The McMurdo Dry Valleys are a row of valleys in Antarctica located within Victoria Land west of McMurdo Sound. The region includes many interesting geological features including Lake Vida and the Onyx River, Antarctica's longest river. It is also one of the world's most extreme deserts. From north to south, the three main valleys are Victoria Valley, Wright Valley and Taylor Valley. The valleys cut through the Beacon sandstone.
The Dry Valleys are so named because of their extremely low humidity and their lack of snow or ice cover. Together, at 4800 square kilometers, they constitute around 0.03 % of the continent, and form the largest relatively ice-free region in Antarctica. The valley floors are covered with a loose gravelly material, in which ice-wedge polygons may be observed.
The unique conditions in the Dry Valleys are caused by katabatic winds (from the Greek word for 'going down'). These occur when cold, dense air is pulled downhill simply by the force of gravity. The winds can reach speeds of 320 km/h (200 mph) evaporating all moisture - water, ice and snow - in the process.
The gravel appears to be derived from two sources. The first is terminal moraines which have formed at the end of glaciers which descend into the Dry Valleys. These glaciers sublime directly to air, for the most part, adding very little liquid water to the valleys. The second potential source of gravel is a rather unusual source. It is believed that during some glacial periods, the quantity of ice in the nearby Ross Sea was so great that it forced its way inland into some of the Dry Valleys, in a kind of reverse glacier and deposited its own terminal moraine.
Endolithic plants have been found living in the Dry Valleys, sheltered from the dry air in the (relatively) moist interior of rocks. Summer meltwater from the Valleys' overhanging glaciers provides the primary source of soil nutrients. Scientists consider the Dry Valleys perhaps the closest of any terrestrial environment to Mars, and thus an important source of insights into possible extraterrestrial life.
Part of the Valleys was designated an environmentally protected area in 2004.