Glacial
geology
The scientific study of the effects of glaciers on the broad land areas, on the oceans, and on climate, of their erosion and deposition, and of their modification of the Earth's surface in detail. Included in the realm of glacial geology is the history of glacial theory, consideration of the origin of glacial ages, extent and times of past glaciations, erosion and sculpturing of plains and mountains, deposition of ice-contact and meltwater sediments, and the consequences of glaciers on worldwide climate, and also on local climate around their edges. Quite distinct from glacial geology, however, is the separate, growing subscience of glaciology, the study of glaciers themselves, the physics of glacier ice, and the accumulation, flow, wastage, budget, dynamics, and internal structures of glaciers. See also: Glaciology
Fig. 1 Large-scale glacial grooves, carved in soft limestone, Kelleys Island, Ohio.
Fig. 2 Severe valley-wall erosion by main trunk glaciers. (a) Before glaciation sets in, the region has smoothly rounded divides and narrow, V-shaped stream valleys. (b) After glaciation has been in progress for thousands of years, new erosional forms are developed. (c) With the disappearance of the ice, a system of glacial troughs is exposed. (After A. N. Strahler, Introduction to Physical Geography, 3d ed., John Wiley and Sons, 1973)
Fig. 3
End moraines in western
Features on the Earth's surface explained by former worldwide glaciation are numerous, embracing, for example, glacially eroded and molded valleys and mountains; ice-transported and deposited sediments and nonglacial sediments; abandoned stream channels with associated floodwater deposits; elevated silts and clays that collected around continental edges when sea level was higher; valleys eroded across and into continental shelves and slopes when sea level was much lower; communities of plants and animals similar to each other but separated by shallow seaways where land bridges once existed; fossil shells and microorganisms in deep-sea sediments reflecting colder or warmer water temperatures than today; vegetated sand dunes aligned to wind systems no longer operating; ancient shorelines and beach ridges ringing dry empty lake basins far inland; and orderly patterns of stones and fine sediments next to glacier margins in polar regions and high mountains.
Glacial ages
There are many theories on the history of glacial ages. Several hypotheses are under consideration, including combinations of the following: changes in solar flares and variations in sunspot number with time and correlative climatic changes; climatic variations on an astronomical time scale resulting from periodic perturbations of Earth's orbit, explaining fluctuations of climates but not their cause; variations in amounts of carbon dioxide or ozone and volcanic dust in the atmosphere, reducing solar heat received; localized uplift and regional mountain building producing highlands and lower temperatures, explaining glacial ages but not climatic fluctuations; drifting of continents toward polar positions, producing land areas at both poles and consequent glaciation, at least since 2 × 107 years ago; and changes in atmosphere-to-ocean circulation involving heat-and-cold cold exchange and ocean current disposition between high and low latitudes.
During the glacial ages of the last 7 ×
106 years, ice sheets and mountain glaciers expanded and waned with a
periodicity of about 10,000 years. Eight major glaciations occurred during the
Pleistocene Epoch, a time that has become synonymous with the glacial ages. It
took many thousands of years for ice sheets and glaciers to build to maximum
size, but only hundreds of years to disappear. Although many large mountain
glaciers still remain, only the
The great Pleistocene glaciers scoured the land, eroded valleys deeper or filled them in, deposited till under the ice along their margins, and left sand and gravel as water flowed away from the melting ice. Accumulated glaciers, weighing millions of tons, bent the land down beneath them, in some places below sea level. When the ice sheets rapidly wasted away, seawater flooded over the recently evacuated land and fine marine sediments settled over the continental edges. Long after the heavy ice had gone, the land slowly recovered, lifting these sediments 30 to 300 ft (10 to 100 m) above sea level. When the ice sheets were growing, water evaporated from the seas, precipitated as snow and changed into glacier ice, remaining for thousands of years on the land. If all the present glaciers melted, sea level would rise by approximately 330 ft (110 m). See also: Glacial epoch; Pleistocene
Glacial erosion
Two fundamental processes of glacier erosion are abrasion and plucking. Abrasion is the scouring of bedrock by rock debris carried in and near the bottom of a glacier; clean ice does not abrade solid rock. Conditions that lead to increased and more efficient rock abrasion include more debris in the glacier base, faster ice velocity, greater renewal of coarser debris, thicker heavier ice, less water pressure at the glacier base to reduce ice buoyancy, the presence of rock particles harder than the bedrock, and more angular than round particles. Plucking is the removal of fractured, jointed, or layered rock. Together, abrasion and plucking reduce a landscape to streamline molded forms with polished, striated surfaces showing small grooves and gouges; large-scale grooves, however, may be carved in soft limestone (Fig. 1). Larger rock knobs and small rock hills abraded by ice contain a gentle slope on the up-ice (stoss) side and a plucked or quarried steeper slope on the down-ice (lee) side, giving stoss-and-lee forms, often whaleback in appearance, which serve as a guide to ice direction. Drumlins, streamline hills made of till and created by the work of both erosion and deposition by glacier ice, show ice direction and usually occur in groups as drumlin fields.
Glacially sculptured landscape
Large ice sheets were long thought capable of molding rigorously the landscape across which they moved, yet some land areas known to have been glaciated bear little or no signs of ice erosion. It is now known that if an ice sheet is thin or develops in a cold continental climate and moves slowly, it is most likely cold-based and frozen to the ground with very little or no erosion beneath it. On the other hand, an ice sheet that is thousands of meters thick or that develops in a warm maritime climate is thicker, moves faster at its edges, and is most likely warm-based, therefore sliding across the ground with much erosion. Broad areas of low relief are uniformly abraded, softer rocks are scoured into deep troughs (with lakes in them later on), and high rock knobs are severely cut away. Ice erosion is concentrated in valleys which will deepen. Slopes in between are essentially unmodified, and plateau tops with fragile rock remnants, or tors, on their highest ridges survive all glacier erosion.
Alpine landscapes undergo selective
concentrated erosion, and deep valleys will be further deepened into glacial
troughs with their profiles changed from the preglacial V shape to a glacial U
shape by harsh valley-wall erosion. Such great deepening took place during the
Pleistocene in
In glacially sculptured mountains, valleys may be cut hundreds of meters deep, valley-floor profiles are accentuated by sharp rock steps, and glaciated tributary valleys are abandoned and left hanging above the main valleys as hanging troughs. Severe valley-wall erosion by main trunk glaciers may plane off or truncate the ends of spurs of tributary ridges leading to the main valley floor, leaving vertical rock cliffs as truncated spurs (Fig. 2).
Above the hanging tributary valleys, cliffed semicircular rock basins, known as cirques, may be carved from the valley head. Cirques may contain small lakes, or tarns, if the bedrock basin holds in the water, or they may contain moraine-dammed lakes, if the till and rock debris of a moraine hold in the water. Upper slopes and ridge crests between valleys and along divides may be steepened into sharp arêtes by the valley-wall ice erosion. The highest peaks along arêtes are horns, and the lowest places are cols. See also: Glaciated terrain
Glacial deposition
Fine-grained rock debris, rock flour, and coarse rock fragments are picked up or entrained within the base of a glacier and then transported and deposited from either active or stagnant ice. This product of glacial deposition, known as till, consists of particles that follow complicated routes, being deposited on the top or along the sides of the glacier bed, entrained again, and finally dropped. As a sediment, till has certain distinctive features: it exhibits poor sorting, is usually massive, and consists of large stones in a fine matrix of minerals and rock types.
Any ridge of till with a linear surface expression related to the glacier terminal position is a moraine. Although most moraines are made of till, some may contain stratified sediments as well. Lateral moraines are built along edges or sides of mountain glaciers. Medial moraines are made by the joining of two lateral moraines. End moraines (Fig. 3) are moraine ridges built transverse to the glacier margin, indicating the former ice front position. A terminal moraine is an end moraine in the farthest forward position that a particular glacier ever reached. See also: Moraine
As a cold ice sheet melts, meltwater transports rocks and particles through the glacier and away. This water either flows in surface channels off the ice front, drops into crevasses, or melts openings such as tunnels in the ice. As dirt-laden ice melts, rocks and finer debris drop into the meltwater streams, eroding and widening the channels and tunnels. The water eventually becomes turbid with boulders, pebbles, sand, silt, and clay, depositing sediment whenever the streams slow down, even momentarily. These deposits are known as stratified outwash sediments.
Fig. 4 Development of kames. (a) Melting stagnant ice in a valley, with a marginal stream along one side and a marginal lake along the other side of the valley. (b) After ice melts, a kame terrace with kettles remains along the valley wall, an esker where a tunnel formerly existed, two kames where the stream flowed on the ice, and a kame delta where sediment built into a marginal lake. (After A. N. Strahler, Introduction to Physical Geography, 3d ed., John Wiley and Sons, 1973)
Fig. 5 Patterned ground. (a) Horizontal cross section through sorted nets, or sorted polygons. Distance across net or polygon could be 10 cm to 10 m (4 in. to 33 ft). (b) Cross section through nonsorted stripes, view normal to slope. Distance between depressions could be 20 cm to 10 m (8 in. to 33 ft). (c) Horizontal cross section of nonsorted polygons or ice-wedge polygons. Distance across polygons could be 10 cm to 100 m (4 in. to 330 ft). (After J. Brown, Soils of the Okpilak River Region, Alaska, Cold Regions Res. Eng. Lab. Res. Rep. 188, 1966)
If the gravel and sand accumulate in pools within the ice or near the ice base and become exposed as the ice around it melts, round hills, termed kames, remain standing above the surrounding land. Crevasse fillings are short ridges with nearly flat or wavy tops that remain if ice melts around old gravel- and sand-filled crevasses parallel to or at right angles to former ice motion. Gravel and sand remain as winding, serpentine ridges termed eskers if a drainage system of tunnels existed within the ice. Where the tunnels emerge, gravel fans may be built beyond the ice front, or into a lake as small deltas at the ice front. If meltwater flows along the ice margin against an adjacent hillside, gravel and sand can accumulate on land as well as on the ice forming a sediment, usually with a flat top and slight downstream slope, known as a kame terrace. As an ice sheet disintegrates further, large holes melt out that may be filled with finer sediments by later through-flowing streams. After all the surrounding ice is gone, the sediment remains as kame plains—flat plains with ice-contact slopes all around. Kame deltas are built directly into lakes along glacier margins or at the ice front (Fig. 4). See also: Delta; Esker; Glaciated terrain; Kame
Landscapes adjacent to glaciers
Erosion and deposition by glaciers change the landscape, gouging old stream valleys deeper, filling valleys with till and outwash. Vast areas may be added or subtracted from a river system. Segments of large river valleys used for a while by meltwater streams become abandoned.
A weathering profile, or soil profile, given enough time, forms on any material—rock, till, gravel, sand, or loess. The older moraines and outwash are more deeply weathered and show mature soil profiles on them; younger moraines show only weakly or moderately developed soil profiles. Older soil profiles can be buried by younger deposits, or a soil profile may develop downward through two deposits. With retreat and a long time between glaciations followed by advance of another glacier, a moraine may be weathered under several different climatic conditions. This produces two or more soil profiles impressed on the one moraine, resulting in a complex soil. See also: Soil; Weathering processes
Periglacial phenomena
A subdivision of glacial geology is concerned with the periglacial climate and related phenomena. Most of these phenomena are formed either by repeated freezing and thawing of water in the soil near the ground surface, or by the presence of growth of ice in the ground, with the ground at much greater depth being permanently frozen.
Fig. 6 Pingo or hydrolaccolith. Upper part of ground may be thawed.
The more or less symmetrical patterns taken by rocks, fine debris, or vegetation are known by the group term, patterned ground. In one simple classification, they are either sorted or nonsorted: circles, nets, polygons, steps, and stripes. Sorted forms of large blocks or boulders usually are arranged geometrically, dominating and emphasizing the pattern, with finer particles and soil in their centers, as sorted circles, sorted nets, and sorted polygons (Fig. 5a). Sorted steps and sorted stripes are rocky lobate steps and long, narrow stone stripes. The nonsorted patterned-ground forms, on the other hand, are outlined or dominated by borders or high centers of vegetation, such as peat rings, earth or peat hummocks, and huge ice-cored mounds. Nonsorted stripes are long, parallel, vegetated troughs between small low ridges on a slope (Fig. 5b). Others with a polygonal mesh are controlled by massive wedges of ground ice, as ice-wedge polygons (Fig. 5c). Nonsorted steps appear on slopes as tongue-shaped turf- or grass-covered lobes. The large ice-cored mounds, termed pingos (or hydrolaccoliths), usually occur singly, rising 33 to 230 ft (10 to 70 m) above flat tundra plains, with diameters up to 1980 ft (600 m), sometimes with the ice visible in their center (Fig. 6).
Periglacial phenomena in mountainous regions, mostly as alpine forms of mass movement, occur as rockfall, alluvial, and avalanche talus, tongue-shaped and lobate rock glaciers, block fields, block slopes, block streams, and protalus ramparts.
Solifluction
This process, a slow continuous downslope movement of rock debris and soil saturated with water, is associated with patterned-ground phenomena, and is activated by the same freeze-thaw processes. However, if solifluction occurs above frozen ground, as in most polar regions and in many high mountain ranges, it is known as gelifluction. Solifluction and gelifluction are responsible for extensive denudation of landscapes in periglacial areas all over the world. Study of periglacial phenomena and solifluction features is important because they deal with a process of slope erosion occurring both in the past and present periglacial climates.
- J. T. Andrews, A Geomorphological Study of Post-Glacial Uplift: With Particular References to Arctic Canada, 1980
- J. T. Andrews, Glacial Systems, 1975
- M. J. Clark (ed.), Advances in Periglacial Geomorphology, 1987
- D. E. Sugden and B. S. John, Glaciers and Landscape, 1976
- A. L. Washburn, Geocryology: A Survey of Periglacial Processes and Environments, 1980
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