Geography Reference
In-Depth Information
47
as
crevasses
. Crevasses open where a glacier accelerates and stretches, and are
oriented perpendicular to the direction of maximum extension (Fig. S.2). Where
glaciers make sharp turns, flow over ridges, or drop from hanging valleys, the
geometry of the crevasses may become chaotic, with huge seracs (ice blocks)
tumbling down icefalls, stationary in human time.
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 brittle 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 (Fig. S.3). Then you must decide whether to cross or go around.
I first descended into a crevasse in 1970 about a mile out from our heli-
copter 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 gray. 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
Figure S.2. Crevasses riddle
the surface of Scott Glacier,
one of the major outlet
glaciers that cross the
Transantarctic Mountains
(see Chapter 6).
