MCMURDO DRY VALLEYS REGION, TRANSANTARCTIC MOUNTAINS
by National Science Foundation
Rising high above the western shore of McMurdo Sound and the Ross Sea,
90 kilometers from Ross Island, the glacier-mantled peaks of the Transantarctic
Mountains provide a beautiful background to the McMurdo area. Continually
changing patterns of light and shadow through a 24-hour austral summer
day bring out both the range's gently rounded glacial contours and the
abrupt rock faces. The range is characterized by nearly horizontal layers
of sedimentary rocks--mostly a yellowish sandstone (Beacon Sandstone of
Devonian age) and interlayer of dark dolerite (a lava similar to basalt)
that were injected as sills between the existing sandstone layers.
Several large valley glaciers flow from the polar plateau through gaps
in the range, some joining the Ross Ice Shelf and some flowing directly
into McMurdo Sound. Of greater interest, however, are several ice-free
valleys that contrast with this land of seemingly perpetual and complete
ice coverage. Known collectively as the McMurdo Dry Valleys, they are
within helicopter range of McMurdo Station and have become the object
of much scientific research.
From south to north, these include the Taylor, Wright, McKelvey, Balham,
Victoria, and Barwick Valleys. The valleys have certain common characteristics,
although some have unique features. Generally, they are 5 to 10 kilometers
wide between the crests of intervening ridges and are 15 to 80 kilometers
long. The valleys are ice- free because of the geography of their upper,
or western ends, where high lips slow the entry of ice from the east antarctic
Mean annual sea-level air temperature around McMurdo Sound is -20oC.
In the dry valleys, October to February mean temperatures are -23.7o to
0.7oC. In December and January, temperatures fluctuate around freezing.
There are large variations between years and between localities. In general,
the summer temperatures for the inland ice-free areas exceed those along
the coast; in winter, the situation is reversed.
Annual snow accumulation on Ross Island averages 17.6 centimeters (water
equivalent). Accumulation over the ice plateau, west of the dry valleys,
is considerably less, 3 to 10 grams per square centimeter. Within the
dry valleys, most of the snowfall is associated with humid easterlies
blowing off the Ross Sea.
Winds in the dry valleys range widely in direction and velocity and
are controlled by the local topography, the proximity of the ice plateau
and the ocean, and the season of the year. Onshore winds from the east
dominate during summer with mean speeds of 10 to 15 kilometers per hour.
During the winter, westerly katabatic (or gravity- driven) winds, originating
on the ice plateau, sweep through the valleys.
The winter winds affect the orientations of ventifacts (stones shaped
by the wind) and cause pebble ridges to form. In all of the dry valleys,
ventifacts commonly are positioned with their cut and polished facets
facing west. Pebble ridges with lee slopes facing east are particularly
prominent in the upper Victoria and Barwick Valleys. Pebbles along the
crests are as large as 6 centimeters in diameter. Velocities necessary
to transport such large particles have been calculated to be 200 kilometers
In regions of perennially frozen ground (or permafrost), vertical ice
wedges commonly form. Viewed from above, the wedges form circular, polygonal,
rectangular, stepped, and striped patterns. Further classification is
possible between sorted patterns (where coarse and fine materials are
concentrated spatially) and unsorted patterns. In the dry valleys, the
valley floors generally comprise loose sand and gravel deposits with some
scattered sand dunes. The sand and gravel surfaces often form polygon
patterns, which are most visible after a light dusting of new snow settles
in the cracks and outlines the polygons.
Unsorted polygons are the common pattern throughout the dry valleys.
Polygons have diameters from 10 to 30 meters, and associated wedges grow
in thickness with each annual cycle. Initially, narrow cracks are formed,
with slumping from the sides. As the wedge width increases, the bordering
ground is forced upwards to form a double- rimmed trough, which commonly
forms a trap for wind- driven sediment. With increasing age, the upthrusted
rims reach heights of 1.5 meters above the wedge-under- lain troughs and
the polygon centers.
The presence of a water-rich, active zone with thicknesses on the order
of tens of centimeters promotes the formation of pattern ground. Flood
plains and ice- cored moraines are particularly favorable environments.
However, patterned ground persists through various terrains, including
upland benches, sloping valley walls, and bedrock surfaces. The only unfavorable
regions are those in which a prohibitively thick upper dry zone, 1 to
2 meters or more, exists--for example, the Insel drift of the McKelvey
Valley. There are no meltwater streams, and snow that falls is removed
The dry valleys have two main types of patterned ground--rectangular
networks of narrow ice wedges that develop on alluvial plains associated
with present drainage and polygonal patterns that developed on old morainal
surfaces, alluvial-mantled hillsides, and bedrock surfaces. Development
of thick ice wedges leads to growth of polygonal trenches with bordering
ramparts of upthrusted sediment.
Most polygons are of the unsorted variety, but cobbles and boulders
are concentrated in some trenches over ice wedges. Other trenches are
filled with sand carried there by the wind, as are the interiors of rampart-bounded
Several large lakes occupy parts of some valley floors, their surfaces
frozen much of the year. Some lakes are over 30 meters deep and have perennial
ice covers several feet thick. Lake Vanda, which is typical, has 10 percent
dissolved solids in its lowest few meters- -three times as saline as sea
water--while the upper 50 meters have only 0.1 percent. Scientists have
noted high water temperatures in the lakes, with temperature inversions
resulting in bottom waters as warm as 25oC. These high temperatures are
due entirely to solar heating of the water through the ice and not to
any heat from rocks beneath the lakes. Explorations of lake bottoms by
scuba-equipped limnologists, including core sampling of bottom sediments,
have disclosed the existence of algal life forms that are similar to those
representing the earliest forms of life found on earth.
The ridges flanking the valleys support many small glaciers, most originating
in shallow cirques along the ridge crests. Most of these glaciers flow
only partway down the valley sides, but several, like the Commonwealth
and Canada Glaciers in the lower Taylor Valley, extend onto the valley
floors. In more temperate latitudes, typical mountain glaciers contain
considerable rock material that forms large moraines along their sides
and termini. Unlike these, antarctic valley glaciers are clean and white
with a minimum of rock debris and few crevasses because of the slower
movement of the ice in extreme cold. The glaciers also are old than those
in temperate regions--the Meserve Glacier in Wright Valley has existed
at least 3.4 million years.
The dry valleys have undergone several glaciations. Three major sources
of ice have affected the McMurdo Sound. The Ross Ice Shelf has expanded
several times to form outlet glaciers extending westward into the dry
valleys. The east antarctic plateau ice sheet invaded the valleys from
the west, and alpine glaciers have flowed and continue to flow laterally
into the valleys.
The dry valleys display classical features of glacial erosion. Outlet
glaciers from the plateau ice sheet spill over a protective basement rim
of rock and descended through the valleys to the Ross Sea, sculpting the
valleys as the ice moves. Potassium/argon dates for lava flows indicate
that the valleys have existed in their present form for more than 4 million
A broad, glacially-carved pass through the Olympus Mountains, Bull Pass
connects McKelvey Valley to Wright Lower Valley. Besides the spectacular
alpine views that the pass affords, Bull Pass is best known for the fantastically
shaped granite boulders, some weighing several tons, that decorate the
The magnificent rocks were carved by chemical and mechanical weathering
to produce cavities known as tafoni. Moisture breaks down the rock, gradually
opening pits and cracks. These are then enlarged by windblown sand and
ice, eventually eroding huge boulders into fragile shells before they
collapse completely into sand.
Throughout Bull Pass, the ground is littered with other beautifully
shaped rock fragments called ventifacts, meaning wind-faceted stones.
Found in other arid regions of the earth, these smooth rocks are gradually
worn down and polished by the sand and ice crystals carried by winds off
the polar plateau.
For scientists ventifacts are helpful indicators of glacial history.
The development of facets on the stones can give a good idea of how long
the debris has been exposed to wind, and, therefore, uncovered by glaciers.
Glacial moraines, marking past glacial advances, have often been dated
One of the most interesting dry valleys is Wright Valley. At its head
is the Wright Upper Glacier, fed by ice from the polar plateau via the
Airdevronsix Icefalls. Immediately below the terminus of the 8-kilometer-long
glacier is a much-dissected area of bedrock with numerous deeply cut gullies
and coulees. Known as the Labyrinth, this rugged topography extends down
the valley about 6 kilometers and may have been formed by gigantic floods
of glacier meltwater beneath the ice sheet, similar to the way the famed
Channeled Scabland of eastern Washington State was formed. Scientists
do not agree on a precise explanation for the catastrophic erosion by
water, but the effects are nonetheless startling.
The lower part of the Wright Valley, below Lake Vanda, is zoologically
interesting. Scattered over several miles are mummified, rock-hard carcasses
of Weddell seals. Radiocarbon dating of the remains is difficult, but
scientists estimate that the carcasses are between 2,500 to 3,500 years
old. Their travel inland so far from their coastal habitat is somewhat
of a mystery.
The geologic history of these mountains, revealed through fossil plant
and animal remains in the Beacon Sandstone, indicates that the rocks are
of a similar age and depositional environment and climate as rocks found
today in parts of southern Africa, India, South America, and Australia.
These mountains and the antarctic continent were a part of the single
large landmass known as Gondwanaland. Sometime during the Jurassic Period
(135 to 180 million years ago), the land masses began to break up and
gradually drift apart, eventually to their present continental positions.
The Transantarctic Mountains were uplifted during the Cenozoic Era,
which began about 135 million years ago, with the sandstone and dolerite
sills being brought to elevations between 1,500 and more than 4,000 meters
above sea level. In places, the buckling of the rocks partially cut off
the avenues of ice draining off the eastern margins of the polar plateau,
and some areas became devoid of ice in the process to form the present