Keywords Landforms

In areas that have experienced longer histories of nonglacial conditions, eg, northern interior Yukon, northwestern BANKS ISLAND and other high-arctic islands, landscapes are more likely to be in equilibrium with current periglacial conditions. Processes unique to periglacial environments include formation of permafrost, development of thermal contraction cracks, thawing of permafrost (formation of thermokarst), and formation of wedge and injection ice. Other processes, not necessarily restricted to periglacial regions, are important because of their high magnitude or frequency in cold, nonglacial environments. These include ice segregation, seasonal frost action and various forms of instability and rapid mass movement. Nearly all frost-action processes operate in conjunction with the freezing of water.

The most distinctive periglacial landforms are those associated with permafrost. The most widespread, tundra polygons formed by thermal-contraction cracking, divide the ground surface up into polygonal nets 20-30 m in dimension. Water often penetrates the cracks to form ice wedges several metres deep and up to 1-2 m wide near the surface. In drier environments, mineral soil infills the cracks and sand wedges result. Ice-cored hills (PINGOS) are a less widespread periglacial landform. Pingos form when water moves to the freezing plane under a pressure gradient that may be hydraulic or hydrostatic in nature. Pingos are not typical of all periglacial landscapes but result from specific geomorphic and hydrologic conditions that severely limit their occurrence. Other aggradational landforms such as palsas and peat plateaus are usually associated with ice segregation rather than injection. Ground-ice slumps, thaw lakes and irregular depressions (thermokarst) resulting from the melt and erosion of ice-rich permafrost constitute a further group of related periglacial landforms.

Many periglacial phenomena result from frost wedging and the cryogenic weathering of exposed bedrock. Frost wedging is associated with the freezing and expansion of water which penetrates joints and bedding planes. The details of cryogenic weathering are still poorly understood. Coarse, angular rock debris (block fields), normally attributed to frost wedging or cryogenic weathering, occur widely over large areas above the treeline and in the arctic islands. In addition, frost-heaved bedrock blocks and extensive talus (scree) slopes are common. Angular, frost-shattered rock protuberances (tors) may stand out above the debris-covered surfaces, reflecting more resistant bedrock. These are most frequent in SEDIMENTARY ROCKS, especially in the arctic islands (eg, SOMERSET ISLAND); however, in the unglaciated interior of the Yukon (Klondike Plateau) tors are formed on very old and extremely resistant metamorphic rocks. Flat, erosional surfaces (cryoplanation terraces) are sometimes associated with tors but can also occur independently.

Agents of transport include frost creep, the ratchetlike movement that occurs when soil, during a freeze-thaw cycle, expands normally to the surface and settles in a more nearly vertical direction; and solifluction, the slow downslope movement of water-saturated debris. Where solifluction occurs on or above frozen ground, it is termed gelifluction. Solifluction lobes, sheets and terraces are especially well developed above the treeline and below sites of perennial snowbanks.

The small-scale relief of periglacial regions is characterized by various patterned ground phenomena. These are often related to cryoturbation, the lateral and vertical displacement of soil that accompanies seasonal or diurnal freezing and thawing. The most widespread are nonsorted circles or nets, typically 1-2 m in diameter and up to 0.5 m high. In the Mackenzie Valley, they occur over extensive areas, wherever fine-textured and poorly drained sediments exist. In Keewatin, morphologically similar forms caused by density differences in saturated sediments are termed "mud boils." A wide range of nonsorted and sorted forms of patterned ground are described from other parts of northern Canada, and no single explanation is applicable to all.

See also PHYSIOGRAPHIC REGIONS.

Permafrost Polygon
NWT. The polygonal pattern is the result of winter contraction cracks that have become infilled with ice from percolating summer meltwater (photo by Stephen Krasemann).

Author HUGH M. FRENCH

Suggested Reading
Hugh M. French, The Periglacial Environment (1976).

Links to Other Sites
Yukon Territory
An extensive visitors guide to all there is to see and do in the scenic and historic Yukon. From the Department of Tourism & Culture, Government of Yukon.

Soil Landscapes of Canada
This site features photographs of typical soil landscapes found in various regions of Canada. From Agriculture and Agri-Food Canada.

A Journey to a New Land
The purpose of this website is to examine and explore the issues around the first arrival of humans in the Americas. The activities are designed to present students with opportunities to learn more about archaeological methods and the human past. From the SFU Museum of Archaeology and Ethnology and the Virtual Museum of Canada.