques may form back to back and erode their backwalls until only a narrow ridge, called an [[arête]] is left. This structure may result in a [[mountain pass]]. If multiple cirques encircle a single mountain, they create pointed [[pyramidal peak]]s; particularly steep examples are called [[Glacial horn|horns]].<ref name=dkp/>

=== Roches moutonnées ===
Passage of glacial ice over an area of bedrock may cause the rock to be sculpted into a knoll called a ''[[roche moutonnée]],''{{sfn|Huggett|2011|loc=Glacial and Glaciofluvial Landscapes|pp=271}} or "sheepback" rock. Roches moutonnées may be elongated, rounded and asymmetrical in shape. They range in length from less than a meter to several hundred meters long.<ref>{{Cite book|title=Glaciers & Glaciation |publisher=Arnold |location=London |date=1998 |first1=Douglas |last1=Benn |first2=David |last2=Evans |pages=324–326}}</ref> Roches moutonnées have a gentle slope on their up-glacier sides and a steep to vertical face on their down-glacier sides. The glacier abrades the smooth slope on the upstream side as it flows along, but tears rock fragments loose and carries them away from the downstream side via plucking.

=== Alluvial stratification ===
As the water that rises from the ablation zone moves away from the glacier, it carries fine eroded sediments with it. As the speed of the water decreases, so does its capacity to carry objects in suspension. The water thus gradually deposits the sediment as it runs, creating an [[alluvial plain]]. When this phenomenon occurs in a valley, it is called a ''valley train''. When the deposition is in an [[estuary]], the sediments are known as [[bay mud]]. Outwash plains and valley trains are usually accompanied by basins known as "[[Kettle (landform)|kettles]]". These are small lakes formed when large ice blocks that are trapped in alluvium melt and produce water-filled depressions. Kettle diameters range from 5&nbsp;m to 13&nbsp;km, with depths of up to 45&nbsp;meters. Most are circular in shape because the blocks of ice that formed them were rounded as they melted.<ref name="britannica">{{cite web |title=Kettle geology |publisher=Britannica Online |url=https://www.britannica.com/EBchecked/topic/315739/kettle |access-date=2009-03-12}}</ref>

=== Glacial deposits ===
[[File:Receding glacier-en.svg|thumb|Landscape produced by a receding glacier|300x300px]]
When a glacier's size shrinks below a critical point, its flow stops and it becomes stationary. Meanwhile, meltwater within and beneath the ice leaves [[Stratigraphy|stratified]] alluvial deposits. These deposits, in the forms of columns, [[Terrace (geology)|terraces]] and clusters, remain after the glacier melts and are known as "[[glacial deposits]]". Glacial deposits that take the shape of hills or mounds are called ''[[kame]]s''. Some kames form when meltwater deposits sediments through openings in the interior of the ice. Others are produced by fans or [[river delta|deltas]] created by meltwater. When the glacial ice occupies a valley, it can form terraces or kames along the sides of the valley. Long, sinuous glacial deposits are called ''[[esker]]s''. Eskers are composed of sand and gravel that was deposited by meltwater streams that flowed through ice tunnels within or beneath a glacier. They remain after the ice melts,<ref name=dkp/> with heights exceeding 100&nbsp;meters and lengths of as long as 100&nbsp;km.

=== Loess deposits ===
Very fine glacial sediments or rock flour{{sfn|Huggett|2011|loc=Glacial and Glaciofluvial Landscapes|p=264}} is often picked up by wind blowing over the bare surface and may be deposited great distances from the original [[fluvial]] deposition site. These [[Eolian processes|eolian]] [[loess]] deposits may be very deep, even hundreds of meters, as in areas of China and the [[Midwestern United States]]. [[Katabatic wind]]s can be important in this process.