The Dry
Valleys
The Dry Valleys of Antarctica get virtually no snowfall or moisture
of any kind. Researchers come not only to study one of the world's
most delicate, yet simple ecosystems, but also to learn more about
the unique geological formations and processes occurring there.
From south to north, the principal ice-free valleys include the
Taylor, Wright, McKelvey, Balham, Victoria, and Barwick Valleys.
Similar but smaller valleys also occur farther south, along the
coast of McMurdo Sound and the western margin of Koettlitz Glacier;
these include, from south to north, Miers, Marshall, and Garwood
Valleys, and the Salmon Stream valley.
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Glacier-Scoured
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Antarctic Dry Valley glaciers are clean
and white, with a minimum of rock debris, few crevasses, and
near-vertical terminus walls.
This is due largely to the relatively
dry-based nature of these Antarctic glaciers and the fact
that they do not move by slipping and sliding on the underlying
bedrock, but more by shearing and plastic flow within the
ice itself.
Also, at such low temperatures, the ice
is not able to incorporate rock debris by the pressure-induced
melting common to glaciers in temperate climates.
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The major Dry Valleys have
certain characteristics in common, and some have unique features.
They are generally 5-10 kilometers wide (between ridge crests) and
15-50 kilometers long. Only the Taylor and upper Wright Valleys have
glaciers at their heads, which connect with the ice of the polar plateau;
the other valleys have either barren upper reaches or small alpine
glaciers. Only Taylor Valley exits directly to the sea ice of McMurdo
Sound, whereas the others are blocked by the Wilson Glacier.
Several lakes occupy
parts of some valley floors, their surfaces frozen most of the year.
Some lakes are over 30 meters deep and have perennial ice covers
several meters thick.
As in most regions of perennially frozen
ground, permafrost patterns often form in the loose sand and
gravel of the Dry Valleys.
Ice polygons have diameters of between
10 and 30 meters, and associated frost wedges grow in thickness
during each annual cycle.
Initially, narrow cracks are formed in
the ground, with slumping from the sides.
As the wedge width increases, the bordering
ground commonly forms a trap for wind-driven sediment.
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Lake Vanda, which is
typical, has 10 percent dissolved solids content in its lower few
meters--three times as saline as sea water-while the upper 50 meters
has only 0.1 percent. Scientists
have noted high water temperatures in the lakes, with temperature
inversions resulting in bottom waters as warm as 25°C (75°F). These
high temperatures are due entirely to solar heating of the water
through the ice, and not to any heat from rocks at depth beneath
the lakes.
The lakes are by far
the most interesting and diverse habitats in the Dry Valleys. Organisms
are found growing on and in the ice cover, in the water, and on
the bottom of the lakes. Exploration of lake bottoms by SCUBA-equipped
divers, including core sampling of bottom sediments, have disclosed
the existence of algal mats on lake floors; in certain respects
these are analogous to some of the  Earth's
earliest life forms. The mats produce gases which render them buoyant
in marginal zones of the lake. There they form columns, which detach
from the bottom, rise, and then work their way upward through the
surface ice layers-as much as 5 meters thick-after which they dry
out and blow away, sometimes to colonize in other locations.
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The Labyrinth
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Immediately below the terminus of the
Wright Upper Glacier is a much-dissected area of dolerite
bedrock, with numerous deeply cut gullies and coulees, known
as the Labyrinth.
This rugged topography extends down the
valley about 6 kilometers and is believed to be the product
of either (1) the large, steady flow of subglacial streams,
or (2) a sudden release of extremely large volumes of glacial
meltwater that had been trapped beneath the ice cap.
Scientists do not all agree on the precise
explanation for the apparently catastrophic erosion by water,
but the effects are nonetheless startling.
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