From Benn 1. The measurement of the arrangement of clasts within a diamicton can be a powerful tool in the analysis of Quaternary sediments 3 , and it is traditionally used, in conjunction with striae data, as a standard quantitative tool in the analysis of past ice flow directions.
More recently, till fabric data has been used to infer process 4. Till fabric data can be used together with striae data to reconstruct ice flow direction, and can be used to help interpret depositional processes. The resulting data are three mutually orthogonal eigenvectors V1, V2 and V3 , with the principal eigenvector, V1, being parallel to the axis of maximum clustering in the data. V3 is normal to the preferred plane of the fabric. The degree of clustering about the eigenvectors is given by the eigenvalues S1, S2, and S3, with their relative magnitudes reflecting the fabric shape 5.
A-axis fabric data has a long history of research 3,6,7. Coulomb plastic behaviour involves slippage between clasts and the surrounding, faster-flowing matrix. Therefore, more elongate clasts assume a minimum cross-sectional area, orientating the a-axis parallel to main stress direction.
This makes strong, consistent till macro-fabrics in tills useful in interpreting palaeo ice-flow directions 8. Andersen et al. They interpreted the glacigenic sediments as lodgement tills. The upper till had a strong, unimodal clustering of clast axes around the mean axis, resulting in a high significance value.
The clast fabric from the lower till had a weak, equatorial, near random orientation of clast axes 8.
The lower fabric was, however, probably influenced by cobbles and boulders, leading to a local fabric probably unrelated to glacier flow. Recently, eigenvalues S values and vectors V values have been used to infer the genesis of glacial materials, indicating factors such as the rheology of the sediment.
For example, debris-rich basal ice subjected to high cumulative strains tends to have strongly clustered clast macro-fabrics, whereas tills formed under low strain can have either strongly clustered or highly variable clast macro-fabrics 3. Other researchers have found strong fabrics at low strains 10, In Jeffery Rotation clasts are continually rotated as a result of vertical velocity gradients, whereas in March Rotation B , clasts passively trace the deformation of the surrounding medium from Benn, b.
Hicock et al. Eigenvalues cannot be used alone, given the complexity of the subglacial environment 7. Some researchers have argued that Jeffery-type rotation Figure 1A is incompatible with the deforming bed hypothesis March-type rotation Figure 1B through plastic deformation has been identified as the dominant mode of clast orientation in deforming tills Weak clast macro-fabrics have often been reported as typical of deforming bed tills 14 , suggesting that particles are here free to rotate in a viscous medium Inhomogeneous deformation may produce a range of clast macro-fabric strengths, and localised fabric patterns reflect the deformation history and local strain conditions of the sediment Therefore it is important to limit the size range in the sample 4.
To obtain the clast measurements, clasts in the approximate size range mm should be excavated and the long axis a-axis and dip angle of 50 clasts per exposure recorded, using a compass-clinometer Benn, b. The data are presented in equal-area stereonets and rose diagrams, according to procedures in Evans and Benn and Benn b.
Clasts should be sampled from a 2 m 2 area. All three eigenvalues should be given. Striae are used in conjunction with till-fabric analysis to reconstruct past ice-flow directions. Striae on individual in situ clasts and boulders were measured using a compass-clinometer. Up to 50 striae sets were collected per exposure, and at least 10 per clast. Three conditions are necessary to form a glacier: 1 Cold local climate polar latitudes or high elevation.
Glaciers can only form at latitudes or elevations above the snowline , which is the elevation above which snow can form and remain present year round. The snowline, at present, lies at sea level in polar latitudes and rises up to m in tropical areas. Glaciers form in these areas if the snow becomes compacted, forcing out the air between the snowflakes.
As compaction occurs, the weight of the overlying snow causes the snow to recrystallize and increase its grain-size, until it increases its density and becomes a solid block of ice.
A glacier is actually a metamorphic rock. A glacier can change its size by Accumulation , which occurs by addition of snowfall, compaction and recrystallization, and Ablation , the loss of mass resulting from melting, usually at lower altitude, where temperatures may rise above freezing point in summer. Thus, depending on the balance between accumulation and ablation during a full season, the glacier can advance or retreat see figure Glaciers move to lower elevations under the force of gravity by two different processes: Internal Flow - called creep, results from deformation of the ice crystal structure - the crystals slide over each other like deck of cards.
This type of movement is the only type that occurs in polar glaciers, but it also occurs in temperate glaciers. Basal sliding - meltwater at base of glacier reduces friction by lubricating the surface and allowing the glacier to slide across its bed. Polar glaciers are usually frozen to their bed and are thus too cold for this mechanism to occur. The upper portions of glaciers are brittle, when the lower portion deforms by internal flow, the upper portions may fracture to form large cracks called crevasses.
Crevasses occur where the lower portion of a glacier flows over sudden change in topography see figure The velocity of glacial ice changes throughout the glacier. The velocity is low next to the base of the glacier and where it is contact with valley walls. The velocity increases toward the center and upper parts of the glacier see figure Glaciation : is the modification of the land surface by the action of glaciers.
Glaciations have occurred so recently in N. America and Europe, that weathering, mass wasting, and stream erosion have not had time to alter the landscape. Thus, evidence of glacial erosion and deposition are still present. Since glaciers move, they can pick up and transport rocks and thus erode.
Since they transport material and can melt, they can also deposit material. Glaciated landscapes are the result of both glacial erosion and glacial deposition. Glacial Erosion - Glaciers erode in several ways. Plucking — Ice breaks off and removes bedrock fragments Ice melts by pressure against the up-ice side of an obstruction. Entering cracks in bedrock, this water re-freezes to the ice.
Glacial movement plucks away bedrock chunks see figure Small scale erosional features note: most of this material will be presented as slides in class. Landforms produced by mountain glaciers see figure Landforms produced by Ice Caps and Ice Sheets. Glacial Deposition and Deposits.
Since glaciers are solid they can transport all sizes of sediment, from huge house-sized boulders to fine-grained clay sized material. The glacier can carry this material on its surface or embedded within it.
Thus, sediment transportation in a glacier is very much different than that in a stream. Thus, sediments deposited directly from melting of a glacial can range from very poorly sorted to better sorted, depending on how much water transport takes place after the ice melts. All sediment deposited as a result of glacial erosion is called Glacial Drift.
Stratified Drift - Glacial drift can be picked up and moved by meltwater streams which can then deposit that material as stratified drift. The weight of glacial ice sheets depress the lithosphere into the mantle causing the crust to subside.
After the ice melts, the depressed lithosphere rebounds. The rebound process is still taking place today see figures When glacial ice forms, it can block existing drainages causing the formation of new lakes and forcing streams to find new pathways that develop into new drainage networks. Once the ice melts, the new drainage network become well established and the old drainage networks are often abandoned.
Such a change in drainage networks took place as a result of the last ice age in North America see figure Prior to glaciation, streams in the northern U. Because the glacial ice retreated toward the north, the Mississippi drainage system became the major drainage system for much of the U. During the Pleistocene Epoch, large lakes formed both as result of ice dams and melting of glaciers.
Examples include the Great Lakes of the northern U. As ice melted, lakes were also formed in the western U. For example in the Basin and Range Province, basins were filled with large lakes formed by internal drainage. One of these lakes. Lake Bonneville, covered much of western Utah, eventually draining and evaporating leaving the remnant called the Great Salt Lake. The last glaciation ended about 11, years ago.
But the period between 11, years ago and 2 million years ago the Pleistocene epoch was a time of many glacial and interglacial ages. Based on evidence from glacial deposits and glacial erosion features geologists have been able to document at least 4 glaciations during the Pleistocene, two of which are poorly documented. But recent studies of deep-sea sediments and dating of these deposits suggest that there were at least 30 glaciations that occurred during the Pleistocene. This evidence comes from studies of fossils found in deep-sea sediment cores, and what they tell us about ocean surface temperatures in the past.
The results come from studies of the isotopes of oxygen. The record for the past two million years is shown here and in figure The data suggests about 30 glaciations separated by interglaciations during the past 2 million years. This means that glaciers transport everything from large boulders to tiny grains smaller than sand. These rocks and sediments are all mixed together in a jumble after they are deposited.
In contrast, rocks and sediments deposited by rivers settle out as the water speed slows, so big boulders are often dropped before small grains of sand. Rather than jumbling sediments of every size, rivers sort them out in a way that viscous glaciers cannot.
Glacier flour describes the component of glacier sediment that is much finer than sand. This material has a similar consistency to flour, which is the reason for its name. Because this sediment is so fine, it is easily transported by and suspended in water. It is responsible for the cloudy or milky appearance of the streams, rivers, and lakes that are fed by glaciers. Glacier lakes can have a wide range of beautiful colors that arise as sunlight scatters when it hits sediment particles in the water.
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