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=== <u>Erosional</u> === |
=== <u>Erosional</u> === |
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Glacial drift is the coarsely graded and extremely heterogeneous [[sediment]] of a glacier; till is the part of glacial drift deposited directly by the glacier. Its content may vary from [[clay]]s to mixtures of clay, [[sand]], [[gravel]], and [[boulder]]s. This material is mostly derived from the subglacial [[erosion]] and [[Glacial motion|entrainment]] by the moving ice of the [[glacier]]s of previously available unconsolidated sediments. Bedrock can also be eroded through the action of glacial [[Plucking (glaciation)|plucking]] and [[Abrasion (geology)|abrasion]] and the resulting [[clasts]] of various sizes will be incorporated to the glacier's bed. 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 scoring or striation of the bedrock by coarse grains or larger rocks moved by the glacier, or by polishing of the bedrock by smaller grains such as silt. Glacial plucking is the fracturing of larger pieces of rock from the bedrock surface by meltwater infiltration of preexisting fractures and joints freezing, presence of cavities between the glacier and bedrock surface, or simply loose rock able to be dislodged.<ref>{{Cite web|last=Bendle|first=Jacob|date=05/10/ |
Glacial drift is the coarsely graded and extremely heterogeneous [[sediment]] of a glacier; till is the part of glacial drift deposited directly by the glacier. Its content may vary from [[clay]]s to mixtures of clay, [[sand]], [[gravel]], and [[boulder]]s. This material is mostly derived from the subglacial [[erosion]] and [[Glacial motion|entrainment]] by the moving ice of the [[glacier]]s of previously available unconsolidated sediments. Bedrock can also be eroded through the action of glacial [[Plucking (glaciation)|plucking]] and [[Abrasion (geology)|abrasion]] and the resulting [[clasts]] of various sizes will be incorporated to the glacier's bed. 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 scoring or striation of the bedrock by coarse grains or larger rocks moved by the glacier, or by polishing of the bedrock by smaller grains such as silt. Glacial plucking is the fracturing of larger pieces of rock from the bedrock surface by meltwater infiltration of preexisting fractures and joints freezing, presence of cavities between the glacier and bedrock surface, or simply loose rock able to be dislodged.<ref>{{Cite web|last=Bendle|first=Jacob|date=05/10/20|title=Subglacial erosion|url=http://www.antarcticglaciers.org/glacial-geology/glacial-landforms/glacial-erosional-landforms/subglacial-erosion/|url-status=live|archive-url=|archive-date=|access-date=11/25/20|website=Antarctic Glaciers}}</ref> The combination of these two processes at varying rates result in a wide range of sediments and rock fragments produced and transported by the moving glacier, resulting in the loamy texture present in glacial tills. Rarely, eroded unconsolidated sediments can be preserved in the till along with their original [[sedimentary structures]]. More commonly, these sediments lose their original structure through the mixture processes associated with subglacial transport and they solely contribute to form the more or less uniform [[Matrix (geology)|matrix]] of the till. |
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=== <u>Depositional</u> === |
=== <u>Depositional</u> === |
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Revision as of 21:18, 25 November 2020
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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.
Processes
Erosional
Glacial drift is the coarsely graded and extremely heterogeneous sediment of a glacier; till is the part of glacial drift deposited directly by the glacier. Its content may vary from clays to mixtures of clay, sand, gravel, and boulders. This material is mostly derived from the subglacial erosion and entrainment by the moving ice of the glaciers of previously available unconsolidated sediments. Bedrock can also 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. 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 scoring or striation of the bedrock by coarse grains or larger rocks moved by the glacier, or by polishing of the bedrock by smaller grains such as silt. Glacial plucking is the fracturing of larger pieces of rock from the bedrock surface by meltwater infiltration of preexisting fractures and joints freezing, presence of cavities between the glacier and bedrock surface, or simply loose rock able to be dislodged.[1] The combination of these two processes at varying rates result in a wide range of sediments and rock fragments produced and transported by the moving glacier, resulting in the loamy texture present in glacial tills. Rarely, eroded unconsolidated sediments can be preserved in the till along with their original sedimentary structures. More commonly, these sediments lose their original structure through the mixture processes associated with subglacial transport and they solely contribute to form the more or less uniform matrix of the till.
Depositional
Eventually, the sedimentary assemblage forming this bed will be abandoned some distance down-ice from its various sources. This is the process of glacial till deposition. 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. The different types of till can be categorized between subglacial (beneath) and supraglacial (surface) deposits. Subglacial deposits include lodgement, subglacial meltout, and deformation till. Supraglacial deposits include supraglacial meltout and flow till.[2] Supraglacial deposits and landforms are of critical importance, because they make up a majority of the land surface at their localities. They typically sit at the top of the stratigraphic sediment sequence, which has a major influence on land usage.[3] Till is deposited as the terminal moraine, along the lateral and medial moraines and in the ground moraine of a glacier. As a glacier melts, especially a continental glacier, large amounts of till are washed away and deposited as outwash in sandurs by the rivers flowing from the glacier, and as varves (annual layers) in any proglacial lakes which may form. Till may also be deposited as drumlins and flutes. Interpreting the glacial history of landforms can be difficult due to the tendency of overprinting landforms on top of each other. Till may contain detectable concentrations of gems or other valuable ore minerals picked up by the glacier during its advance, for example the diamonds found in the U.S. states of Wisconsin and Indiana, and in Canada. Prospectors use trace minerals in tills as clues to follow the glacier upstream to find kimberlite diamond deposits and other types of ore deposits.
Tillite
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.
Types of Till
There are various types of classifying tills:
- primary deposits – laid down directly by glacier action
- secondary deposits – reworked by fluvial transport, erosion, etc.
Traditionally (e.g. Dreimanis, 1988[4]) a further set of divisions has been made to primary deposits, based upon the method of deposition. Van der Meer et al. 2003[5] 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 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 can contain preglacial sediments (non glacial or earlier glacial sediments), which have been run over and thus deformed, or deposited syn-sedimentary in context, by meltout processes or lodgement. The constant reworking of these deposited tills leads to a highly homogenized till.
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. 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.[2]
See also
- Boulder clay – Geological deposit of clay
- Diamictite – Type of sedimentary rock
References
- ^ Bendle, Jacob (05/10/20). "Subglacial erosion". Antarctic Glaciers. Retrieved 11/25/20.
{{cite web}}: Check date values in:|access-date=and|date=(help)CS1 maint: url-status (link) - ^ a b Bennett, Mathew (2009). Glacial Geology: Ice Sheets and Landforms. Wiley-Blackwell. ISBN 978-0-470-51690-4.
- ^ Hilt Johnson, W.; Menzies, John (2002-01-01), Menzies, John (ed.), "10 - Supraglacial and ice-marginal deposits and landforms", Modern and Past Glacial Environments, Oxford: Butterworth-Heinemann, pp. 317–333, ISBN 978-0-7506-4226-2, retrieved 2020-11-25
- ^ Dreimanis, A., 1988. Tills: Their genetic terminology and classification, pp. 17–83. In R. P. Goldthwait and C. L. Matsch, éd., Genetic classification of glacigenic deposits. A. A. Balkema, Rotterdam
- ^ Meer, J.J.M. van der, Menzies, J. and Rose, J. 2003. Subglacial till: The deforming glacier bed. Quaternary Science Reviews 22, pp. 1659–85.