GLACIATED TERRAIN: WHAT YOU ARE ACTUALLY RIDING OVER

The Technique

1. A glacier is ice that moves. Snow lands, compacts into firn, and turns to ice under the weight of the snow above it. That mass then flows downhill under its own weight, slowly and continuously, all year. Deep inside the glacier the ice is under enough pressure to deform like thick treacle, bending rather than breaking. The upper layer cannot do that. It is brittle, and when the ice below it stretches, the top cracks. That crack is a crevasse. The brittle layer is often described as roughly the top thirty metres, which is sometimes offered as reassurance about depth. Treat it as no comfort at all. Thirty metres is far more than enough to kill you, and real crevasses hold wedged ice blocks, meltwater, and narrowing throats that stop a falling body long before the bottom, in positions from which nobody can be pulled easily.
2. Crevasses are tension cracks, so they form where the ice is being pulled apart. The ice gets pulled apart where the glacier speeds up, where it spreads sideways, where it bends, and where it passes over a change in the shape of the bed beneath it. This matters more than any other single idea in this lesson: crevasse patterns are not random. They are a readable consequence of the ground under the ice and the shape of the valley. Because the bed does not move, the same glacier tends to crack in the same places year after year. Individual crevasses open, shift down-glacier and close again, but the zones stay. That is what makes late season imagery so valuable, and it is why local knowledge on a particular glacier is worth more than general knowledge about glaciers.
3. Learn the standard concentrations by name and by cause. Convex rolls, where the ice flows over a bulge in the bed and the surface is stretched, produce transverse crevasses across the line of flow. Steepenings and icefalls are the same effect turned up until the surface breaks into a chaos of blocks and towers. Bends produce crevasses on the outside of the turn, where the ice travels further and faster. Valley margins produce marginal crevasses, angled up-glacier, caused by the drag of rock against moving ice. Where a glacier widens or spills out of its valley, it splays, and the crevasses splay with it. Rock outcrops, nunataks and buried bedrock knobs split the flow and shear it, so the ice around and downstream of any rock sticking through the surface is suspect. At the head of the glacier, the bergschrund marks where flowing ice pulls away from the ice and snow frozen to the headwall. At the bottom, the terminus and any ice cliff is broken ground.
4. Understand the difference between a dry glacier and a snow covered one, because they are two different problems. A dry glacier is bare ice, typically low on the glacier in late summer, and its crevasses are visible. You can see them, map them, and ride or walk around them. Visible is not the same as safe. Some crevasses on a dry glacier carry thin ice lids or rotten old snow that looks like ground. Some hold running or standing water. Wet ice is slick, and a slip on a dry glacier can slide you into a hole you could see perfectly well. A snow covered glacier is the accumulation zone, or the whole glacier in winter and spring. Every crevasse that exists is still there, and every one of them is hidden. That is the state you ride in, and it is the reason this module exists.
5. Snow bridges form in a few ways, usually together. Wind drives snow across an open slot and builds it out from the downwind lip like a small cornice, sometimes from both lips until they meet. Storms lay slabs straight across a gap that is already partly spanned. Then time and cold do the rest: the crystals sinter, bond to each other and to the layers above and below, and the bridge gains strength. Think of a bridge as an arch or a beam spanning a gap. It carries load by its own structure, not by resting on anything, and it fails when the load exceeds what that structure can carry across that span, or when warmth, water or a weak buried layer takes the structure away.
6. Bridges fail for reasons you can mostly anticipate. Warm air, strong sun and rain all destroy bonding. A buried weak layer, the same kind that concerns you in avalanche terrain, can sit inside an otherwise convincing bridge and let go under load. Repeated loading cracks a bridge internally before anything shows on the surface, which is why the third and fourth machine across a marginal bridge are in more danger than the first. And a bridge can simply be too thin for the width it spans. When a bridge does fail it usually fails suddenly and over a much larger area than the footprint of whatever loaded it, because once the arch breaks, the whole span comes down. Sagging is a genuine warning sign when it is there. Plenty of bridges collapse without ever showing it.
7. Season changes everything, in both directions. Early season is dangerous because the crevasses have not been filled. A snowpack that measures well on the valley floor tells you nothing about how much snow has gone into a slot several metres wide, and a thin, uniform white surface over unfilled crevasses is one of the most treacherous conditions there is. A poor start to winter can leave a glacier in this state deep into the season. Late season is dangerous for the opposite reason: bridges that were thick have been melting from above and from below all summer, and a surface that still reads as continuous can be sitting on very little. On top of both, a fresh fall of snow at any time of year covers open holes and sag marks, and makes an actively dangerous surface look freshly perfect. New snow on a glacier reduces your information, it does not improve your safety.
8. Within a season, temperature runs the day. Cold keeps bridges strong, and the strongest state a bridge reaches is after a long, clear, cold night with a good refreeze. A warm airmass, a rain event, a foehn wind or a long day of strong sun does the reverse, and the effect builds through the afternoon. This is not a small variation. The same crossing can be routine at dawn and unjustifiable by mid-afternoon, on the same day, with nothing else changed. Build your plans around that fact rather than around your own schedule.
9. Finally, learn to recognise glaciated terrain before you are standing on it. On topographic maps, look for ice and snow shading, contours printed in blue, named glaciers, névé and icefields, moraine and ice cliff symbols, and lakes at the snout. On imagery, find pictures taken in late summer when the glacier is dry: the crevasse fields show up as lines and scars, and that pattern is roughly where the crevasses will be under the winter snow. In person, look for lateral moraine ridges and the clean trimline above them, scoured bedrock, an ice snout, patches of blue ice, meltwater, an absence of rock and vegetation on the surface, and the moment the ground under the snow stops being ground. If you are not certain whether you are on a glacier, you are not qualified to be making that call alone, and the correct response is to stop and get someone who is.

Common Mistakes

Reading a smooth, unbroken white surface as evidence of safety. Fix: judge the surface by what you know is underneath it from map, imagery and local knowledge, and treat surface appearance as almost no information at all.

• Assuming a big snow year means the crevasses are safely filled. Fix: check where the snow actually went. Deep snow on the valley floor and a well filled crevasse field are different things, and only local, current knowledge tells you which you have.
• Treating a dry glacier as the safe option because the holes are visible. Fix: respect thin ice lids, water filled slots and slippery wet ice, and keep the same discipline about where you put people and machines.
• Believing new snow has improved conditions. Fix: recognise that fresh snow hides open holes, erases sag lines and adds an unbonded load on top of old bridges. It makes the ground less readable, not safer.
• Riding a glacier in the afternoon because that is when you got organised. Fix: plan around the refreeze, be on the glaciated section early, and accept a shorter day rather than a warm one.

The Drill

Pick a real glacier you might ride. Find late summer satellite imagery of it and a topographic map of the same ground. On the map, mark the direction of flow, then mark every zone where you would expect crevasses from theory alone: convex rolls, the icefall, the outside of each bend, both margins, any splaying section, the ground around every rock outcrop or nunatak, and the bergschrund. Then overlay the late summer imagery and mark the crevasses that are actually visible in it. The standard is that your theory map and the imagery agree on the major zones before you look at the imagery, and that you can explain every zone the imagery shows that you missed. Do this for three different glaciers. Then send it to a qualified mountain guide who knows that ground and ask what you got wrong.

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