    thumb|location of valleys (indicated by red dot) within the ross dependency 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. ==climate==  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 , 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 , heating as they descend, and evaporating all water, ice and snow. ==geology==  the valleys cut through the beacon supergroup, as well as older granites and gneisses of the ross orogeny. deposited directly from ice, tills dot this bedrock landscape. these 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 we
Antarctic_Specially_Managed_Areas Landforms_of_Victoria_Land McMurdo_Dry_Valleys Oases_of_Antarctica