Alpine permafrost and the Body Bag Bowl rock glacier
Permafrost, that ground condition up north which raises havoc with road and pipeline construction, causes undue headaches in mining the placer gold in the Klondike, and has slowly been melting under Dawson to bring crazy tilts to its historic buildings.
Permafrost is defined as ground that remains permanently frozen for at least two years. It may be dry ground or rock, and thus there is no ice in any form, but the ground temperature is below 0 °C. Or it may be moist soil with segregated ice crystals in the pore spaces or actual ice lenses or cracks filled with ice. In the latter case, the uppermost layer thaws during summer, which is the so-called "active-layer" lying over the permafrost table.
Permafrost coupled with the freeze-thaw action of the active layer creates unusual landforms, both minute or large, collectively known as "periglacial" features. That is, the climate is of glacier-producing temperatures but due to the dryness of the atmosphere, a glacier cannot be generated.
So what does this have to do with Body Bag Bowl, backed by glaciers on the other side of Disease Ridge?
Permafrost is not confined to continental northern latitudes where the average annual temperature is below 0 ° C. Alpine permafrost, altitude controlled by the elevation of the annual 0 ° C isotherm, wends its way through all mountain chains southward to the point where the annual 0° C marker is higher than the mountains. Southward then, the permafrost becomes a string of disconnected islands reducing in areal extent. Of course the freezing isotherm is controlled by exposure to the sun, hence its lower on north facing slopes, and by type of ground insulation (thick moss cover is important) and other disturbance factors.
Around Whistler the lower limit of permafrost is at about 1,800 metres – higher on south slopes and lower on the north. Walking through Symphony Bowl to the Musical Bumps reveals small scale ground features which owe their presence to permafrost and the action of the overlying seasonal active layer.
However, turn around and look north to Blackcomb and adjacent Disease Ridge. There is a big bulge in Body Bag Bowl between the two peaks – a mega feature produced by permafrost called a rock glacier, a mass of slowly moving rock debris which resembles a glacier. Such features are found everywhere in the east or leeward side of the Coast Mountains and can be enormous tongues of debris extending to valley bottoms farther north – the one at Atlin fills up a postcard photo for example.
How do rock glaciers form and why? The basic ingredient is permafrost, which may be several metres below ground surface initially. The usual mechanism calls for blocky rock fall from cliffs (in this case its Disease Ridge) which roll downslope over an apron of winter residual snow stopping at its base. When the accumulating pile reaches the appropriate thickness to act as an insulator the permafrost table moves up in the underlying soil (or rock) and into the oldest boulders at the bottom of the accumulating pile. Then seasonal snow melt, summer rain or moist air percolates down to the cold debris forming a skin-thin film of ice around each individual grain and block of rubble. The debris is lying on a slope, the deep debris is now lubricated with ice to reduce friction with each frozen particle now acting as a pseudo ball bearing, primed for movement as more rock debris falling off Disease Ridge is dumped onto the mass to provide the driving force. And so the entire mass moves down slope, though slowly at less than one tenth the speed of a conventional glacier, or about 10 to 20 centimetres per year for the most active ones.
If the annual air temperature rises the rock glacier stops moving; the interstitial ice has disappeared, the level of permafrost has retreated deeper into the mountain or disappeared altogether. The sharp frontal rubbly profile of its snout becomes a stabilized convex slope and darkens with weathering and lichen and other growth. As seen by the photo the Body Bag Bowl is very active; the snout is of light-coloured, fresh, unstable rubble lying at the angle of repose (30 ° ) and the top of the glacier of dark lichen-covered rubble near the snout becomes progressively lighter with fresher debris in the active area of rock debris supply. The glacier currently measures out to a length of 400 metres, a width of 120-150 metres and a thickness of 20-30 metres.
So the "Body Bag" in aptly-named Body Bag Bowl is a rock glacier, and come to think of it, the feature is large enough to be a great place to be buried if posterity of your cadaver is on one’s mind! Permafrost is a proven preserver of corpses – just ask the ghosts of Sir John Franklin.
Written by: Karl Ricker