Glacial deposits

Glacial deposits are a distinctive association of landforms produced by glacier ice or the water derived from it. Understanding how these deposits form allows geologists and physical geographers to reconstruct the extent, shape, and other characteristics of former glaciers that covered areas currently free from ice.

Glacial Expansion and Contraction

Glacial deposits and the landforms produced by them are extremely varied but have a single origin—they are all products of deposition by glacier ice, either through direct deposition from the ice to the land surface or through deposition from water derived from the ice. Glacial deposits are being formed in places such as Greenland and Antarctica, where large glaciers called continental ice sheets still exist. Continental ice sheets are huge accumulations of ice thousands of kilometers in diameter and up to four kilometers thick that completely engulf the underlying topography. Glacial deposits are also being formed in some high-altitude mountainous regions where small, tongue-shaped alpine glaciers occupy valleys leading down from upper mountain slopes. The upper slopes of Mount Rainier in Washington State contain twenty such glaciers.

At various times during the past two million years, continental ice sheets and alpine glaciers were much more extensive than they are today. This period of glacier expansion and contraction is known as the Pleistocene epoch, which ended only about 10,000 years ago. Whether a particular area was glaciated at some time during the Pleistocene epoch can be determined by examining the glacial deposits left behind. These constitute a distinctive and recognizable association of landscape features, especially if glaciation was relatively recent. Much of the northern United States north of 40 degrees latitude and most of Canada contain these deposits.

Glacial deposits are classified according to mode of origin and environment of deposition. They can be subdivided into unstratified deposits that show little or no evidence of water transport and stratified deposits that have been transported by and deposited in water. These two categories are, in effect, end members of a continuum, with some deposits showing minor effects of water reworking and sorting. Glaciers produce huge amounts of water, referred to as meltwater, especially during the summer months, when air temperatures are at a maximum.

Till

Glaciers also carry tremendous amounts of eroded rock and soil debris, which get incorporated into the moving ice either by falling onto the glacier surface from adjacent valley sides or, more commonly, by freezing onto the base of the glacier as it moves forward. Once incorporated, the debris is crushed and abraded in transport until it is finally deposited. Unstratified debris deposited directly from glacier ice and not subsequently transported is called till, or ground moraine. Till is characterized by lack of layering or stratification and a wide range of particle sizes from clay (less than 0.002 millimeter) to boulders many meters in diameter. Different types of till may be deposited by a glacier depending on whether the temperature is above or below freezing at the base of the glacier, the amount and distribution of debris within the glacier, and the style of deglaciation (melting of the glacier).

Debris that has been eroded and incorporated into the glacier may be subsequently deposited beneath it by a plastering-on process as the ice moves forward. This produces lodgment till, a compact, relatively thin deposit (usually fewer than a few meters thick) that may cover thousands of square kilometers. Three other varieties of till are produced when the glacier melts. When debris melts out from a stagnant glacier (no longer moving), it is gradually let down onto the ground surface. If there is no subsequent meltwater reworking and downslope transport of the debris, the resultant deposit is called meltout till. Meltout till tends to be discontinuous but locally quite thick, especially in high-latitude regions where the base of the former glaciers was well below freezing. Debris progressively exposed on the glacier's surface as it melts may get remixed by slumping and landsliding. The result is a jumble of partially washed debris called superglacial, or ablation, till. It is a very extensive surface deposit in some formerly glaciated regions and often forms hummocky topography such as that found in parts of the Canadian prairies and the Dakotas in the United States.

A fourth type of till, called flowtill or sediment flow diamicton, is deposited by flowage of water-saturated debris from the glacier surface or, less commonly, its base. This thick slurry of mud and debris forms discontinuous, lobate deposits on the land surface. If the flow is into a body of water abutting the front of the glacier, the resulting deposit is termed subaqueous flowtill. Unlike lodgment, meltout, and superglacial till, flowtill shows evidence of water sorting and mass flowage. It is, therefore, not till in the strict sense because it has been reworked subsequent to deposition from glacier ice. Each of the four till types may be recognized by its position relative to the other till types, its grain-size distribution, its geometry, its compactness, and its internal features.

Moraines

In general, the deposition of till produces three types of landforms: a relatively flat surface dominated by lodgment till and often containing elongate, streamlined hills or ridges called drumlins that are all oriented in the direction of glacier flow; a hummocky surface dominated by superglacial till and flowtill; or one or more elongate, arcuate ridges called moraines that are often arranged in a nested series separated by flat areas.

Moraines are usually composed of till but may include other deposits. They tend to have a hummocky or pitted surface. Moraines are formed at the margin of a glacier primarily by a plowing-up of debris in front of the ice combined with deposition of sediment by water flowing off the steep front of the glacier. Moraines form only when the adjacent margin of the glacier is stationary for long periods (the longer the stationary position, the larger the moraine). Moraines deposited by continental ice sheets are usually tens of meters high, up to a few kilometers wide, and tens to hundreds of kilometers long. Long Island, New York, is, in part, a large moraine deposited by the most recent continental ice sheet. Moraines formed by alpine glaciers tend to be much steeper and shorter than those formed by continental ice sheets. Although moraines formed by alpine glaciers sometimes reach heights of 200 meters, their lengths are no more than a few kilometers, since the glaciers that produce them are confined by valley walls.

Moraines are classified according to where they form. Ridges of debris accumulated along the sides of alpine glaciers are called lateral moraines, whereas those formed at the forward margin of any glacier are collectively called end moraines. The end moraine marking the farthest extent of the glacier is called the terminal moraine, while those formed at various distances, usually many kilometers, behind the terminal position are called recessional moraines. Like the terminal moraine, recessional moraines are constructed over long periods and require a stationary glacier front for formation. They, therefore, represent periods of relative climatic stability in which the amount of forward movement of the ice front is exactly balanced by the amount of ice melting. The state of Illinois in the United States features a beautiful example of terminal and recessional moraines. The terminal moraine curves across the central part of the state for more than 400 kilometers. Behind it to the north and east are a series of twelve to fifteen recessional moraines spaced from three kilometers to more than seventy kilometers apart.

Stratified Deposits

Stratified glacial deposits, otherwise known as stratified drift, are different from till in that they have been extensively reworked by and deposited in water. They are not as uniformly extensive as till and are confined mainly to lower-elevation areas such as stream valleys, where they form a large variety of individual landforms. Perhaps the most curious of these landforms are eskers, which are discontinuous, steep-sided, sinuous ridges of sand and gravel usually a few meters to tens of meters in height and up to two kilometers wide. They tend to snake through the landscape in a discontinuous fashion for tens or hundreds of kilometers. Eskers are actually stream sediments deposited in tunnels partly or wholly within glacier ice. When the bounding ice melts, the esker sediments are left as ridges on the land surface. Unlike normal stream deposits, these sit on the land surface and may locally rise over topographic obstructions. Eskers superficially resemble moraines but are much narrower and steeper, contain little or no till, and tend to be oriented perpendicular to the glacier's margin rather than parallel to it as in moraines. Tunnels emanating from the margin of a glacier mark the downstream ends of eskers-in-the-making. Such tunnels can be seen at the front of Malaspina Glacier in Alaska.

Eskers are related to stratified deposits called kames. These are isolated mounds, hills, ridges, deltas, or terraces composed of collapsed meltwater-deposited sediment. Debris falling down crevasses or holes in the glacier or debris deposited by meltwater streams along the margin between the glacier and a valley side will, upon final ice melting, form kames. Where streams carrying large amounts of sediment drain from the ice into a lake abutting the glacier front, kame deltas may be formed. They resemble ordinary stream deltas except that they have no streams behind them, and they may be perched above the surrounding land surface if the meltwater streams feeding them were within or on the surface of glacier ice. In the Appalachian Mountains of southern Quebec, Canada, kame deltas perched on valley sides are often used as cemetery sites, since they are the only flat surfaces to be found. The different types of kames may be recognized by their geometry, coarse grain size, elevation relative to other deposits, and the disruption of layering (stratification) produced by collapse following melting of the bounding ice. Both kames and eskers are termed ice-contact stratified drift since both form in at least partial contact with glacier ice. Ice-contact deposits are numerous in areas where the glacier stagnated and rapidly melted from the top downward.

Outwash and Glaciolacustrine Deposits

Some stratified sediments of glacial origin are not deposited in direct contact with ice. Rather, they form in meltwater streams or lakes in front of the glacier. The most abundant of these deposits is outwash, a sorted and stratified layer of meltwater-transported sand and gravel deposited as a wide, thin sheet by numerous meltwater streams emanating from the front of a glacier. Outwash is usually associated with moraines, filling valleys in front of them to form what are called valley trains or, where particularly wide, outwash plains. These are forming in Iceland, where small continental-type glaciers called ice caps are in the process of melting. The thickness of outwash deposits is usually measured in meters or a few tens of meters. The top surface is commonly flat but may be riddled with holes called kettles. Kettles form when isolated blocks of ice in front of a glacier become surrounded by outwash and become partially or completely buried by it. When the blocks finally melt, which may take hundreds of years, holes are created in the outwash surface. Kettles commonly are filled with water and form ponds a few meters to tens of meters in diameter. As time passes, the kettles gradually fill with sediment or organic matter and turn into bogs and swamps. The New England region of the United States is famous for its kettles.

Another type of stratified deposit not formed by ice contact is glaciolacustrine (glacial lake) silt and clay. Many glaciers, especially continental ice sheets, block the drainage of streams flowing toward them. Water thus becomes ponded up against the front of the glacier or its moraine. Streams of meltwater flowing from the glacier into the adjacent lake carry huge amounts of fine-grained sediment (silt and clay) that become suspended in the lake water and gradually sink to the bottom, forming a layer that may reach several tens of meters in thickness. When the lake finally drains due to the melting of the blocking glacier, the glaciolacustrine deposits are left as isolated flat areas. Large areas in North America were occupied by such lakes during the melting of the last continental ice sheet. The largest was probably Glacial Lake Agassiz, which covered an area of more than 200,000 square kilometers in the Canadian provinces of Manitoba, Saskatchewan, and Ontario and extended into North Dakota and Minnesota in the United States. This lake was estimated to be more than 200 meters deep at its maximum extent.

No discussion of glacial deposits is complete without mentioning loess. Loess is a massive (unstratified) accumulation of silt deposited not by ice or water but by wind. It forms a widespread surficial blanket unless buried by younger deposits, from less than one meter to more than 300 meters thick. It occupies an area of more than 200,000 square kilometers in the central United States and more than 800,000 square kilometers in central China. Where present in sufficient thickness (more than a few meters), it forms a distinctive landscape of steep, sometimes vertical bluffs cutting a flat upland surface.

The origin of loess has been debated since the nineteenth century. The prevailing opinion is that it is a windblown deposit derived from two sources: deserts and outwash surfaces. The latter source appears to be the major one since loess deposits are thickest near outwash sources and get progressively thinner in a downwind direction. Loess may be considered a variety of glacially derived sediment not deposited in direct contact with ice.

Study of Glacial Deposits

Geologists and physical geographers who study glacial deposits are primarily interested in reconstructing the geometry, flow directions, ages, and melting patterns of former glaciers. The first step in the process is to identify the different types of glacial deposits in an area. This is accomplished by finding sites where these deposits are exposed and analyzing the exposed deposits for such things as grain size, internal structure and stratification, geometry, and composition. This information allows determination of deposit type. Next, the areal extent of the deposit must be established and its boundaries plotted on a topographic map or aerial photograph, both of which are small-scale, two-dimensional representations of the topography of an area. The spatial relations among deposit types can then be used to establish the geometry of the former glacier at various times during its existence. The shape of the glacier at various times, for example, can be determined by examining the positions and shapes of its terminal and recessional moraines, since they are outlines of its margin.

Ice-flow directions can be established by plotting the orientations of linear features produced by the flowing ice, such as drumlins or striations on bare rock surfaces. In some cases, an investigator wishes to know which kind of terrain a glacier has passed over. Analysis of the distribution of key rock and mineral types in till can yield this information. A sampling grid is drawn on a topographic map, and the investigator attempts to collect at least one sample from each square in the grid. The samples are then analyzed for types of rocks and minerals present and their relative abundance. The spatial change in abundance of these constituents can then be used to establish which terrain the glacier passed over. This method is used extensively by geologists interested in locating valuable ore deposits hidden under a cover of till.

The age of a glacial deposit can be determined by relative age dating and absolute age dating techniques. The former refers to whether one glacial deposit is younger or older than another, whereas the latter establishes the age of the deposit in years. A commonly used relative age dating technique compares the relative amounts of weathering (chemical alteration) of glacial deposits as expressed by depth of soil development or depth of weathering on surface boulders. There are several absolute age dating techniques, the most popular being the carbon-14 method. This method measures the age of organic matter associated with a glacial deposit. Old soils or peat deposits buried by glacial debris can be dated, thereby establishing a maximum age for the overlying deposits and the glacier advance that produced them. Near South Africa's gold fields, scientists confirmed the existence of glacial deposits dating back 2.9 billion years using these dating tehniques.

Patterns of ice melting (deglaciation) are determined by the analysis of the spatial patterns of till and associated stratified drift deposits. Melting glaciers produce typical sequences of these deposits that can be traced in the direction of glacier movement. For example, a particular valley may contain outwash leading up-valley to a terminal moraine ridge behind which is an esker bordered by kames, both of which are underlain by till. The sequence records a glacial advance to the position of the moraine, followed by stability of the ice front while the moraine forms and outwash is deposited down-valley. Subsequently, the glacier stagnates, and the esker and kames form beneath or on the stagnating ice. Rapid melting of the ice finally exposes the till. Many river valleys in the New England region of the United States contain such sequences.

The vertical sequence of glacial deposits at any one spot is often determined by taking core samples using a coring device. Coring devices are long, hollow tubes that are inserted vertically into the ground. When extracted, they contain a continuous vertical sequence of glacial sediments that may be observed by cutting the core tube lengthwise. This is often the only method of studying the history of glaciation in areas where exposures are few.

Principal Terms

Alpine glacier: a small, elongate, usually tongue-shaped glacier commonly occupying a preexisting valley in a mountain range

continental ice sheet: a glacier of considerable thickness that completely covers a large part of a continent, obscuring the relief of the underlying surface

drumlin: a smooth, elongate, oval-shaped hill or ridge formed under a moving glacier

glacier: a mass of snow and ice that persists for two or more years and is capable of movement by internal deformation

meltwater: water derived from the melting of glacier ice

moraine: an arcuate ridge consisting of till, stratified drift, or both, often deformed, deposited at the margin of a glacier

Pleistocene epoch: a geologic time period beginning between two and three million years ago and ending approximately 10,000 years ago, featuring several expansions and contractions of continental ice sheets

stratified drift: a sorted, layered sediment derived from glacier ice but subsequently reworked and resedimented by meltwater

till: unsorted, unconsolidated sediment consisting of clay to boulder-sized particles that are deposited directly by glacier ice without subsequent reworking by meltwater

Bibliography

Bennett, Matthew M., and Neil F. Glassner, editors. Glacial Geology: Ice Sheets and Landforms. 2nd ed., Wiley-Blackwell, 2013.

Bloom, Arthur G. Geomorphology: A Systematic Analysis of Late Cenozoic Landforms. Rawat Booksellers, 2022.

Hambrey, Michael, and Jurg Alean. Glaciers. 2nd ed., Cambridge UP, 2004.

Hess, Darrel et al. Physical Geography: A Landscape Appreciation. 13th ed., Prentice-Hall, 2022.

Klempe, Herald. “Identification of Quaternary Subsurface Glacial Deposits Using 3D Databases and GIS.” Norwegian Journal of Geography, vol. 58, 2004, pp. 90-95. doi.org/10.1080/00291950410006823. Accessed 20 Apr. 2023.

Menzies, John. Modern Glacial Environments: Processes, Dynamics, and Sediments. Butterworth-Heinemann, 1996.

Plummer, Charles C., et al. Physical Geology. 17th ed., McGraw-Hill, 2022.

"Scientists Find Evidence of World's Oldest Glaciers." Phys Org, 13 July 2023, phys.org/news/2023-07-scientists-evidence-world-oldest-glaciers.html. Accessed 31 July 2024.

Tulenko, Joseph P., et al. "Clues from Glacier Debris: Dating and Mapping Glacial Deposits Since the "Last Ice Age in the Western Alaska Range." National Park Service, 18 May 2021, www.nps.gov/articles/000/aps-20-1-2.htm. Accessed 1 Aug. 2024.

Williams, M. A. J. The Nile Basin: Quaternary Geology, Geomorphology and Prehistoric Environments. Cambridge UP, 2019.