Why Does That Desert Ridge Point Downwind? The Wind Carved It

Wind alone cannot erode rock to any significant degree, since moving air carries too little energy. What does the work is the sand the wind carries, which travels mostly by bouncing along close to the surface rather than being lifted high, so the cutting is concentrated in the first metre or so above the ground. That produces undercutting at the base of any obstacle, which is the characteristic signature. Over long periods the abrasion carves softer rock away and leaves harder material standing, and because the sand travels with the wind, the resulting shapes align with the prevailing direction rather than with any feature of the rock.

The form is consistent enough to be recognised from the air:

• •A long ridge aligned with the prevailing wind direction
• •Blunt and steep at the upwind end, tapering downwind
• •Frequently described as an inverted boat hull
• •Undercut along the base, where sand concentration is highest
• •Occurring in parallel fields rather than singly
• •Separated by corridors scoured to a flat floor

The conditions needed are restrictive, which is why the landform occurs in a limited number of places. There must be a strongly dominant wind direction sustained over a very long period, since a shifting wind produces rounded forms rather than aligned ones. There must be a supply of sand to do the cutting. The rock must be soft enough to be abraded, so the landform forms typically in weakly cemented sediments, ancient lake beds and volcanic ash rather than in hard crystalline rock. And there must be almost no rain, since water erosion works far faster and would destroy the form before the wind could complete it.

Wind abrasion produces a family of features and distinguishing them matters when reading a landscape. A ventifact is an individual stone cut and polished by sand, frequently with two or three flat facets meeting in sharp edges where the stone has been turned over at some point in its history. A mushroom rock stands on a narrowed stem, because the cutting is concentrated near the ground where sand travels. Pedestal rocks and undercut cliffs record the same concentration on a larger scale. Desert pavement is the lag of stones left where wind has removed everything finer. Each records the same process acting on a different scale and material.

The largest field is in the Lut desert of Iran, covering many thousands of square kilometres with ridges up to eighty metres high, cut into ancient lake sediments and aligned with a wind that blows consistently for months. Comparable fields occur in the Sahara, in the Taklamakan, in Peru and in Antarctic dry valleys, where the same process works in extreme cold. The landform is also abundant on Mars, where a thin atmosphere and very long timescales have produced enormous fields, and studying the terrestrial ones is one of the ways the Martian surface is interpreted, since the shapes record wind direction over periods no measurement could cover.

Sand bouncing along within a metre of the ground does the cutting rather than the wind itself, which is why these ridges are undercut at the base and aligned with the prevailing wind rather than with the rock. They need a dominant wind sustained for a very long time, a sand supply, soft rock and almost no rain. The Lut desert field covers thousands of square kilometres, and Mars carries enormous ones.