Crevasses, caused by strains from variations in
the rate of motion of different parts of the glacier and convexities in
the channel, are mere cracks when they first open, so narrow as hardly
to admit the blade of a pocket-knife, and gradually widen according to
the extent of the strain and the depth of the glacier. Now some of these
cracks are interrupted, like the cracks in wood, and in opening, the
strip of ice between overlapping ends is dragged out, and may maintain a
continuous connection between the sides, just as the two sides of a
slivered crack in wood that is being split are connected. Some crevasses
remain open for months or even years, and by the melting of their sides
continue to increase in width long after the opening strain has ceased;
while the sliver-bridges, level on top at first and perfectly safe, are
at length melted to thin, vertical, knife-edged blades, the upper
portion being most exposed to the weather; and since the exposure is
greatest in the middle, they at length curve downward like the cables of
suspension bridges.
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