The McMurdo Dry Valleys are a row of snow-free valleys in Antarctica located within Victoria Land west of McMurdo Sound. The region is one of the world’s most extreme deserts, and includes many interesting features including Lake Vida and the Onyx River, Antarctica’s longest river.
The Dry Valleys are so named because of their extremely low humidity and their lack of snow or ice cover. They are also dry because, in this location, the mountains are sufficiently high that they block seaward flowing ice from the East Antarctic ice sheet from reaching the Ross Sea. At 4,800 square kilometres (1,900 sq mi), the valleys constitute around 0.03% of the continent, and form the largest ice-free region in Antarctica. The valley floors are covered with loose gravel, in which ice wedge polygonal patterned ground may be observed.
The unique conditions in the Dry Valleys are caused, in part, by katabatic winds; these occur when cold, dense air is pulled downhill by the force of gravity. The winds can reach speeds of 320 kilometres per hour (200 mph), heating as they descend, and evaporating all water, ice and snow.
The valleys cut through the Beacon sandstone, as well as older granites and gneisses of the Ross Orogeny. Tills, deposited directly from ice, dot this bedrock landscape. These tills are relatively thin and patchy, and differ markedly from the extensive, mud-rich tills of the Laurentide ice sheet in the Northern Hemisphere. One reason for the difference is that most of the tills in the Dry Valleys were deposited from cold-based ice (ice with basal temperatures below 0 °C (32 °F)), whereas the Laurentide ice sheet was largely wet-based, with significant melting at the base and at the glacier surface.