Introduction to geomorphology

1. Structure: refers to the nature of solid material that forms the surface, its composition, texture, fabric, architecture, mechanical strength, and other physical attributes.

2. Process: refers to the physical, chemical, or biological processes that shape the surface into landforms. Broadly speaking, processes are either depositional (constructive) or erosional (destructive).

3. Time: refers both to the rate at which a process modifies the surface and to the length of time or duration that a process has operated at a site.

The processes that shape landforms can be categorized as endogenetic or exogenetic. Endogenetic processes are related to plate tectonics and to the surface effects of plate movements, both horizontally and vertically, as well as to other processes originating from the Earth's interior. Exogenetic processes develop at or above the surface in the atmosphere, hydrosphere, cryosphere, or biosphere. They involve wind, water, ice, mass movements, or living organisms that modify landforms. Geomorphic processes associated with glaciation are among the most variable and complex of any environments at the Earth's surface.

Endogenetic and exogenetic processes combine with structure and time to produce the observed landforms at the Earth's surface. Most landforms involve a considerable mass of material, and so are slow to adapt when environmental changes take place. The geomorphology of a region, therefore, represents a long-term integration of environmental conditions and trends. A region's geomorphology is, thus, a reflection of both past and present environments.

Erosion or deposition beneath a glacier result from a complex relationship between ice velocity, basal melting/freezing, and substratum conditions. In general, erosion takes place under most of the accumulation zone, whereas deposition predominates in the ablation zone. The greatest amount of erosion takes place where ice velocity is highest, immediately inside the equilibrium line or in concentrated flow of ice streams--see Fig. 3-2.

Glacial landforms comprise many kinds of features created by glacial destruction and/or construction. Destructive landforms are the results of erosion or deformation in which material is removed from the local landscape. Constructive landforms are created by building up material due to deposition or deformation. Both glacier ice and glacial melt water are active agents of glacial landform genesis.

Large blocks of the substratum may be removed by a process called plucking or quarrying. This takes place when blocks of the substratum become frozen onto the ice base and are lifted up into the glacier. Freezing on may occur where ice is advancing from thawed-bed to frozen-bed zones either locally or regionally, and the removal of sediment is facilitated by compressive (upward) ice flow.

Subglacial melt water is another important agent of erosion. High-pressure water charged with sediment may be highly erosive, and can carve potholes and distinctive hydraulic channel forms. Sudden outbursts or floods of subglacial melt water, may remove huge volumes of sediment from beneath the glacier very rapidly (Booth 1994). It should be emphasized that all mechanisms of glacial erosion may operate in close association or may alternate through time at a particular site.

	Glacial polish on granite surface, island of Bornholm, Denmark. Subglacial grinding produced this polished surface. Ice movement from left to right; scale pole marked in 20-cm intervals. Photo date 6/79; © by J.S. Aber.
	Giant glacial groove on Kelleys Island, Ohio. Many giant grooves were once present on Kelleys Island. Photo shows about half of a preserved groove. Photo date 8/77; © J.S. Aber.
	Glacial grooves on Kelleys Island are cut into Paleozoic dolostone. The grooves were 2-6 m deep, 5-20 m wide, and 100-400 m long (Goldthwait 1979). This view depicts intricate striations and small grooves within the main channel. Photo date 8/77; © J.S. Aber.
	Crescentic gouges on Sioux Quartzite, southwestern Minnesota. Such gouges are produced by ice pressing and dragging a boulder over the bedrock surface. Ice movement in direction of needle (35 cm long); gouges are convex in the downice direction. Photo date 6/82; © by J.S. Aber.
	Crescentic gouges, shallow grooves and striations on Sioux Quartzite, southwestern Minnesota. Ice movement in direction of needle (35 cm long); gouges are convex in the downice direction. Photo date 6/82; © by J.S. Aber.
	Small potholes on a rock knob in front of Nigardsbreen, an outlet glacier of Jostedalsbreen, western Norway. These potholes were carved in crystalline bedrock by subglacial melt water when the glacier extended over the area. Photo date 6/87; © by J.S. Aber.

1. Begin shallow and narrow, and steadily increase in width and depth to a maximum, and then suddenly terminate.
   
2. Begin and increase, as in the first case, reach a maximum size, and then gradually taper out.
   
3. Start abruptly and then gradually taper out.

A characteristic kind of erosional landform is called stoss-and-lee topography. The stoss (upice) side is smoothed, striated and polished, in contrast to the lee (downice) side, which is rough and irregular. Smoothing of the stoss side is a result of pressure-melting and abrasion; the lee side is formed by regelation and plucking of angular blocks. Stoss-and-lee topography is developed at several scales: individual boulders--see Fig. 3-5, small bedrock knobs--called roche moutonées, and sizeable hills.

	Crossing sets of striations on Sioux Quartzite, Jeffers Petroglyphs, southern Minnesota. Needle and compass mark ice movement directions, which differ by about 30°. Ice movement toward bottom of view. Photo date 6/82; © by J.S. Aber.
	Crossing crescentic gouges on Sioux Quartzite, southwestern Minnesota. Part of the upper gouge is truncated by the lower gouge; the upper gouge is older, the lower one is younger. Needles indicate directions of ice movements, which differ by about 50°. Photo date 6/82; © by J.S. Aber.
	Glaciated crystalline bedrock of the Fennoscandian shield, Stockholm, Sweden. This view is directed downice; note polished and smoothed stoss surfaces (compare with image below). Glacial grinding and abrasion took place on the stoss sides of bedrock knobs. Photo date 4/87; © by J.S. Aber.
	Glaciated crystalline bedrock. This view is in upice direction; note rugged "stairstep" form of lee surfaces (compare with image above). Plucking (quarrying) occurred on lee sides of obstacles. Photo date 4/87; copyrighted by J.S. Aber.
	View of small roche moutonée formed on Sioux Quartzite near Jeffers, southern Minnesota. View is parallel to long axis of knob, which stands about 1 m above the adjacent glacial pavement. Smoothly streamlined upice end, top, and sides are visible. Ice movement toward background of view. This bedrock knob was freshly exposed when photographed. Photo date 7/96; © by J.S. Aber.

	Glaciated valley, near Voss, western Norway. View from head of the valley in downice direction. Note steep valley sides and gently rounded valley floor--classic U-shaped valley cross profile. Also notice highway zig-zag turns up the valley head in foreground. Photo date 6/87; © by J.S. Aber.
	Waterfalls spill from a hanging valley on the side of a deeper valley. Geisdalsfossen, western Norway. Photo date 6/87; © by J.S. Aber.
	Okanagan Lake, British Columbia. NASA space-shuttle photograph, STS068-155-011, 10/01/94, 5-inch format. Low-oblique view to south. Lake Okanagan valley is a deeply eroded glacial trough in the Canadian Rocky Mountains. NASA Johnson Space Center, Imagery Services.
	Small, lake-filled cirques in Tatra Mountains, southern Poland. The upper cirque (right) is separated from the lower one by a rock barrier. Each rock basin was excavated by a small cirque glacier. For more information and excellent views of the Tatras--go to Tatra Mountains virtual tourist. Photo date 8/93; © J.S. Aber.
	Morskie (Lake) Oko, Tatra Mountains, southern Poland. This alpine lake is dammed by a massive end moraine, located on the far side of the lake in this view over one of Poland's most popular tourist destinations--more views of Morskie Oko. Photo date 8/93; © J.S. Aber.
	View of Píco Aguila, Andes Mountains, Venezuela. This steep peak is a horn created by erosion of cirque glaciers in surrounding valleys. The peak reaches more than 4000 m altitude. Photo date 6/96; © by J.S. Aber.

Where montane glaciers or ice sheets descend into the sea, spectacular fjord valleys may be carved 100s to 1000s of m below sea level. The world's deepest fjords are in Antarctica: Vanderford, Vincennes Bay (2287 m) and Skelton Inlet (1933 m), and many fjords cut through the mountains of coastal Greenland. Fjord entrances are usually quite shallow with shoals and small islands; deep basins are located well inland from the mouth, as demonstrated by the classic fjords of western Norway--see Fig. 3-7.

The locations of fjords may be related to preglacial valleys, structural (bedrock) control, or crustal fractures (Holtedahl 1967). Fjords are the results of combined glacier erosion and erosion by high-pressure melt water flowing beneath the ice--see Fig. 3-6. The positions of fjord mouths mark the points where the valley glaciers either began to float or could spread out laterally, so that erosion was much less than in the confined inland valley.

	Space-shuttle photograph of southern tip of Greenland. Low-oblique view toward north, color-visible, 70-mm format, 12/88. All land areas are snow covered in this mid-winter scene. Deep glacier-carved fjord valleys cut through the coastal mountain ranges. The fjords define straight paths and meet at angular junctions, which indicate that glacial erosion followed crustal fractures. NASA Johnson Space Center, Imagery Services, STS027-36-49.
	View over interior portion of Hardangerfjord, western Norway. This is the deepest portion of the fjord--water depth more than 800 m, located 100 km inland from its opening to the sea. Note ship on far side of fjord for scale. Photo date 6/87; © by J.S. Aber.
	Giant "half potholes" on side of Granvin valley, a tributary to Hardangerfjord, western Norway. Sculpted side of valley was carved by high-pressure melt water flowing between the glacier and valley wall. Photo date 7/87; © by J.S. Aber.
	Closeup view of single pothole, which is carved into metamorphic bedrock, Granvin valley, Norway (see above). Photo date 7/87; © by J.S. Aber.
	Overview of Bolstadfjord, an interior fjord of western Norway. The main fjord basin is 100s of m deep in center. The fjord's entrance is located at the far end of this view (see below). Photo date 5/87; © by J.S. Aber.
	Closeup view of Bolstadfjord's mouth beneath the bridge. The entrance is narrow and quiet shallow; notice tidal current flowing out of fjord (to left). Photo date 5/87; © by J.S. Aber.

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