Tricks For Crevassed Terrain

Article by Barry Roberts
Monday 24th May 2010

 A very heavily crevassed area of the Geant Glacier, Mont Blanc, France: moving safely in this kind of terrain require in-depth knowledge about glacier and crevasse structure. Photo: copyright David Pickford / www.davidpickford.com

1. Knowing Where To Look

The first and most obvious trick is to avoid walking into a crevasse in the first place. In summer when the snowbridges have melted away and the crevasses are open, this is a pretty easy task. In winter, when we will be doing most of our off-piste travel, snow covers all but the widest crevasses making the task of spotting a slot very much harder.

Trick number one is working out where on a glacier a crevasse is likely to be. Crevasse location is never completely predictable, but the simple trick of remembering that crevasses form when the ice comes under tension can show us where most crevasses are likely to be and in which way they will point.

Four different places are likely to lead to crevasse formation:

  • Over a drop: when the slope angle steepens, the glacier ice tends to crack as the surface ice has further to travel. Slots appear perpendicular to the direction of the glacier’s flow. 
  • Around a bend: radial cracks appear as the ice has to bend around a corner. These are often then pulled into “herringbone” diagonal slots as the faster-moving ice in the centre of the flow pulls away from the slower ice scraping against the glacier walls.
  • As the valley base widens: as the glacier opens up, as it often can towards the bottom of a slope, the ice has to spread out and vertical cracks appear running parallel to the direction of the glacier’s flow.
  • Over a steep drop: when the angle gets too steep, the glacier behaves just like a waterfall on a river and breaks up into a random chaotic jumble. Unlike avalanches where condition give some an indication of the potential danger, huge blocks of ice can tumble without warning down an icefall driven simply by the slow underlying movement of the ice below. This means that icefalls can be incredibly dangerous to cross as the ice is in constant but completely unpredictable motion. The famous Khmba Icefall on the route up from basecamp at Mount Everest, for example, claimed 19 lives between 1963 and 1995 - one for every 25 people who crossed successfully and claimbed to the top.

2. Spotting the slots

On a “wet” glacier in winter, the main risk is unknowingly stepping onto a weak snowbridge and falling through. With fresh snowfall it can be very hard to spot the hidden crevasses (see below) but there are a number of signs which can help to reveal their location. These are:

  • Terrain: crevasses are caused by tension in the surface of the ice, and can often be caused by quite small changes in the local terrain. Watch out in particular for small areas where the slope steepens. These can often hid a longitudinal slot, or a mini icefall.
  • Dips in the ground: snowbridges tend to sag over time so that crevasses will often be revealed by a gentle dip in the terrain, especially after a long warm period. Avoid these dips!
  • Changes in surface colour: after a snow free period, windblown dust tends to collect in the small dips over a crevasse. This makes the surface appear darker than the surrounding safe snow. Conversely, after fresh snowfall (especially with wind), more snow will tend to collect in the dips making them appear lighter than the surrounding safe area. Avoid therefore any patches of dark or light snow which could be hiding a crevasse.

3. Route finding

A skier negotiates a safe route in heavily crevassed terrain, staying clear of crevassed areas. Photo: copyright David Pickford / www.davidpickford.com

The critical point about route finding on a glacier is taking a route which keeps you as far away as possible from the likely crevasse areas. The key to this is preparing a careful plan before you set foot on the glacier, and then following the plan once you are on the glacier.

The four key steps are:

  • Study the map: most good maps show likely crevasse areas, and all good maps show contour lines that can help you to plot a route avoiding potentially dangerous steep areas. Ski touring maps often have suggested routes marked on them. The key point with all this is to study the map carefully the night before setting off and decide the route you are going to follow.
  • Think likely danger areas: in plotting your route, and in travelling on the glacier once you’ve started, remember to look out for all the likely stress points in the glacier which could hide a crevasse. Use all the information about slot type and slot indications to plan an overall route, and a local micro route that keeps you as much away from trouble as possible.
  • Follow your plan: once you’ve started on the glacier, stick to the plan you designed unless there’s an obvious reason to change (in which case prepare a new plan before you set off again). Be aware of how easy it can be to drift off course on featureless ground (and in particular in a whiteout). Use all the navigation techniques described in the section on Staying Found to keep you on track.
  • Check your route when high: take advantage of the times when your are high above the glacier and can see clearly to check, and if necessary re-evaluate, your route. It is much easier to do this when you can see the slots from above than when looking across at them from ground level!

 

4. Roping up

The advantage of roping up is that a crevasse fall is potentially less serious as you should be able to stop the victim before they fall too far in. The disadvantage is that it can make travel very inconvenient, and there is a risk of a single fall pulling more than one victim into the crevasse. The circumstances when you should rope up are as follows:

  • Winter walking: always rope up. Winter walking is the most dangerous travel of all since the pressure from a boot is much higher than from a ski which spreads the load. Roped travel when walking is relatively straightforward and additional safety easily outweighs the minor inconvenience.
  • Winter skiing: only consider roping up in conditions of very poor visibility when you consider the risk of straying unknowingly into a slot to be unacceptably high. A skier is safer than a walker as the ski spreads the load, and the difficulty of skiing roped up is so extreme that most parties vow “never again” after their first attempt.

If you do ever have to rope up, the technique is as follows (we assume a group of 3 people on a 30m glacier rope):

  1. The second strongest person should be at the front and ties in to the end of the rope using a figure of eight.
  2. The weakest person ties an overhand knot in the rope ten metres back from the front person and clips this into their harness.
  3. The strongest person ties an overhand a further 10 metres back, clips this in, and coils the spare rope over their shoulder.
  4. If walking you now proceed as a group, 10 metres apart, making sure that there is only a small amount of slack between each person. This minimises the distance anyone will fall whilst still allowing a natural travelling rhythm.
  5. If skiing, good luck! The only way practical way to proceed is in a very cautious and slow snowplough, taking particular care not to speed up or slow down around corners as this tends to catapult the following skiers at alarming speeds.

You definitely want to try this out with your group before deciding to use it for real. Another option worth considering is just to rope up the first two skiers with the lead skier (or "canary") supported by an experience second. The second needs to be capable of holding a fall on skis!

5. Crevasse Rescue

Crevasse rescue is not a difficult skill to learn, but it is quite technical and it can be all too easy to get it wrong. As with transceiver rescue, the key is practice. It is also unfortunately an area where everyone seems to do it a little bit differently. It is much better to learn one method really well, rather than to try to learn multiple difficult techniques and end up confused about what goes where while your friend is stuck at the bottom of a slot.

If the worst does happen and one of your party falls down a crevasse, your immediate priority is to get them out again. Time is less critical than in avalanche rescue, since there is rarely a risk of suffocation, but it is still important to work efficiently and quickly to minimise the dangers from shock or hypothermia.


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