Chapter 17b
Glaciers and Glacial Landforms (cont.)
Geosystems, 4CE, pp. 533-564
Geosystems, 3CE, pp. 515-549
Glaciers can carry sediment in three different ways:
on the surface - called supraglacial sediment
within the ice - called englacial sediment
at the base of the ice - called subglacial sediment
Glaciers vary in terms of how much sediment they carry in each location. They way sediment is carried will affect the glaciers appearance, how it erodes its bed, and depositional forms it may create.
A. Processes of Glacial Erosion
1. Detachment
a. Some sediment comes from other gradational processes such as chemical or physical weathering or mass wasting. These processes may loosen bedrock and provide debris that a glacier can pick up. This material is already detached by the time it gets to the glacier. It often falls onto the surface from surrounding cliffs. This appears to have been the case with the Foothills Erratic Chain and the Okotoks/Airdrie Erratics. The amount and size of debris likely resulted from a landslide in the Rockies.
b. Abrasion occurs when ice and debris transported by the ice grinds against the bed, scraping and prying away fragments of rock. The amount of abrasion is related to the velocity of the glacier, its thickness, and the amount of sediment it is transporting.
c. Freeze - thaw processes are important where water on the base of warm-based glaciers penetrates cracks in the bedrock and refreezes. As the glacier moves, it can pull apart the rock to which it is frozen from the bed.
2. Entrainment
a. some blocks of rock and debris may fall directly onto the glacier's surface from surrounding cliffs.
b. some materials may be carried onto the glacier by wind or water.
c. debris at the base may be "picked up" as ice that has been pressure-melted refreezes around rocks and particles.
d. debris may be "ploughed ahead" of the glacier by the snout and dragged along underneath the moving ice.
3. Transport
a. Subglacial material is often dragged along; this material is called a ground moraine. If a glacier is in retreat the ground moraine is left behind as mounds of rock, and loose debris called till. Till is simply the loose deposits left behind by a glacier – an unsorted mixture of rock, sand, silt, and clay.
b. Englacial material flows with the ice (unlike wind or water, in which the mass of transported material is related to velocity, ice can carry very large material as easily as small, light material). If a glacier is retreating, it is dropped and left behind, adding to the ground moraine as more till.
c. Supraglacial material is carried as moraines on the surface.
Moraines along the edges of the glacier, composed of debris that falls from, or is scraped away from the rock walls are called lateral moraines.
When two glaciers merge, two lateral moraines merge, forming a moraine in the middle of the glacier, called a medial moraine.
** See pages 542-3 (or Figure 17.7 “A retreating alpine glacier” (3CE, p. 521)). Find the lateral moraine and medial moraines! Understand how, why, and where they are formed.
If a glacier is retreating, these lateral or medial may be left behind as long ridges of debris.
Moraines at the snout (lowest end) of a glacier are called terminal moraines. If the glacier is retreating, several older terminal moraines may be evident further down the valley. These are referred to as recessional moraines.
Check out lateral and recessional moraines on the Athabasca Glacier.
B. Erosional Landforms
1. Small-Scale Effects
a. striations are grooves or deep scratches in the bedrock, caused by rough rocks and particles caught in the glacier's base gouging the rock. They line up with the direction of ice movement.
b. chatter marks are lateral (right-angle) grooves in the bedrock caused by flakes or chips of the bedrock being pried away by the ice. These area at right angles to the direction of ice movement.
c. polishing - fine clay and silt particles carried by glaciers over hard bedrock can create very smooth surfaces that look like they have been polished.
d. potholes and stream channels may be cut by subglacial streams.
2. Alpine Glacial Landforms
Study Figures 17.10 & 17.11 4CE, pp.545-6 "An alpine valley..." & "The geomorphic handiwork of alpine glaciers” (Figure 17.11 & 17.12, 3CE, p. 525-6 “Alpine glaciers occupy valleys" & "The geomorphic handiwork of alpine glaciers” ).
Alpine glaciers create highly eroded landscapes:
a. deep, straight U-shaped Valleys are created as glaciers widen and deepen existing river valleys (stream valleys are typically V-shaped).
b. Glaciers tend to cut away any ridges that protrude into the valley, producing truncated spurs.
c. Between glacial valleys are steep sided, sharp-edged ridges called arêtes, and pointed peaks called horns. Alpine glaciers erode back into the cliff, creating knife-edged aretes and sharp horns. Click here for Banff examples, and Washington State examples.
d. Side valleys feeding into a larger, deeper valley are referred to as hanging valleys. Because tributary glaciers in these side valleys tended to be smaller, they did not erode their bed as deeply as the larger, heavier main glacier. Thus, when all the glaciers melt, the main glacial U-shaped valley is cut much deeper than the side valleys. The side valleys appear to “hang” part way up the side of the main valley. Streams running our of these hanging valleys may form spectacular hanging waterfalls.
e. Alpine glaciers tend to erode bowl-shaped basins called cirques. If the glacier melts a small lake often remains in the cirque, called a tarn. Some cirques contain a series of small, circular tarns arranged in steps down the valley, called paternoster lakes (“Pater noster” is Latin for “Our Father,” the first phrase of the prayer Roman Catholics use with the Rosary – these lakes, arranged like beads on a chain, supposedly look like Rosary beads).
f. Most cirque glaciers have a permanent crevasse near the top called a bergschrund. The thin, uppermost stretch of the glacier remains frozen to the upper rock face. The thick, heavy, lower 95+% of the glacier flows downhill. In essence, the glacier pulls apart -- the uppermost part frozen tight to the rock; the lower majority of the glacier flowing downhill. Rock falling from the cliffs above may be transported to the glacier base down this large crevasse. Mountain climbers are very aware of these crevasses. Cirque glaciers are often easier to climb than the surrounding rock, but the bergschrund can be a formidable obstacle.
g. When two cirques cut back towards each other far enough, a col, or semi-circular pass, or may be formed. ("col" is the French word for pass)
h. when a U-shaped glacial valley is flooded by rising sea levels, a long, narrow, steep-sided bay called a fjord (or fiord) is formed.
Study these landforms! They make great exam material!
3. Continental Glacial Landforms
a. Continental glaciers tend to reduce topography over wide areas (lowering mountains to rounded hills, like the Appalachians, Atlantic Canada, and the Canadian Shield).
b. Continental glaciers cause widespread striations, polishing, etc.
c. In areas of softer rock they carve out hollows, creating lakes when retreat occurs (northern Ontario and Quebec are dotted with tiny lakes, carved by glaciers out of the Canadian Shield).
d. Roche moutonnées are rocky hills of resistant bedrock: typically they have a smooth, rounded stoss side (the side from which the glacier came), and an irregular, blocky, steep lee side caused by the ice plucking out large blocks. Figure 17.9 "Roche moutonnee" (fig. 17.18, 3CE, p. 530) “Glacial erosion streamlined rock”
Continental glaciers deposit material in the ways:
i. as glacial till, rock and debris deposited directly by the glacial ice (ranging from fine particles to huge boulders)
ii. as glaciofluvial (glacial river) deposits (till)
iii. as glaciolacustrine (glacial lake) deposits (till)
Glacial rivers and lakes are never clear, but often muddy or green/blue because the water contains many fine, suspended sediments and particles, many of which reflect specific wavelengths of light, creating dramatic colours. Many glacial lakes are dramatic blue or green colours, because the fine glacial sediment suspended in the water reflects light in these wavelengths.
Erratics are rocks that geologically belong in one region, that have been transported (often thousands of kilometres) to a location that is completely unrelated to their geologic structure.
1. Subglacial Landforms
Study Figure 17.16, 4CE, p. 548 (fig. 17.17; 3CE, p. 529) “Continental glacier depositional features”
i. flutes - small ridges of till (less than 1 metre high); these are the remnants of small subglacial stream beds. (A stream, flowing under the ice, deposits sediment on its bed. The stream level "rises." The stream, however, is trapped side-to-side by ice. So the stream bed sediment piles up in the subglacial stream channel. When the glacier melts, this stream bed sediment remains as a sinuous mound)
ii. eskers - long sinuous ridges of till (often several meters high) which may be 100's of kilometres long and several metres high; these are the remnant river beds of subglacial rivers. Eskers are the same as flutes except are MUCH larger in scale. Flutes are small (less than 1 metre high), and tend to relatively short (a few tens or hundreds of meters long). Eskers are much larger (up to several metres high), and longer (hundreds of kilometres). Flutes are from subglacial creeks; eskers from subglacial rivers!
iii. drumlins - large, streamlined hills (up to 200 m high and up to 5 km long), formed by subglacial floods piling sediments in glacial caverns. These are made of till, not solid rock. Often many drumlins are found in the same location. (Streams flowing under the ice tend to carve out hollows or caverns. Sediment can accumulate in these subglacial chambers. When the ice melts, these mounds of sediment remain).
2. Landforms Along Glacial Margins
i. lateral moraines - till along edges of glacier. Like alpine glaciers, continental glaciers accumulate sediment along their edges. Continental glacial lateral moraines are obviously much bigger than alpine glacial lateral moraines!
ii. terminal moraines - till mounded up at the snout. Like alpine glaciers, as a glacier retreats, several terminal moraines may form parallel ridges charting the retreat (referred to as recessional moraines). Again, the continental versions are much more extensive than alpine ones!
iii. ice-cored moraines - as ice melts, a block of ice may be trapped in the middle of till. When this block of ice eventually melts, it may form a depression with a small lake in it, called a kettle lake. The ridges between kettle lakes are called kames. Kames also do occur at the snouts of alpine glaciers, but are much more common, and much larger, when associated with continental glaciers. Kettles/kames result in hummocky terrain, with a series of depressions (kettles) and gravelly hills (kames).
iv. An outwash plain is the area that is affected by runoff from the glacier. If the glacier is retreating, it is often made up of till left behind as a ground moraine. Because this is loosely consolidated, unstable material, and runoff varies tremendously seasonally, braided rivers are most common. Again, this is similar to the material at the snouts of alpine glaciers. But outwash plains are much larger and more extensive when associated with continental glaciers.
Worth reflecting on ...
Ian Hutchinson is a professor of nuclear physics at MIT. Check out his biography:
http://www.youtube.com/watch?v=_dY8scOeXLA&list=PL1F3AB913D99D2230
http://www.youtube.com/watch?v=MO9OSbJgMjo&list=PL1F3AB913D99D2230
http://www.youtube.com/watch?v=o3sLdosL5OE&list=PL1F3AB913D99D2230
If you have not yet done so, please do read the article by Peter Harris in the 17a notes: Caring for Creation - the new frontier of mission
How do you feel about Harris's comments? Do you agree? Feel free to discuss this quote on the course discussion site (see the syllabus for details ...)
To review …
Check out the resources at the text web site for Chapter 17:
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