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 ice sheet.

Climate

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 per hours.

Patterned ground

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 by sublimation.

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 polygons.

Lakes

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.

Glaciers

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.

Former glaciations

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 years ago.

Bull Pass

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 landscape.

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 this way.

Labyrinth

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.

Mummified seals

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.

Transantarctic Mountains

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 dry valleys.