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Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as matrix support, is diagnostic of till. Geschiebemergel.JPG
Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as matrix support, is diagnostic of till.
Glacial till with tufts of grass Glacial till exposed in roadcut-750px.jpg
Glacial till with tufts of grass
Till after avalanche, Norway Till after avalanche in Norway (2).jpg
Till after avalanche, Norway

Till or glacial till is unsorted glacial sediment.

Till is derived from the erosion and entrainment of material by the moving ice of a glacier. It is deposited some distance down-ice to form terminal, lateral, medial and ground moraines.


Till is classified into primary deposits, laid down directly by glaciers, and secondary deposits, reworked by fluvial transport and other processes.


Till is a form of glacial drift , which is rock material transported by a glacier and deposited directly from the ice or from running water emerging from the ice. [1] It is distinguished from other forms of drift in that it is deposited directly by glaciers without being reworked by meltwater. [2] [3] [4] [5]

Till is characteristically unsorted and unstratified, and is not usually consolidated. Most till consists predominantly of clay, silt, and sand, but with pebbles, cobbles, and boulders scattered through the till. The abundance of clay demonstrates lack of reworking by turbulent flow, which otherwise would winnow the clay. [6] Typically, the distribution of particle sizes shows two peaks (it is bimodal) with pebbles predominating in the coarser peak. [4]

The larger clasts (rock fragments) in till typically show a diverse composition, often including rock types from outcrops hundreds of kilometers away. Some clasts may be rounded, and these are thought to be stream pebbles entrained by the glacier. Many of the clasts are faceted, striated, or polished, all signs of glacial abrasion. The sand and silt grains are typically angular to subangular rather than rounded. [4]

It has been known since the careful statistic work by geologist Chauncey D. Holmes in 1941 that elongated clasts in tills tend to align with the direction of ice flow. [7] Clasts in till may also show slight imbrication, with the clasts dipping upstream. [4]

Though till is generally unstratified, till high in clay may show lamination due to compaction under the weight of overlying ice. Till may also contain lenses of sand or gravel, indicating minor and local reworking by water transitional to non-till glacial drift. [2]

The term till comes from an old Scottish name for coarse, rocky soil. It was first used to describe primary glacial deposits by Archibald Geikie in 1863. [8] Early researchers tended to prefer the term boulder clay for the same kind of sediments, but this has fallen into disfavor. [9] Where it is unclear whether a poorly sorted, unconsolidated glacial deposit was deposited directly from glaciers, it is described as diamict or (when lithified) as diamictite. [4] Tillite is a sedimentary rock formed by lithification of till. [10]



Glacial till is mostly derived from subglacial erosion and from the entrainment by the moving ice of previously available unconsolidated sediments. Bedrock can be eroded through the action of glacial plucking and abrasion, and the resulting clasts of various sizes will be incorporated to the glacier's bed. [11]

Glacial abrasion is the weathering of bedrock below a flowing glacier by fragmented rock on the basal layer of the glacier. The two mechanisms of glacial abrasion are striation of the bedrock by coarse grains moved by the glacier, thus gouging the rock below, and polishing of the bedrock by smaller grains such as silts. Glacial plucking is the removal of large blocks from the bed of a glacier. [11]

Much of the silt in till is produced by glacial grinding, [4] and the longer the till remains at the ice-bedrock interface, the more thoroughly it is crushed. However, the crushing process appears to stop with fine silt. Clay in till is likely eroded from bedrock rather than being created by glacial processes. [7]


The sediments carried by a glacier will eventually be deposited some distance down-ice from its source. This takes place in the ablation zone , which is the part of the glacier where the rate of ablation (removal of ice by evaporation, melting, or other processes) exceeds the rate of accumulation of new ice from snowfall. As ice is removed, debris are left behind as till. [12] [13] The deposition of glacial till is not uniform, and a single till plain can contain a wide variety of different types of tills due to the various erosional mechanisms and location of till with respect to the transporting glacier. [14]

The different types of till can be categorized between subglacial (beneath) and supraglacial (surface) deposits. Subglacial deposits include lodgement, subglacial meltout, and deformation tills. Supraglacial deposits include supraglacial meltout and flow till. [15] Supraglacial deposits and landforms are widespread in areas of glacial downwasting (vertical thinning of glaciers, as opposed to ice-retreat. They typically sit at the top of the stratigraphic sediment sequence, which has a major influence on land usage. [14] Till is deposited as the terminal moraine, along the lateral and medial moraines and in the ground moraine of a glacier, and moraine is often conflated with till in older writings. [16] Till may also be deposited as drumlins and flutes, though some drumlins consist of a core of stratified sediments with only a cover of till. [17] Interpreting the glacial history of landforms can be difficult due to the tendency of overprinting landforms on top of each other. [18]

As a glacier melts, large amounts of till are eroded and become a source of sediments for reworked glacial drift deposits. These include glaciofluvial deposits, such as outwash in sandurs, [19] and as glaciolacustrine and glaciomarine deposits, such as varves (annual layers) in any proglacial lakes which may form. [20] Erosion of till may take place even in the subglacial environment, such as in tunnel valleys. [19]

Types of till

There are various types of classifying tills:

Traditionally (e.g. Dreimanis, 1988 [21] ) a further set of divisions has been made to primary deposits, based upon the method of deposition. Van der Meer et al. 2003 [22] have suggested that these till classifications are outdated and should instead be replaced with only one classification, that of deformation till. The reasons behind this are largely down to the difficulties in accurately classifying different tills, which are often based on inferences of the physical setting of the till rather than detailed analysis of the till fabric or particle size.

Subglacial till

Lodgement till

Subglacial lodgement tills are deposits beneath the glacier that are forced, or “lodged” into the bed below. As glaciers advance or retreat, the clasts that are deposited by the ice may have a lower velocity than the ice itself. When the friction between the clast and the bed exceeds the forces of the ice flowing above and around it, the clast will cease to move, and it will become a lodgement till.

Meltout till

Subglacial meltout tills are tills that are deposited via the melting of the ice lobe. Clasts are transported to the base of the glacier over time, and as basal melting continues, they are slowly deposited below the glacier. Since the rate of deposition is controlled by the rate of basal melting, it is worth considering the factors that contribute to melting. These can be the geothermal heat flux, frictional heat generated by sliding, ice thickness, and ice-surface temperature gradients.

Deformation till

Subglacial deformation tills refer to the homogenization of glacial sediments that occur when the stresses and shear forces from the moving glacier rework the topography of the bed. These contain preglacial sediments (non glacial or earlier glacial sediments), which have been run over and thus deformed by meltout processes or lodgement. The constant reworking of these deposited tills leads to a highly homogenized till. [15]

Supraglacial till

Meltout till

Supraglacial meltout tills are similar to subglacial meltout tills. Rather than being the product of basal melting, however, supraglacial meltout tills are imposed on top of the glacier. These consist of clasts and debris that become exposed due to melting via solar radiation. These debris are either just debris that have a high relative position on the glacier, or clasts that have been transported up from the base of the glacier. Debris accumulation has a feedback-loop relationship with melting. Initially, the darker colored debris absorb more heat and thus accelerate the melting process. After a significant amount of melting has occurred, the thickness of the till insulates the ice sheet and slows the melting process. Supraglacial meltout tills typically end up forming moraines.

Flow till

Supraglacial flow tills refer to tills that are subject to a dense concentration of clasts and debris from meltout. These debris localities are then subsequently affected by ablation. Due to their unstable nature, they are subject to downslope flow, and thus named “flow till.” Properties of flow tills vary, and can depend on factors such as water content, surface gradient, and debris characteristics. Generally, flow tills with a higher water content behave more fluidly, and thus are more susceptible to flow. There are three main types of flows, which are listed below.

  • Mobile flows: Thin, fluid, and rapid flows that significantly contribute to erosional processes. These cause strong clast orientation in the direction of flow.
  • Semi-plastic: Thick, slow moving “tongues” of debris. These are also erosive, and clast sorting is more organized than in mobile flows.
  • Creep: Very slow movement of debris, downslope in direction. Flow rate is slow enough not to be seen on relatively short timescales, as observed by humans. Particle orientation is often random and not associated with the direction of flow. [15]


In cases where till has been indurated or lithified by subsequent burial into solid rock, it is known as the sedimentary rock tillite. Matching beds of ancient tillites on opposite sides of the south Atlantic Ocean provided early evidence for continental drift. The same tillites also provide some support to the Precambrian Snowball Earth glaciation event hypothesis.

Economic resources

Tills sometimes contain placer deposits of valuable minerals such as gold. [23] [24] Diamonds have been found in glacial till in the north-central United States [25] and in Canada. [26] Till prospecting is a method of prospecting in which tills are sampled over a wide area to determine if they contain valuable minerals, such as gold, uranium, silver, nickel, or diamonds, and the flow direction indicated by the till is then used to trace the minerals back to their bedrock source. [27] [28]

See also

Related Research Articles

<span class="mw-page-title-main">Glacier</span> Persistent body of ice that is moving under its own weight

A glacier is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such as crevasses and seracs, as it slowly flows and deforms under stresses induced by its weight. As it moves, it abrades rock and debris from its substrate to create landforms such as cirques, moraines, or fjords. Although a glacier may flow into a body of water, it forms only on land and is distinct from the much thinner sea ice and lake ice that form on the surface of bodies of water.

<span class="mw-page-title-main">Drumlin</span> Elongated hill formed by glacial action

A drumlin, from the Irish word droimnín, first recorded in 1833, in the classical sense is an elongated hill in the shape of an inverted spoon or half-buried egg formed by glacial ice acting on underlying unconsolidated till or ground moraine. Assemblages of drumlins are referred to as fields or swarms; they can create a landscape which is often described as having a 'basket of eggs topography'.

<span class="mw-page-title-main">Moraine</span> Glacially formed accumulation of debris

A moraine is any accumulation of unconsolidated debris, sometimes referred to as glacial till, that occurs in both currently and formerly glaciated regions, and that has been previously carried along by a glacier or ice sheet. It may consist of partly rounded particles ranging in size from boulders down to gravel and sand, in a groundmass of finely-divided clayey material sometimes called glacial flour. Lateral moraines are those formed at the side of the ice flow, and terminal moraines were formed at the foot, marking the maximum advance of the glacier. Other types of moraine include ground moraines and medial moraines.

<span class="mw-page-title-main">Oak Ridges Moraine</span> Mountain in Canada

The Oak Ridges Moraine is an ecologically important geological landform in the Mixedwood Plains of south-central Ontario, Canada. The moraine covers a geographic area of 1,900 square kilometres (730 sq mi) between Caledon and Rice Lake, near Peterborough. One of the most significant landforms in southern Ontario, the moraine gets its name from the rolling hills and river valleys extending 160 km (99 mi) east from the Niagara Escarpment to Rice Lake, formed 12,000 years ago by advancing and retreating glaciers during the last glaciation period. Below the approximately 200 meter thick glacial derived sediments of the moraine lies thick bedrock successions of Precambrian rocks and up to 200 meters of Ordovician aged rock, capped by a regional unconformity of erosion and non-deposition to the Quaternary period. Rivers and lakes scatter the landscape and are important for creating habitat for the rich diversity of species of animals, trees and shrubbery. These are also the supply of fresh water to aquifers in the moraine through complex subterranean connections. Construction development nearby, and with expansion of communities around the moraine in need of potable water, it is a contested site in Ontario, since it stands in the path of major urban development. This region has been subject to multiple decades of scientific research to study the origins of formation, and how early communities used the land. A larger focus currently is how to source potable water without removing the aquifer entirely.

<span class="mw-page-title-main">Glacial erratic</span> Piece of rock that has been moved by a glacier

A glacial erratic is glacially deposited rock differing from the type of rock native to the area in which it rests. Erratics, which take their name from the Latin word errare, are carried by glacial ice, often over distances of hundreds of kilometres. Erratics can range in size from pebbles to large boulders such as Big Rock in Alberta.

<span class="mw-page-title-main">Conglomerate (geology)</span> Coarse-grained sedimentary rock composed mostly of rounded to sub-angular fragments

Conglomerate is a clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts. A conglomerate typically contains a matrix of finer-grained sediments, such as sand, silt, or clay, which fills the interstices between the clasts. The clasts and matrix are typically cemented by calcium carbonate, iron oxide, silica, or hardened clay.

<span class="mw-page-title-main">Glacial landform</span> Landform created by the action of glaciers

Glacial landforms are landforms created by the action of glaciers. Most of today's glacial landforms were created by the movement of large ice sheets during the Quaternary glaciations. Some areas, like Fennoscandia and the southern Andes, have extensive occurrences of glacial landforms; other areas, such as the Sahara, display rare and very old fossil glacial landforms.

Parent material is the underlying geological material in which soil horizons form. Soils typically inherit a great deal of structure and minerals from their parent material, and, as such, are often classified based upon their contents of consolidated or unconsolidated mineral material that has undergone some degree of physical or chemical weathering and the mode by which the materials were most recently transported.

The Oak Ridges Moraine is a geological landform that runs east-west across south central Ontario, Canada. It developed about 12,000 years ago, during the Wisconsin glaciation in North America. A complex ridge of sedimentary material, the moraine is known to have partially developed under water. The Niagara Escarpment played a key role in forming the moraine in that it acted as a dam for glacial meltwater trapped between it and two ice lobes.

<span class="mw-page-title-main">Terminal moraine</span> Type of moraine that forms at the terminal of a glacier

A terminal moraine, also called end moraine, is a type of moraine that forms at the terminal (edge) of a glacier, marking its maximum advance. At this point, debris that has accumulated by plucking and abrasion, has been pushed by the front edge of the ice, is driven no further and instead is deposited in an unsorted pile of sediment. Because the glacier acts very much like a conveyor belt, the longer it stays in one place, the greater the amount of material that will be deposited. The moraine is left as the marking point of the terminal extent of the ice.

<span class="mw-page-title-main">Drift (geology)</span> Material of glacial origin

In geology, drift is a name for all sediment transported by a glacier and deposited directly by or from the ice, or by glacial meltwater. Drift is often subdivided into unstratified drift that forms moraines and stratified drift that accumulates as stratified and sorted sediments in the form of outwash plains, eskers, kames, varves, and so forth. The term drift clay is a synonym for boulder clay. Both are archaic terms for glacial tills with a fine-grained matrix.

<span class="mw-page-title-main">Boulder clay</span> Geological deposit of clay

Boulder clay is an unsorted agglomeration of clastic sediment that is unstratified and structureless and contains gravel of various sizes, shapes, and compositions distributed at random in a fine-grained matrix. The fine-grained matrix consists of stiff, hard, pulverized clay or rock flour. Boulder clay is also known as either known as drift clay; till; unstratified drift, geschiebelehm (German); argile á blocaux (French); and keileem (Dutch).

<span class="mw-page-title-main">Plucking (glaciation)</span> Glacial erosion of bedrock

Plucking, also referred to as quarrying, is a glacial phenomenon that is responsible for the weathering and erosion of pieces of bedrock, especially large "joint blocks". This occurs in a type of glacier called a "valley glacier". As a glacier moves down a valley, friction causes the basal ice of the glacier to melt and infiltrate joints (cracks) in the bedrock. The freezing and thawing action of the ice enlarges, widens, or causes further cracks in the bedrock as it changes volume across the ice/water phase transition, gradually loosening the rock between the joints. This produces large pieces of rock called joint blocks. Eventually these joint blocks come loose and become trapped in the glacier.

<span class="mw-page-title-main">Tunnel valley</span> Glacial-formed geographic feature

A tunnel valley is a U-shaped valley originally cut under the glacial ice near the margin of continental ice sheets such as that now covering Antarctica and formerly covering portions of all continents during past glacial ages. They can be as long as 100 km (62 mi), 4 km (2.5 mi) wide, and 400 m (1,300 ft) deep.

<span class="mw-page-title-main">Diamictite</span> Type of sedimentary rock

Diamictite is a type of lithified sedimentary rock that consists of nonsorted to poorly sorted terrigenous sediment containing particles that range in size from clay to boulders, suspended in a matrix of mudstone or sandstone. The term was coined by Richard Foster Flint and others as a purely descriptive term, devoid of any reference to a particular origin. Some geologists restrict the usage to nonsorted or poorly sorted conglomerate or breccia that consists of sparse, terrigenous gravel suspended in either a mud or sand matrix.

<span class="mw-page-title-main">Rogen moraine</span> Landform of ridges deposited by a glacier or ice sheet transverse to ice flow

A Rogen moraine is a subglacially formed type of moraine landform, that mainly occurs in Fennoscandia, Scotland, Ireland and Canada. It is one of the three main types of hummocky moraines. They cover large areas that have been covered by ice, and occur mostly in what is believed to have been the central areas of the ice sheets. Rogen moraines are named after Lake Rogen in Härjedalen, Sweden, the landform's type locality. Rogen Nature Reserve serves to protect the unusual area.

Fluvioglacial landforms are those that result from the associated erosion and deposition of sediments caused by glacial meltwater. These landforms may also be referred to as glaciofluvial in nature. Glaciers contain suspended sediment loads, much of which is initially picked up from the underlying landmass. Landforms are shaped by glacial erosion through processes such as glacial quarrying, abrasion, and meltwater. Glacial meltwater contributes to the erosion of bedrock through both mechanical and chemical processes.

<span class="mw-page-title-main">Overdeepening</span> Characteristic of basins and valleys eroded by glaciers

Overdeepening is a characteristic of basins and valleys eroded by glaciers. An overdeepened valley profile is often eroded to depths which are hundreds of metres below the deepest continuous line along a valley or watercourse. This phenomenon is observed under modern day glaciers, in salt-water fjords and fresh-water lakes remaining after glaciers melt, as well as in tunnel valleys which are partially or totally filled with sediment. When the channel produced by a glacier is filled with debris, the subsurface geomorphic structure is found to be erosionally cut into bedrock and subsequently filled by sediments. These overdeepened cuts into bedrock structures can reach a depth of several hundred metres below the valley floor.

<span class="mw-page-title-main">Edaga Arbi Glacials</span> Palaeozoic geological formation in Africa

The Edaga Arbi Glacials are a Palaeozoic geological formation in Tigray and in Eritrea. The matrix is composed of grey, black and purple clays, that contains rock fragments up to 6 metres across. Pollen dating yields a Late Carboniferous to Early Permian age.

<span class="mw-page-title-main">Glaciofluvial deposits</span> Sediments/deposits formed from ice sheets or glaciers

Glaciofluvial deposits or Glacio-fluvial sediments consist of boulders, gravel, sand, silt and clay from ice sheets or glaciers. They are transported, sorted and deposited by streams of water. The deposits are formed beside, below or downstream from the ice. They include kames, kame terraces and eskers formed in ice contact and outwash fans and outwash plains below the ice margin. Typically the outwash sediment is carried by fast and turbulent fluvio-glacial meltwater streams, but occasionally it is carried by catastrophic outburst floods. Larger elements such as boulders and gravel are deposited nearer to the ice margin, while finer elements are carried farther, sometimes into lakes or the ocean. The sediments are sorted by fluvial processes. They differ from glacial till, which is moved and deposited by the ice of the glacier, and is unsorted.


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