Glacial Erosion Processes
Erosion by glaciers takes place mainly by abrasion and quarrying. Abrasion occurs when fine particles and fragments held in the ice, at or near the base of a glacier, move across the underlying material, commonly bedrock. This process can striate and polish fragments in the ice and the underlying bedrock. In addition, abrasion may form elongated, gutterlike channels (flutings) in the bedrock.
Quarrying, the removal of blocks of bedrock by overriding ice, usually occurs where the bedrock is easily fractured, or where planes of weakness, such as joints, are present. Roches moutonnées, large, knoblike bedrock features with streamlined sides tapering up-glacier, and steep, abruptly broken sides down-glacier, result from abrasion and quarrying. Relatively more erosion and removal of material takes place in valley glaciers, where the ice is confined by topography, than in the less constrained ICE CAPS and ice sheets. U-shaped valleys, such as the Bow Valley in the Rocky Mountains, are formed where glacial erosion has deepened and widened pre-existing river valleys.
As erosion takes place in one area, deposition may occur in another. Forms such as DRUMLINS (streamlined, oval mounds commonly 15-25 m high and about 1500 m long) and certain kinds of ground MORAINE (low-relief, undulating terrain commonly up to 6 m high) can form under moving ice. However, most glacier deposition takes place near the terminus (boundary) during retreat of the ice. Features which can occur include hummocky moraine, high-relief forms up to about 10 m high, consisting of mounds, ridges and knobs, some doughnut-shaped; series of arcuate (bow-shaped) ridges of varying heights and lengths, named (according to their form, origin and position) cross-valley, ribbed, washboard, De Geer, push, ice-thrusted and recessional moraines; single, prominent ridges marking the limit of a glacial advance, called end or terminal moraines; and ground moraine. Most of these features contain a high percentage of glacial till.
Till, in the strictest sense, is unstratified, unsorted material deposited directly from a glacier. It usually consists of a heterogeneous mixture of clay, silt, sand, pebbles, cobbles and boulders, with most constituents closely reflecting the composition of local bedrock. Commonly, larger particles are angular to well-rounded, striated, and show a preferred orientation. Till can be subdivided into several types depending on the location of debris in the ice and how it was deposited.
Glaciers are also directly or indirectly responsible for various other deposits. Meltwater, originating on the surface, inside a glacier, or at its base, may form braided streams beyond the glacier margin. These streams display an interconnecting network of shallow channels which carry and deposit gravel and sand. Gravel is an important industrial resource in Canada, and some of the largest deposits have resulted from glacier-derived braided streams. An outstanding modern example is the Donjek River, YT, fed from the Donjek Glacier in the St Elias Range. Kames and ESKERS (knoblike features and sinuous ridges, respectively) result from the deposition of SAND AND GRAVEL by ice-content glacial streams.
Glacially associated lake deposits form large plains that cover wide areas of Canada. The lakes were formed by direct damming by the glacier or by impedances to pre-existing drainage. Manitoba's former glacial Lake AGASSIZ is an outstanding example of a glacially dammed lake. Most sediments deposited in glacial lakes consist of silt and clay, which commonly form varves, ie, pairs of coarse and fine layers deposited in one year. Beach ridges composed of gravel and sand occur along the margins of some former glacial lakes.
The absence of vegetation in recently deglaciated areas and the exposure of unconsolidated sand and silt (such as occurs in interchannel areas of braided rivers and former lake beds) allow wind to form sand dunes and deposit loess. Dunes are formed by the shifting of sand by saltation and traction; loess deposits, consisting of fine sand and silt, originate from suspended material that may have been carried hundreds of kilometres (see AEOLIAN LANDFORM).
Major Advances and Retreats
The areal extent and succession of glacial deposits indicate how far and how often glaciers expanded in the past. Most is known about glacial activity of the past 2-3 million years, although evidence indicates that glaciation took place several times during GEOLOGICAL HISTORY. During the Pleistocene ICE AGE, as much as 30% of Earth's surface was covered by glaciers. Glaciers formed and expanded in mountainous regions throughout the world. In northern latitudes (eg, Canada and northern Europe) ice caps developed, expanding to ice sheets. About 97% of Canada was covered; hence, this country contains more glaciated terrain than any other.
The number of major glaciations that occurred during the Ice Age is open to question. Traditionally, 4 glaciations were recognized, each lasting approximately 100 000 years, separated by long, warmer periods. From oldest to youngest, these are known in North America as Nebraskan, Kansan, Illinoian and Wisconsinan. Within these major glaciations, minor glacier retreats and advances are recognized. New evidence and reinterpretation of old data suggest that ice did expand and retreat many times, but the complexity of the data is such that it is not even possible to say with certainty that there actually were 4 major glaciations. Much is known about the Wisconsinan, less about previous glaciations.
Last Major Glaciation
Since the Wisconsinan was the latest glaciation, evidence (eg, moraines) is relatively well preserved. In addition, the time of the Wisconsinan glaciation can be estimated, mainly through radiocarbon dating of organic matter from below, within and above Wisconsinan glacial deposits (see GEOLOGICAL DATING). Although radiocarbon dating is by far the most important method for determining when glaciers expanded, it is useful only for material less than about 50 000 years old.
More extensive glaciations took place in Canada prior to the late Wisconsinan, although there is evidence in Western Canada that parts of the Laurentide ice sheet flowed beyond any earlier limits. It is not known if the earlier, more extensive ice took place during the early Wisconsinan or if it represents a major glaciation such as the Illinoian. It is evident, however, that during the Pleistocene (1.65 million to 10 000 years ago), ice never flowed far beyond the late Wisconsinan limits and, in general, conformed roughly with the configuration of the latest glaciation. Glaciers never extended into the northern Yukon and parts of the Northwest Territories. In addition, the highest peaks of western Canada and the higher hills on the prairies (eg, the CYPRESS HILLS) have never been glaciated (see NUNATAK). Although the climate was severe enough to support glaciers, there was not enough moisture to nourish their expansion.
Enough information is available from glacial and associated deposits and from radiocarbon-dated organic samples to give a reasonable account of the Wisconsinan glacier complex in Canada. Sometime after about 100 000 years ago, ice caps formed and expanded in several parts of Canada. Major areas of accumulation included the Keewatin Sector, the Labradorian Sector and the Foxe-Baffin Sector. Minor ice caps formed in the Atlantic provinces and the arctic islands. In time, these ice caps coalesced, forming the Laurentide Ice Sheet. Apparently, at about the same time, valley glaciers expanded in the western mountains and, in time, formed the Cordilleran Ice Sheet.
There is conflicting evidence about how far the ice sheets expanded initially. At least one retreat occurred before the final onslaught, which probably began about 25 000 years ago and reached the areas illustrated on the map. The maximum limit reached by the late Wisconsinan ice sheets is under debate. In addition, there is evidence that the time of maximum expansion of the ice sheets varied from region to region. The Laurentide Ice Sheet probably had a maximum ice thickness close to 4000 m; that of the Cordilleran Ice Sheet may have been close to 2000 m.
As recession of the ice sheets took place, most of the glacial landforms seen today across Canada were formed. There were minor readvances during the overall retreat, but in general the retreat was relatively rapid, with ice withdrawn from large parts of Canada by 10 000 years ago. Since that time, glacial and other landforms have been modified by various agents such as water and wind. However, these changes have been minor, and the preservation of the present glacial landscape is ensured for thousands of years to come.
Author N.W. RUTTER
Links to Other Sites
A definition of the term "cryosphere" from the "Climate and Cryosphere" website, a project of the World Climate Research Program. Browse other sections of this site for summaries of scientific research programs concerning cryosphere regions and their impact on global climate.
Life of a Rock Star
This site tells the story of an extraordinary group of scientists who tramped, paddled and rolled across Canada in the nineteenth century to study the geology of Canada's varied terrain.
A Journey to a New Land
A multimedia website about the arrival of early humans in the Americas. Features activities that help students learn about archaeological research techiques and discoveries. From the SFU Museum of Archaeology and Ethnology and the Virtual Museum of Canada.
Glossary: Glacial Features
A glossary of terms that relate to glacial features. Check the rest of the site for additional information. From the Department of Earth Sciences, Simon Fraser University.
Take a virtual tour of Qu'Appelle Valley to explore the fascinating wildlife and geological history of this picturesque region in southern Saskatchewan. Also provides clear explanations of basic ecological and geological concepts related to the natural features of this area. From the University of Regina.
Sea Ice Cycle
A very readable description of environmental factors that impact on the formation, structure, and disintegration of sea ice. With many illustrations. From Environment Canada's "Educational Resources."
Archaeological Potential Study – Gatineau/Ottawa Area
A well-illustrated report about the archaeological and geological history of the Ottawa-Gatineau region. From the website for the Interprovincial Crossings Environmental Assessment Study.
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