Permafrost, the permanently frozen ground that underlies large areas of northern Canada, is generally out of sight beneath a surface layer of soil. But permafrost leaves its mark on the landscape. You just have to know what to look for.

There are four principal landforms that appear only in permafrost areas, says Chris Burn, a geographer at Carleton University in Ottawa. Burn has studied permafrost in several parts of the Yukon.

The best-known of the four is the pingo, a cone-shaped hill with a core of ice, he says. Pingos form when underground water under pressure comes in contact with permafrost. The water freezes, expands, and pushes the ground surface upward, forming a hill. As more water freezes, the hill grows.

"Pingos are the landform feature most like people," Burn says. "They have a birth date, and they grow quickly while they're young. Then their growth slows, they deteriorate, and finally they die."

Some pingos form in the sediments of drained lakes.

"There are several pingos of this kind in northern Yukon, near the Dempster Highway, and a famous one north of Aishihik Lake," says Burn.

Others are formed at the base of slopes where groundwater accumulates. They are particularly common in the Dawson area, he says, and more than 400 such pingos have been identified in the west Yukon.

A less dramatic but more widespread permafrost phenomenon is the ice wedge polygon, a geometric shape in the ground bounded by straight ice-filled cracks. The polygons often have five or six sides, each side as long as 10 metres.

They are created by the contraction and expansion of permafrost through the changing seasons. When permafrost shrinks in the cold, the ground above it sometimes cracks. Water pours into the cracks, then freezes and becomes a wedge of ice, pushing up a ridge on either side of the crack and forming geometric shapes where the cracks meet.

"In Yukon north of treeline, it's often possible to see polygons on the ground surface," says Burn. "South of treeline the wedges aren't as visible, but you can see them in riverbanks, especially after the spring flood."

Thermokarst lakes are more easily visible features, even if they aren't as obviously the result of permafrost action.

"Thermokarst lakes are water bodies that occupy a depression in the ground surface formed by subsidence after melting of ground ice," explains Burn. They can result from human disturbance of the permafrost, but more often they are formed when natural events like forest fires warm the ground.

Once the lakes are deep enough that they don't freeze to the bottom in winter, the lake bottom temperature stays above freezing and the permafrost continues to melt, he says. The lakes are often surrounded by trees that tilt toward the water as the ground beneath them continues to subside.

"Thermokarst lakes are abundant in the Yukon," Burn says. "There are lots east of the Klondike Highway in the valley bottoms near Pelly Crossing and Mayo. They can be seen from the Silver Trail north of Halfway Lakes, and from the Alaska Highway near Beaver Creek."

The fourth and most active landform related to permafrost is the thaw slump. Like thermokarst lakes, thaw slumps result from disturbance of permafrost, but in this case the disturbance is generally caused by erosion.

The Yukon's North Slope is a prime example. Wave action erodes the shoreline and exposes the permafrost, which melts when the temperature rises above freezing. The ice-rich soil turns almost liquid, flows away as mud, and the soil above and around it slumps. When the waves wash away the mud and expose more permafrost, the slumping continues.

"The long-term retreat of the Yukon coast, at between two and ten metres a year, is due to this process," says Burn. In other areas, where erosion is less constant, the build-up of mud or surface material will often cover the permafrost and eventually stop the slumping process.

For more information about permafrost and its effect on the landscape, contact Chris Burn at the Department of Geography, Carleton University, Ottawa, Ontario, K1S 5B6.