Some Definitions for Glacial Geology (see also glacier definitions on syllabus)
cirque: an amphitheatre-shaped bowl scoured by a glacier into the side of a mountain. It may contain a cirque glacier, or it may be ice free, the glacier forming it long since melted away. Beautiful cirques with and without glaciers are found in the Alps and Glacier National Park, to name a few.
compressive flow: Flow condition where the ice in front is moving slower than the ice behind, so the ice behind compresses into the ice in front. This results in upward ice flow along shear planes in the ice, and longitudinal crevasses.
crevasse: brittle fractures formed in glacial ice due to stress conditions above some material strength of the ice that results in "failure", i.e. loss of cohesion across the fracture plane. Because failure can occur in a number of different ways (e.g. shear fractures and extension fractures), there are a variety of different measures of both the stress condition and the related material strength of ice that is ultimately exceeded. However, when stress exceeds the yield stress (a.k.a. the elastic limit), plastic deformation occurs and the ice flows as a ductile substance rather than breaking as a brittle substance. We discussed three types of crevasses: 1) marginal crevasses that are related to shearing flow conditions along the ice edge in a valley glacier, and that form an acute angle with the valley wall opening up-ice-flow. 2) longitudinal crevasses that form in compressive flow conditions and run parallel to ice flow, 3) and transverse crevasses that form in extending flow conditions and run across the ice perpendicular to ice flow.
deformation: a change in the size and/or shape of an object, a.k.a. strain. Deformation can be the result of volumetric strain (due to changes in length called extensional strain) or shear strain (due to changes in shape called shear strain). Glacial ice deforms principally in shear when it flows. The shear strain RATE (how quickly it deforms) is given by Glen's law, which relates the rate to the shear stress given by the Nye equation.
drumlin: a hill formed by glacial deposition and erosion, streamlined into the direction of ice flow, with a steep slope on the up-ice-flow direction side, and a gentle slope on the down-ice-flow direction side.
esker: 1. deposit sense: an ice contact stratified deposit--usually of sand and gravel, though coarser grain sizes such as sorted boulders are known--formed in a subglacial meltwater channel. 2. landform sense: a long, sinuous (i.e. winding) ridge.
esker tube: a subglacial meltwater channel.
glacial hydrology: From Websters, "hydrology is the science dealing with the waters of the Earth, their distribution on the surface and underground, and the cycle involving evaporation, precipitation, flow to the seas, etc." Glacial hydrology refers to the hydrologic processes affecting glaciers, including precipitation of snow, sublimation and/or melting of snow and ice, evaporation of water, and meltwater runoff. It is the last process that we have discussed the most. Meltwater leaves a glacier from both supraglacial streams and subglacial tunnels, depositing ice contact stratified deposits. Beyond the ice margin, braided streams deposit outwash sand and gravel.
Glen Law: (squiggle with dot on top) = A T ^n, where A and n are constants, but A is NOT the same constant as in profiling equation. The shear strain RATE (how quickly it deforms) is related to the shear stress, T (see Nye equation). Internal deformation of ice determines only part of the velocity of ice observed on the surface of a glacier. Another component of the observed velocity is basal sliding.
ice sheet: A thick, extensive body of glacial ice that is not confined to valleys. Ice sheets cover an area 1,000,000 and 10,000,000 square kilometers. Ice flow is radial from the centers of ice domes that merge to form an ice divide. The ice domes form a centrally located accumulation zone. Modern examples include Greenland and the East and West Antarctic Ice Sheets of Antarctica.
ice cap: A thick, extensive body of glacial ice that is not confined to valleys. Ice caps cover an area 100 to 10,000 square kilometers. Ice flow is radial from the center of 1 or more ice domes that may merge to form an ice divide. The ice domes form a centrally located accumulation zone. Modern examples include the Barnes and Penny Ice Caps on Baffin Island, ice caps on Spitsbergen, and Vatnajokull, Hofsjokull, Langjokull, and
Myrdalsjokull ice caps on Iceland.
ice contact stratified deposits: deposits of water-laid, sorted and stratified sediment formed in contact with the ice, including kames and eskers, and deposits in ice marginal channels and tunnel valleys.
ice marginal channel: A channel cut by flowing water along the ice margin. These channels are often left dry, or with remnant lakes, after the ice has melted away. Parallel sets of ice marginal channels may form when the ice margin retreats (melts back) down a slope.
ice-cored moraine: a mixture of debris and ice with ice forming the matrix. Ice-cored moraine is a remnant of "dirty glacial ice", debris rich glacial ice that has buried itself in the debris eroded from the basal zone, and transported through the englacial to supraglacial zones.
ice thickness: the shortest length from the surface of the glacier to the bed, usually measured perpendicular to the bed because the bed is relatively horizontal. The ice thickness perpendicular to a horizontal bed can be predicted from the profiling equation if the profiling constant is known.
kame: any of two ice contact stratified deposits leaving a hill or mound landform. 1. moulin kame: a conically stratified hill or mound composed of sorted sand and gravel, formed at the base of a moulin under the ice. 2. delta kame: a wedge-shaped hill or mound, composed of sorted sand and gravel, and stratified in one direction, formed from supraglacial drainage pouring off the ice margin.
kettle: a depression formed by the melting of buried ice blocks. Kettles are an example of inverted topography, in this case inverted by the melting of ice-cored moraine: where there was a high, now there is a low.
moulin: a shaft extending from the top to the base of the ice. Moulins are usually formed, or at least started, by intersecting crevasses. They remain open due to the heat liberated by supraglacial meltwater that collects into them.
Nye equation: T = pgh sin(a) Determines the shear stress, which is a "directed pressure" that causes a shear strain. Shear stress makes one layer flow over the top of a layer below.(see Glen law)
outwash: sand and gravel deposited by the running water of meltwater streams beyond the ice margin. These streams often form braided stream networks. Outwash is sorted, stratified, and usually crossbedded (remnant bedding from underwater dunes and sandbars). See Chapter 7 of Eyles for details.
profiling equation: h = A d ^(0.5) = A sqrt( d), see below
radial flow: Flow parallel to a radius of a circularly shaped object. For ice caps and ice sheets, the radial glacial flow is outward from the centrally located accumulation zone toward the ice margin; the direction of ice flow therefore depends upon which side of the ice cap or ice sheet is being considered.
Roche Moutonnee: a bedrock knob eroded by glacial ice, with an abraded, gentle slope on the up-ice-flow direction side, and a plucked (quarried), steep slope on the down-ice-flow direction side.
supraglacial: refers to the top of the glacier. The supraglacial zone transports debris without much physical interaction between the, or breakdown of the, fragments (therefore less abrasion, fracturing, etc.). The only way for this debris to get deposited is by melting the ice. Therefore, supraglacial material, including large erratics, becomes part of ablation till.
tunnel channel: A large cavern, partly in the ice and partly scoured into the bed, extending into and under the ice from the ice margin, from which subglacial water drains. The remnant of a tunnel channel is a scoured, dry valley (tunnel valley), usually half-filled with outwash sand and gravel. The filling with sand and gravel often gives the valley a flat floor. Several examples in Wisconsin and Michigan have scoured valleys hundreds of feet deep (with maybe all but 150 to 200 feet filled with sediment), a quarter to a half mile across, and run several miles in length. The tunnel channel roof was probably as high up into the ice as the valley floor was scoured deep. Another peculiar feature of tunnel channels is that the floor of the channel may go up and down in elevation (a.k.a. up-and-down long profiles), unlike a scoured river valley with a steadily sloping floor. At times, the whole cross-sectional area of the tunnel may have delivered torrents of water. Sugden and John (1976) point out, "up-and-down long profiles are most easily explained by water flowing under hydrostatic pressure in enclosed conduits." Tunnel channels typically form where the ice margin was frozen to the bed. The channels may have provided the only outlet for trapped subglacial water.