Crevassing

Crevasses are like rattlesnakes--not a problem if you know where they are, but if you do not see them, they can catch you by surprise. The danger of a crevasse is that it may be covered by a bridge that conceals a yawning space below. A crevasse opens in tiny increments with each additional fracture separating the ice by a millimeter or so. As the crack opens, blowing snow sifts down into it, sealing up the gap, and building a bridge that widens at the same pace as the opening of the crevasse. The bridge is typically thinnest at the edges and droops in the middle. To detect subtle crevasses, you need to look for faint linear offsets in the snow, and, if you find one, probe it with an ice axe or a pole to see how thin and wide it is. Then you must decide whether to cross or go around.

Probing a bridged crevasse is the only way to tell whether it is safe to cross.

I first descended into a crevasse in 1970 about a mile out from our helicopter camp on McGregor Glacier. On an overcast day, I was belayed on a rope from above and climbed down a “crevasse ladder,” (a flexible, wired ladder for crevasse rescue) into a world of deep, soft, and subtle grey. The crevasse was narrow and not more than six feet wide at the top. The walls reached twenty feet below to an irregular surface of blocks that had dropped years before from the underside of the bridge that we had chopped open for our fun. The paired walls undulated gracefully in symmetrical curves that transcended simple math, then played off to the right into a slightly larger room that pinched to nothing at its bottom. I wedged my foot across the bottom of the crevasse and looked back up. I was surrounded by a sculpture illuminated from without. The walls were translucent gray, strewn through with layers of fine, white bubbles, configured in blocks that had been broken and then fused into a brittle/ductile, mish-mash of rehealed ice. I felt as if I were underwater—it was rapture.

Crevasse interior 1

I was hooked. Unless there was a particularly good 16-mm movie showing at the camp that night, several of us would hike over to the crevasse field to fool around in our newfound world within the glacier. When the sun was shining, the grayness that I had experienced in my first crevasse was transcended by pervasive blue, pale and bright near thin spots in an overhanging bridge, dark and rich, deep down in. The blue color is due to absorption of light in the red portion of the visible spectrum by molecules of water. It is the same in water and in ice. Crevassing is indeed an underwater experience. The deeper down one goes, the purer becomes the blue.

Crevasse interior 2

For the second half of the field season, we moved camp to the west side of Nilsen Plateau. Soon enough we found a promising crevasse about a mile from camp. It was a big one, marked by subtle sags and cracks in the snow surface, with its opposing sides separated by more than 100 feet. How long the crevasse was, we couldn’t tell. I poked and chopped at the bridge along one of the sides and finally got an opening big enough to fit into. The apparent bridge of the crevasse, rather than spanning a 100-foot opening, was a solid plug of ice as far down as I could see. It sat about three feet away from the glacier wall, producing a narrow crevasse that was littered with blocks broken from the underside of its overhanging bridge.

Crevasse interior 3

Once I was down in the crevasse, I wormed my way laterally over a series of blocks, drawn toward a dark blue spot deep in the crevasse. After maybe fifty feet of this crawl, I came to the threshold of a gigantic room that opened abruptly. As I stood up on a big, wobbly block of ice, I gasped. At first glance I couldn’t see any walls, only a deep, empty space of diffuse blue. What was this place? As my eyes adjusted to the low light, I realized I was at the edge of a room at the termination of this broad crevasse. The walls were smooth and perfectly vertical, and must have been more than 100 feet high where they pinched together on the far side of the room. At that point the bridge was at its thinnest, glowing white high above. From there down to where I was standing, the bridge sagged in a graceful curve, its underside pocked by the loss of blocks that littered the floor of the crevasse. At that point the blocks met the bridge and merged into the plug of ice in the broad part of the crevasse that I had been crawling along to get here.

Diagram of crevasse interior described in the post.

I signaled to let me have more rope and climbed forward and down over a half dozen of the big blocks till they dropped off steeply and I felt it would be unsafe to go further. Standing close to the middle of the room, I could feel the immensity of the space, a nearly perfect tetrahedron, crafted by Nature, hidden away. It was like being in the hull of a giant ship, looking toward the bow. I lingered longer than I thought was fair, then crawled back to the surface so the others could descend into the blue. Although I have been in many crevasses since that time, each with its own fascination, there has never been another with such grandeur. (Adapted from The Roof at the Bottom of the World)

Gallery – Ice-cored Moraines

Moraines are accumulations of glacial debris (till) that collect at the margins of glaciers. Typically, rocks that are eroding from outcrops will collect at the bottom of the slope, along the margin of a glacier, and be carried out of the area as the glacier flows onward. However, if accumulation of snow is too slow, ice will not flush out of a valley, or the side of a glacier will flow into a reentrant and ablate. In these cases rock collects on the ice, accumulating in ridges or other patterns indicative of the flow directions. Such features are called ice-cored moraines, the subject of this week’s gallery.

A sombre day in the La Gorce Mountains.

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