Black City: Permafrost
Pingo in Seela Pass, Blackstone Uplands. Note the patterned ground in the foreground.
Frost Mounds/Palsas – peaty, ice-cored mounds a few metres high. These are typically found in areas of discontinuous permafrost, and form and melt each year.
C. Roots, Geological Survey of Canada
Even in summer, when the tundra is green and flowering, ice is never far from the surface. Permafrost is permanently frozen soil that remains below 0°C throughout the year. This frozen ground is discontinuous in the Blackstone Uplands but it has some dramatic permafrost features.
Permafrost has a profound influence on the local topography. The heaving and thawing action of the ice lenses will create tussocks and hummocks. Vegetation affects the presence and thickness of permafrost with its insulating properties. The cold of permafrost in turn limits what can grow and how deep roots may penetrate the earth. Once the insulating surface layers are disturbed, by natural or human forces, the ice-rich soil thaws. This can lead the ground to weaken, settle and erode. If there is a great enough disturbance, the melting can form “thermokarst lakes”.
Some permafrost features that can be seen in the Black City area include:
Pingoes – a pingo is a conical hill or mound with a core of ice. Pingos are formed through the pressure of the water freezing in a contained space, causing the ground to expand upwards. Pingoes can be very large (up to 65 metres high) and persist for many years. The name ‘pingo’ is an Inuit word.
Patterned Ground / Polygons – this is where the topography appears sorted into regular patterns of varying size and often geometrical shape, usually observed as differing elevation, soil composition and vegetation.
Frost Mounds / Palsas – peaty, ice-cored mounds a few metres high. These are usually found in areas of discontinuous permafrost and form and melt each year.